Metrology

METROLOGY FOR NON-METROLOGISTS

Rocío M. Marbán Julio A. Pellecer C.

2002 iii

To contact the authors: 2001 Producción y Servicios Incorporados S.A. Calzada Mateo Flores 5-55, Zona 3 de Mixco Guatemala, Centro América Tel.: (502)431-0662 Fax: (502)434-0692 email: [email protected]

ISBN 99922-770-1-7 © OAS, 2002

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This English version of the second revised edition is published under the sponsorship of SIM.

The Interamerican Metrology System, SIM (Sistema Interamericano de Metrología, Normalización, Acreditación y Calidad) is the regional organization for metrology in America, comprising national metrology institutes from the 34 member nations represented at the Organization of the American States, OAS, which acts as its Executive Secretariat.

The opinions stated in this document are not necessarily opinions of the OAS, its bodies or its staff.

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CONTENTS Acknowledgements Presentation

ix xi

Introduction What we measure and how Characterization of metrology Vocabulary

1 11 19 21

Applications - what is measured and what for Length Mass Temperature Time and frequency Electricity and magnetism Photometry and radiometry Acoustics and vibrations Ionizing radiation Chemistry

27 27 28 29 30 31 32 33 34 35

Standards and reference materials Introduction Length Mass

37 39 45

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INTRODUCTION The initial concept of metrology derives from its etymology: from the Greek metros - measure, and logos - treaty. This concept is certainly as old as human beings: “I have nothing”, “I have something”, “I have much”; these expressions reflect a primitive comparison that is still valid and presently we can say that metrology is the science of measurements and that to measure is to compare with something (a unit) which is taken as the basis for comparison. Measurements for primitive human beings began with the ideas of: near-far, fast-slow, light-heavy, clear-dark, hard-soft, cold-hot, quiet-noisy. At first these were personal perceptions, but experience and life in common gave rise to comparisons between persons and, through the ages, to generally accepted bases for comparison. Thus, after several millennia, it is easy to think of bases for comparison of personal concepts - in other words: measurements and their units.

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Metrology for non-metrologists

Some of these measurements and units are basic: MEASURE

UNIT

length mass time temperature intensity of light electric current amount of substance

metre (meter) kilogram second kelvin candela ampere mol

For other purposes, not covered by the above, it is often necessary to use other measurement units, called derived units because they use or are based on the base units. That is, using mathematical algorithms, a unit is expressed algebraically in terms of other units. To enter the realm of units based on one or more fundamental units, is to enter a world of scientific algorithms for specific purposes, which is why derived units are more numerous. A unit is a value in terms of which a quantity may be described. It must be stressed that, qua unit, it must not be broken down into its elements. Mul2

Introduction

tiples and sub-multiples are used to express quantities larger or smaller than those of the unit per se. We will see later on that in the International System of Units multiples and sub-multiples are decimal, that is, they use powers of 10. We mentioned using something with which to compare; this something is known as a measurement standard or simply a standard. Originally, a standard was considered to be a representation or physical embodiment of a unit. It was necessary to stress that the standard was a trustworthy representation of the unit only under a set of precisely defined conditions, to make sure it was independent of environmental influences such as temperature, humidity, atmospheric pressure, etc. Because of their characteristics, physical standards were not used to directly take measurements. Instead they were the basic reference point for the manufacture and calibration of the instruments that are used for such purposes. Today, thanks to scientific advances, we have more exact and reliable definitions for the units, based on universal physical constants, and now a standard can be defined as: a materialized measure, 3

Metrology for non-metrologists

measurement instrument, reference material or measurement system, whose purpose is the definition, materialization, conservation, or reproduction of a unit, or one or several known values of a quantity, for transmission by comparison to other measurement instruments (2). It is also important that the procedure used to measure give reproducible results and, in fact, there are precise instructions on how to carry out the procedure, which units to use and which standard. In the real world, we usually measure following this sequence: - we decide what we are going to measure, - we select the unit according to the measure, - we select the measuring instrument (calibrated), - we apply the accepted procedure. Before going into details of the main measures, let us have a brief, very brief look, at the history of measurement. Archeological finds show that very ancient civilizations had well-defined concepts of weighing and measuring. Trade, land division, and taxation, 4

Introduction

among others, must have required very soon the uniformity of measurements. The appearance of weights and measures systems goes far back into time. We know little of what was done in the Far East; however, there is no doubt that they existed in the Mesopotamian civilizations and - clearly - it is obvious that the construction of pyramids in Egypt (3000 to 1800 BC) required elaborate systems of measurement. We know, and in some countries we still use, some of the linear measurements of current usage in ancient Egypt (the span, the foot, the pace, the fathom, the cubit). Also in Egypt, scales were used to weigh precious metals and gems. Later on, when coins began to be used as elements of trade, they were simply pieces of gold or silver, stamped with their weight. They gave birth to a monetary system that spread throughout the whole Mediterranean area. The way we measure time is based on the sexagesimal system developed in Mesopotamia, and our calendar is derived from the original 365 days Egyptian calendar. 5

Metrology for non-metrologists

Roman conquest of a large part of the European continent contributed to disseminate the systems of weights and measures. By the beginning of the second millennium AD, the different measures in use had mutiplied uncontrollably. There were, for instance, different measures for capacity according to the product, be it wine or beer, wheat or barley. Measures could also vary from province to province or from town to town. England used Anglo-Saxon measures and gradually tried to improve and simplify its system. For many centuries, the pound-foot-second system was the preferred system in English-speaking countries as well as worldwide for some commercial and technical uses; to date it has not been totally discarded and is still used for many activities in many countries. France created and developed a simple and logical system, based on the most advanced scientific principles known at the time (the end of the eighteenth century) - the decimal metric system, which first came in use during the French Revolution. It owes its name to the use of the decimal system for multiples and sub-multiples and to its 6

Introduction

base unit: the metre, mètre in French, which is itself derived from the Greek metron, meaning measure. In its first version, the metre was defined as one ten-millionth of the length of a quadrant of the earth’s meridian (i.e. one ten-millionth of an arc representing the distance between the Equator and the North Pole) and it was determined by measuring an arc of meridian between Dunkerque, in France, and Barcelona, in Spain. The history, vicissitudes, development and application of this system are amply documented (1,18). Metrologists are very active and there are constantly important changes and improvements in all aspects of measurements. Growing cooperation between metrologists from different countries is also helping to establish internationally accepted work procedures. There are now uniform methods of measurement so we can all work on the basis of the same known quantity or unit, and the results of any calibration, verification and test, in any laboratory or enterprise, are a guarantee of compatibility and quality. In consonance with the global approach, more and more countries are adopting the International System of Units (SI) based on the decimal metric sys7

Metrology for non-metrologists

tem, with the subsequent adoption of the corresponding standards and measurements techniques. Forty-eight countries have subscribed the Metre Convention, that adopted the International System of Units (SI). The Convention gives authority to the Conférence Générale des Poids et Mesures (CGPM - General Conference on Weights and Measures), the Comité International des Poids et Mesures (CIPM - International Committee on Weights and Measures) and to the Bureau International des Poids et Mesures (BIPM - International Office of Weights and Measures), to act internationally in matters pertaining to metrology. CGPM is constituted by representatives of the member countries and it holds meetings every four years in Paris, France for: discussion and examination of agreements for the improvement and dissemination of the International System of Units (SI), validation of advances and results of new fundamental metrological determinations, scientific international resolutions, and decisions pertaining to the organization and development of the BIPM.

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Introduction

In search of a world-wide unification of physical measurements, the BIPM: -

-

establishes fundamental standards and scales for the main physical quantities, carries out and coordinates determinations related to physical constants, preserves international prototypes, coordinates comparisons with standards kept at the National Laboratories of Metrology, ensures coordination of the measurement techniques.

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WHAT WE MEASURE AND HOW The units of the International System of Units (SI) are established by the General Conference on Weights and Measures (CGPM) with authority over the International Office of Weights and Measures BIPM, with headquarters in France. In what follows, the international definitions for the units are those published by BIPM, as of June 2002. CGPM decided to base the SI on seven well-defined units. These are known as base units and they are listed in Table 1. Originally, the base or fundamental units were so called because they were considered to be mutually independent and because, from them, all other units could be derived. The corresponding standards were material embodiments, kept in agreed locations, under strictly determined conditions. Thanks to scientific and technical advances, and the availability of more exact instruments, the base units, with the single exception of the kilogram, are now defined differently, based on physical experiments. It can be argued that in some cases base units are no longer mutually independent. For

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Metrology for non-metrologists

TABLE 1 Base Units of the SI Quantity

Symbol

Unit

length

m

metre

mass

kg

kilogram

time

s

second

electric current

A

ampere

thermodynamic temperature

K

kelvin

amount of substance

mol

mol

luminous intensity

cd

candela

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What we measure

instance, the metre is no longer defined against the former prototype metre - an iridium-platinum bar - and the current definition involves the concept of second, another base unit. Similarly, the candela, the base unit for luminous intensity, is defined in terms of the hertz (s-1) and the watt (m2.kg.s3), both derived units, and of the steradian, a supplementary non-dimensional unit. However, taken as the set of both base and derived units, the SI is considered to be a coherent system because: -

-

-

the base units are defined in terms of physical constants (Appendix 1), with the sole exception of the kilogram, defined in terms of a prototype, each quantity is expressed in terms of a single unit, obtained by multiplication or division of the base units and of the nondimensional derived units, multiples and sub-multiples are obtained by multiplication by an exact power of ten, derived units can be expressed strictly in terms of the base units, that is, they have no numerical factor other than the number 1. 13

Metrology for non-metrologists

The work that is being done for the definition and the improvement of SI units always strives to have units consistent with those that already exist. As we mentioned before, the base units give rise to a large number of other units; Appendix 2 lists some of those considered SI derived units. Two derived units, known formerly as “supplementary” units, deserve special mention. They are the radian (rad), used to measure plane angles, and the steradian (sr), used to measure solid angles. They are also called non-dimensional units. The neper and the bel, of accepted use although they are not integrated into the SI, are also non-dimensional units. The SI also has a set of rules and conventions that have to do with the use of mixed units, how to select and identify prefixes, the use of multiples and sub-multiples, spelling conventions, use of capitalization, use of singulars and plurals, how to group digits of numerical values, decimal marker, rounding out of numerical values, etc.(16,30,37) These rules are not yet uniformly applied; for instance, in several countries of America, the dot and 14

What we measure

not the comma is used as the decimal sign or marker. In any case, it is important to be aware of these rules and for more detailed information on the subject we recommend consulting some of the references (16,37,40,46). There are also some units that do not belong to the SI but that are accepted for use with it. They are sometimes called additional units and are listed in Table 2. Some of those are accepted temporarily, until their use is no longer necessary and they are substituted by the approved units; in some cases their use is limited to specialized fields, as for example the carat (ct) in jewelry. There are other units, outside the SI(40,46), still in use in some countries and some contexts, such as the dyne and the stokes. If we now look at the hierarchical structure of measurement standards, we see we can describe it as a pyramid. At the top, we have the set of standards that corresponds to the SI base units, of which we have already spoken. The second position is taken up by the set of national standards. 15

Metrology for non-metrologists

At the next level, we find the reference standards that will be used to prepare the working standards to be used in turn for operational work. The set of operational standards (working standards) is the base of the pyramid. The chain of organizations in charge of the operation of the SI, is headed by the BIPM, followed by the National Metrology Institutes, the Calibration Laboratories and, finally the working Laboratories. The national metrology institutes have custody of the national standards, and the responsibility for dissemination of the SI units to accredited calibration laboratories in their respective countries. The calibration laboratories are in charge of verifying that measuring equipment as well as reference and working standards comply with the national standards. The testing and assay laboratories, at the operational level, are in charge of evaluating conformity for the products that are to be certified. To do this, they use reference standards, calibrated using the national standards of the upper level. 16

What we measure

TABLE 2 Additional units accepted for use with the SI Name

Time: minute hour day

Symbol

Expression in terms of SI units

min h d

1 min = 60 s 1 h = 60 min = 3600 s 1 d = 24 h = 86 400 s

o

Plane angle: degree minute second

’ ”

1o = (π/180) rad 1’ = (1/60)o = (π/10 800) rad 1”= (1/60)’ = (π/648000) rad

Volume: liter

l, L(a)

1 L = 1 dm3 = 10-3 m3

Mass: ton, metric ton t

1 t = 103 kg

a) The alternative symbol for the liter, “L”, was adopted by the CGPM in order to avoid the risk of confusion between the letter l and the number 1. The script letter is not approved for the liter. b) Other additional units are: the electronvolt (eV), the unified atomic mass unit (u), and the astronomical unit (ua).

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Metrology for non-metrologists

Finally, we find those organizations and entities working with operational or working standards, used by industry and others, and that are normally calibrated against reference standards, which, in turn, have been calibrated using national standards. An important concept in metrology is that of traceability. It refers to the property of a measurement or of the value of a standard, to be related to established references, normally national or international standards, through a continuous chain of comparisons, all of them with known uncertainties. The possibility of determining traceability in any measurement relies on the concept and the actions of calibration and on the hierarchical structure of the standards we have already mentioned. For metrologists, calibration is: a set of operations that establish, under specified conditions, the relation between the values shown by a measuring instrument, a measuring system, the values represented by a materialized measure or by a reference material, and the corresponding values of the quantities established by the standards. The term is sometimes misapplied to a process of comparison or verification that is used to verify that between the values shown by a measuring instrument 18

What we measure

or system and the known values of the measured quantity, the differences are below the maximum tolerance (2). On the other hand, metrologists usually take into consideration the main causes of errors in measurements; they may or may not be known and controllable and can be due to factors of the environment where the measurements are taken, to defects of construction or calibration of the instruments, to operator mistakes, to the interpretation itself of the data, or simply to fortuitous factors.

CHARACTERIZATION OF METROLOGY For convenience, a distinction is often made between the several fields of application of metrology, into: Scientific Metrology, Legal Metrology, and Industrial Metrology. Scientific metrology This is the set of actions taken to develop primary standards of measurement for the base units and the derived units of the International System of Units (SI). 19

Metrology for non-metrologists

Legal metrology According to the International Organization for Legal Metrology (OIML) “legal metrology is the entirety of the legislative, administrative and technical procedures established by, or by reference to public authorities, and implemented on their behalf in order to specify and to ensure, in a regulatory or contractual manner, the appropriate quality and credibility of measurements related to official controls, trade, health, safety and the environment”. Industrial metrology The function of industrial metrology is mainly the proper calibration, control and maintenance of all measuring equipment used in production, inspection and testing. The purpose is to guarantee that the products will comply with quality standards. The equipment is controlled at set times and in such a way that the uncertainty of the measurements will be known. Calibration is done against certified equipment, with a known valid relation to standards such as, for instance, the national reference standards.

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What we measure

VOCABULARY To understand each other, metrologists use an internationally approved vocabulary, the International Metrology Vocabulary (VIM)(54); some of the most common definitions follow: Quantity (measurable) attribute of a phenomenon, body or substance that may be distinguished qualitatively and determined quantitatively. Base quantity one of the quantities that, in a system of quantities, are conventionally accepted as functionally independent of one another. Derived quantity quantity defined, in a system of quantities, as a function of base quantities of that system. Dimension of a quantity expression that represents a quantity of a system of quantities as the product of powers of factors that represent the base quantities of the system.

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Metrology for non-metrologists

Quantity of dimension one, dimensionless quantity quantity in the dimensional expression of which all exponents of the dimensions of the base quantities reduce to zero. Unit (of measurement) particular quantity, defined and adopted by convention, with which other quantities of the same kind are compared in order to express their magnitudes relative to that quantity. Base unit (of measurement) unit of measurement of a base quantity in a given system of quantities. Value (of a quantity) magnitude of a particular quantity, generally expressed as a unit of measurement multiplied by a number. Measurement set of operations having the object of determining a value of a quantity. Measurand particular quantity subject to measurement.

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What we measure

Accuracy of measurement closeness of the agreement between the result of a measurement and a true value of the measurand. Repeatability (of results of measurements) Closeness of the agreement between the results of successive measurements of the same measurand carried out under the same conditions of measurement. Reproducibility (of results of measurements) closeness of the agreement between the results of measurements of the same measurand carried out under changed conditions of measurement. Uncertainty of measurement parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand. Material measure device intended to reproduce or supply, in a permanent manner during its use, one or more known values of a given quantity.

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Metrology for non-metrologists

Measurement standard, etalon material measure, measuring instrument, reference material or measuring system intended to define, realize, conserve or reproduce a unit or one or more values of a quantity to serve as a reference. Standards may be international (recognized through international agreement) or national (recognized by national agreement). Primary standard standard that is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity. Secondary standard standard whose value is assigned by comparison with a primary standard of the same quantity. Reference standard standard, generally having the highest metrological quality available at a given location or in a given organization, from which measurements made there are derived.

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What we measure

Working standard standard that is used routinely to calibrate or check material measures, measuring instruments or reference materials. Transfer standard standard used as an intermediary to compare standards. Traceability property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties. Reference material (RM) material or substance one or more of whose property values are sufficiently homogeneous and well established to be used for the calibration of an apparatus, the assessment of a measurement method, or for assigning values to materials. Certified reference material (CRM) Reference material, accompanied by a certificate, one or more of whose property values are certified by a procedure which establishes traceability to an 25

Metrology for non-metrologists

accurate realization of the unit in which the property values are expressed, and for which each certified value is accompanied by an uncertainty at a stated level of confidence.

Note: because not all countries use the same system to write numbers, it must be stated that in this document we use the comma as the decimal marker and an “x” for the multiplication sign. Thus, for instance, we shall write 6,023 x 1023 and not 6.023 x 1023. Most English-speaking countries use the period or full stop as the decimal marker.

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APPLICATIONS A possible question is: what do we measure for? Without going into details and without any pretense at being exhaustive, let us look at some answers, restricted to main aspects. As can be expected, different applications require different actions which are done with different levels of reliability; in metrology this is known as “uncertainty”, an interval of confidence in the results of the measurements. Length Measurement of length, or the determination of distance, is used in dimensional measurements such as: areas, volumes, capacities, speed and velocity, roundness. Length is present in the definition of the radian and the steradian, the non-dimensional units used to measure angles. In general, we could say that it is used in any determination of the shape of an object. Many fields of human endeavor require dimensional measurements: geodesy, real estate and the property and use of land, construction and maintenance 27

Metrology for non-metrologists

of roads, highways, streets and avenues, building of dwellings, all manufacturing industry, machine tools, odometers to calculate charges for car rentals, many commercial aspects. It is probably in the manufacturing industry where the influence of good length measurements is more striking. The industries of apparel, furniture, automotive, accessories, home appliances, scientific and medical instruments, electronic equipment and many more, require parts that must fit properly into each other, as well as exact measures in the final consumer products. Mass The need to know mass quantitatively is present in most human activities. This explains the wide range of standards and instruments used to determine mass. Without going into details we can mention: industry - administration (purchasing, storehouses, etc.), production (processes and control), sales (orders and shipments); laboratories (research and control); trade (all transactions); science (even in theoretical occupations). The amounts to be determined can go from the mass of the electron to the mass of the universe, through that of mosquitoes, hamburgers, human beings, vehicles, etc. 28

Applications

Normally, everything that is produced, sold or exchanged, is related, directly or indirectly, to mass; which is why we can say that application of metrology in its mass aspect is omnipresent at all levels in everyday life. Temperature The sensation of heat or cold is a common one for living beings and the concept of temperature and its measurement are present in countless human activities. Our first contact with scientific measurement of temperature is usually the home thermometer. We thus think immediately of medical applications and, particularly, measurement of body temperature in sick people, with the importance it can have for the evolution of some ailments. But correct temperature measurements are also required for the manufacture of pharmaceuticals, the use of diagnosis techniques, clinical analysis, sterilization of clinical and hospital materials. Food preparation and the techniques for its conservation require temperature measurements;

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Metrology for non-metrologists

these can be empirical for home use but industry demands accuracy in its measurements. Dyeing of fabrics, manufacture of all types of ceramics, paints and enamel for home appliances and for vehicles, generation of energy, refrigerated transport, air conditioning, and many more human activities, require correct determinations of temperature. Time Measurement of time is useful not only to make sure we are punctual or to determine the winner in a race! There are obvious applications in daily life (getting up at a certain time; buses, trains and airplanes being on time, control of working hours for payment of salaries, control of time for telecommunication charges, etc.). There are also many industrial processes, many medical techniques, that rely on exact measurements of time. Other applications include the use of taximeters (based on time only, or on a combination of time and distance), timekeepers, speedometers.

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Applications

Synchronization of activities such as those on the stock exchange and the military, launching and coupling of spacecraft, etc. demand exact measurements of time. In general terms, we can speak of watches and chronometers (either of type I with digital electronic circuits, or of type II with analog mechanisms or synchronous motors) as well as other timepieces such as those used in vehicle parking meters, automatic car washers, or the timers of home appliances such as washing machines, dryers, microwave ovens. Electricity and magnetism The last two centuries have given birth to countless advances toward our current modern development; electric motors were built and these contributed to industry, transport and all activities that require some type of movement. With incandescent bulbs, artificial light radically changed all of man’s nocturnal activities. To try to enumerate all current applications of electricity, properly offered and used, would mean listing all of mankind’s activities for which electricity 31

Metrology for non-metrologists

has to be controlled (i.e. measured), a control that demands reliable apparatus and systems with a known accuracy. Electricity is fundamental for communications, be it telephones, radio, television, satellite operation. But, what metrology guarantees with its standards and its procedures for electricity and magnetism, is really the reliability in the handling and use of this resource, rather than the availability itself of such a resource. During design, many problems of reliability come up, and the ability to rely on systems that can ensure the proper behavior of equipment, within set limits, is what makes it possible to design, plan and implement complex projects. Also, all of electronics demands reliable (exact, to the layman) measurements, and this reliability and reproducibility are due in great part to the advances in metrology. Photometry and radiometry Man has developed many apparatus and devices that allow him to see no matter what the natural conditions are and, what is more, that can give him light intensities that would be difficult to find in na32

Applications

ture. All these apparatus demand reliable measurement techniques to ensure that the intensity required is effectively being obtained. But, even more important, the techniques for physical and chemical analysis are very often based on extremely exact measurements of light or radiation. Absorption photometers, black body photometers, photoelectric instruments, spectrophotometers and radiation measurement apparatus rely for their accuracy on careful calibrations, based on accepted standards. Photodynamic therapies are currently being used for some ailments, ultraviolet light is used industrially, certain wavelengths of radiation are used for their germicidal properties while others are used for plant growth, etc.; all these applications need reliable measurements. Acoustics and vibration Exact acoustic measurements are crucial for applications such as the design of theaters and auditoria, telecommunications, radio, the manufacture of musical instruments and of sound reproduction and transmission devices (including pho33

Metrology for non-metrologists

nographs, microphones, amplifiers), the elimination of bothersome or dangerous sounds (in offices, production areas, land and air transport), the design of warning systems such as ambulance and firemen sirens and certain industrial indicators, sonar, petroleum exploration, seismographs, echocardiograms and ultrasound in chemistry and in medicine for diagnosis and treatment, in industrial applications such as welding. Ionizing radiation Medical applications of ionizing radiation are possibly the better known, under the form of X rays for diagnosis, and the use of radioactive isotopes for radiotherapy and as tracers in medical and biochemical research. Among industrial applications, we can mention the activation of vitamins, chemical synthesis (such as that for ethyl bromide), polymerization (polystyrene, polyethylene), rubber vulcanization, polymerization of methylmetacrylate, textile finishes for permanent press fabrics and garments, food processing (cooking, drying, pasteurization, etc.), preservation and sterilization of foods, control of germination and of

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Applications

insect infestations in stored grains, “curing” or solidification of finishes in paints and inks, metallurgy, geochemistry, archeology (C14), thickness measurements, electric power generation. Chemistry In scientific and technical activities it is always of importance to be able to know on which basis to calculate which and how much of several substances should be used. An obvious case is that of the laboratory, clinical or industrial, but this can also be said of all types of industrial processes; some, because they handle very large volumes and small variations can imply losses of tons; others, because they use very small amounts and minimal variations can be crucial. That is to say, the use of standards and reference materials is the basis for successful production and the guarantee of quality. As a simple example, the production and marketing of pharmaceuticals is a huge field for the application of metrology.

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STANDARDS AND REFERENCE MATERIALS INTRODUCTION Standards and reference materials are the subjects to be treated in the next sections, in accordance with the following scheme: general considerations of what we measure, the definition of the measurement unit, primary standards, accuracy and uncertainty, and measuring instruments. On the subject of uncertainty, it should be noted that there are two schools among metrologists(2). One of them looks at uncertainty as an element to denote uniformity of the results in repeated measurements. The other school uses the term to mean that we are determining differences among the results. In both cases, we must remember that uncertainty is simply an interval of confidence in the results of the measurement. Both points of view are valid considering the field of application, whether operational or national metrology laboratories. National and secondary laboratories should apply the 1993 ISO “Guide to the expression of uncertainty in measurement”(28). 37

Metrology for non-metrologists

In the Western Hemisphere, the Inter-American System of Metrology, SIM, through the work it is doing, is striving to obtain the highest possible integration and coherence in metrology aspects among its members. In 1999, SIM authorities did a strategic planning exercise. One of the aspects studied was the determination of which areas should be the subject of regional and of national actions. These areas turned out to be: length, mass, temperature, time and frequency, electricity and magnetism, photometry and radiometry, acoustics and vibration, ionizing radiation, and chemistry.

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Standards and reference materials

LENGTH What do we measure We all know intuitively what length is. In practice, what we really measure is the distance or separation between two points and, given that the current definition of standards is nowadays oriented to the use of physical constants, we must be conscious that length implies distance. There are estimates that around 80% of the measurements done in industry have to do with displacements and thus with length. In the year 1800, an accuracy of 0,25mm was considered proper for length measurements; today(18) we speak of required intervals that range from the field of nanotechnology up to that of geophysics.

International definition of the unit for measurement of length History Originally, the metre was defined as one ten-millionth of the length of a quadrant of the earth’s 39

Metrology for non-metrologists

meridian (i.e. one ten-millionth of an arc representing the distance between the Equator and the North Pole) and it was determined by measuring an arc of meridian between Dunkerque, in France, and Barcelona, in Spain, cities both at sea level. The first physical embodiment of the metre, the so-called mètre des Archives, was built in 1799 on this basis. Later, with the approval of the “Convention of the Metre” in 1875, a copy of this prototype became, in 1889, the international prototype of the metre. This prototype metre, an iridium-platinum bar which is still kept in Paris, was considered stable and precise, as well as its copies, and they were used until 1960, when the definition was replaced by one based on the wavelength of a given orangered line of the spectrum of the isotope krypton-86. In 1983, the 17th General Conference on Weights and Measures modified it to the current definition which is related to the speed of light in vacuum (299 792 458 metres per second). Definition The metre (symbol m) is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second.

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Standards and reference materials

Standards For the measurements of length to be practical, it is necessary to transfer from a standard expressed in terms of the velocity of light, to a physical device or standard. For measurements of the order of the metre, interferometric methods are used. The method consists of a comparison between the length to be measured and the wavelength λ of a luminous radiation whose frequency f has been previously determined with great accuracy. The reference used is the wavelength of the radiation produced by a laser, stabilized either in frequency or in wave-length(43). For instance(43), with an helium-neon laser, stabilized in a methane chamber, wavelengths of 3 392,231 397 327 nm can be measured with a relative uncertainty of 3 x 10-12; with an argon laser, stabilized in a iodine chamber, wavelengths of 514,673 466 4 nm, can be measured with a relative uncertainty of the order of 2,5 x 10-10. There are currently portable models of stabilized lasers, and with these it has been possible for BIPM to compare and calibrate in situ in a region without 41

Metrology for non-metrologists

the requirement of several national metrology laboratories having to take their apparatus to Paris for calibration(19). Thanks to these laser-based calibrations, the countries can have their own national standards. By following the chain we have already seen, calibration standards as well as testing and operational standards can be derived from these national standards, and they may include measuring tapes, rulers and all other devices used in everyday life to measure length. In addition to the methods based on light sources, standard gauge blocks are also used. These are highly polished, metallic or ceramic blocks, whose edges have a very high-quality parallelism, and that can be combined in the required number to obtain the desired length, with an accuracy according to their intended use, whether for calibration or operational work. The gauge blocks, calibrated by interferometry, may themselves constitute the physical embodiment of the standard, and, through mechanical comparison, secondary standards may be derived from them. 42

Standards and reference materials

Uncertainties As mentioned before, stabilized laser standards can offer relative uncertainties for the measurement of length of the order of 10-9 and of 10-12. Measuring instruments Length, width, height, thickness, diameter, all these are linear measurements and many instruments and devices have been developed so they can be measured simply and with the required accuracy. We thus find, among others: rulers (wood, metal, fiberglass or plastic, rigid or folding), tape rulers (of metal, plastic or fabric), calipers and dividers (highprecision, for nuts and screws, for gears), micrometers, verniers, gauge blocks, depth and angle gauges, protractors, interior and exterior diameter tapes, roundness or surface gauges, roughness testers, etc. These instruments may be based on mechanical, pneumatic, optical or electronic methods. Accuracy tolerances are established in accordance with the type of instrument and its intended use.

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44

Standards and reference materials

MASS What do we measure The mass of a body can manifest itself in two ways: one is by a change in its state of motion (inertia) and the other by its attraction to other bodies. Let us suppose an imaginary vacuum tunnel, where we have a perfectly lubricated plane surface so that, if we place an object on this surface, there is no friction whatsoever between the surface and the object. If the object is at rest and we want it to move, the effort required to move it is an indication of the mass of the object. In the same tunnel, under the same conditions, if we remove the plane surface, the object falls down, attracted by planet Earth, and this is another manifestation of its mass. In both cases, we would have the measure of the mass of the object: in the first, by the measure of the effort to move the object, and in the second, by the measure of its fall.

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In other words, mass is the quantity of matter contained in a given body while weight is the result of the attraction of planet Earth on that body.

International definition of the unit for measurement of mass History The mass unit, the kilogram, was originally defined as the mass of one liter of water at a temperature of 4o C. This definition was later modified in view of the practical difficulties of obtaining pure water, and because the definition involved another quantity: temperature. It could be argued that the kilogram is really a multiple of the gram and that it is the gram that should be the unit. This has been studied by metrologists but, for practical reasons, it was agreed to continue considering the kilogram as the mass unit. With present knowledge, it has not yet been possible to define the mass unit in terms of universal physical constants; thus, based on the agreements at the 1st and 3rd General Conferences on Weights 46

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and Measures in 1889 and in 1901 respectively, it is currently still defined in terms of a device or prototype. However, the 21st General Conference on Weights and Measure, in October 1999(13), “recommends that national laboratories continue their efforts to refine experiments that link the unit of mass to fundamental or atomic constants with a view to a future re-definition of the kilogram”.

Definition The kilogram (symbol kg) is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

Standards The international prototype is a cylinder, thirty-nine millimetres high and thirty-nine millimetres wide, made from an alloy of ninety percent platinum and ten percent iridium. It has an approximate density of twenty-one and one half grams per cubic centimetre. It is considered the sole primary standard for the kilogram. The original prototype kilogramme des Archives, manufactured at the 47

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same time as the mètre des Archives, is considered to be the historical prototype. From a single smelt, in 1889, were prepared: the international kilogram, four witnesses, and national prototypes (originally 40 of them to fill the needs of the countries then signatories of the Convention of the Metre). These, and those subsequently manufactured by BIPM, are sometimes known as “Nox kilogram”, where “x” is the identification number of one of these standards. Because the definition and construction of the unit are based on an artifact, the unit can never be transferred more accurately than allowed by mass comparison with the international mass prototype. Taking into account the limitations of the comparisons, a hierarchy of the mass standards has been set up, with the following obligatory characteristics:

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INTERNATIONAL PROTOTYPE KILOGRAM Material: Platinum-Iridium; Density: 21,5 g cm-3 PRIMARY STANDARDS OF THE BIPM Material: Platinum-Iridium. NATIONAL PROTOTYPE KILOGRAM Material: Platinum-Iridium. PRIMARY NATIONAL STANDARDS Material: Steel (Brass) Density: 8,0 g cm-3 (8,4 g cm-3) SECONDARY NATIONAL STANDARDS Material: Steel (Brass) REFERENCE STANDARDS WORKING STANDARDS

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Accuracy With the current kilogram standard it is possible to measure mass with an accuracy of 1 in 108. The purpose of standards is to be able to measure exactly the mass of bodies; this requires multiples and sub-multiples of the kilogram so that masses can be exactly determined. The sets of multiples and sub-multiples of the kilogram must also be represented in the form of mass standards and compared with one or more kilogram standards. To do so, multiples and sub-multiples are grouped in decades related to at least 4 standards; the most common representation is 1 2 2 5; thus, a one kilogram mass, m1kg, can be determined as: m 100 + m200 + m200 + m500 where: m100 m200 m200 m500

= mass of the 100 grams standard. = mass of the 200 grams (N O 1) standard. = mass of the 200 grams (NO 2) standard. = mass of the 500 grams standard. 50

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Clearly, an analytical laboratory balance requires a different degree of accuracy than a truck weight controller scale. Accuracy of mass standards can be categorized as Ei , Fi , Mi with values going usually from one milligram to 50 kilograms. High accuracy masses correspond to class Ei , fine accuracy to class Fi , and medium accuracy to class M i . When studying the accuracy of m1kg, the first composition to estimate variability would be: m1kg - (m100 + m200 + m200 +m500 ) = x where m1kg is the one kilogram mass standard and the value of x can belong to any of the E, F or M classes. OIML Recommendation R111(41) gives the different tolerance levels for accuracy of different standards masses for classes Ei , Fi y Mi. Quality of measurement will be characterized by its uncertainty. Measuring instruments The beam balance is the oldest known instrument to have been used to measure mass. As long as the definition of the kilogram remains unchanged, 51

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we can only compare mass, never measure it directly. With contemporary techniques it is possible to build countless numbers and capacities of instruments, adequate for their intended use, be that in laboratories, industries, commerce, government agencies, etc. Basic requirements for balances are that they be stable, exact, sensitive and subject to calibration. High accuracy metrology uses mass comparators. The mass comparator for a national standard must have a limited interval and good sensitivity (for instance, 1 microgram). In the past, we spoke of simple balances, with equal or unequal arms, with or without gliding weights, combination balances, platform-scales, roman scales, crane scales, deflection balances, spring balances and automatic balances with multiple equilibrium positions; today we also use electromechanical scales which send electrical signals to determine weight. In view of all possible combinations, we nowadays speak of weighing instruments, without making distinctions between, for instance, a balance and a scale.

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TEMPERATURE What do we measure In the case of measurements of what we call temperature, what we are looking for is an indicator of the heat of a given body. But heat is not the same as temperature. We can define heat as a form of energy associated with and proportional to the molecular motion. What we know as temperature is really the value of a reading on a measuring device such as a thermometer. For this reason, we say that temperature is a manifestation of heat.

International definition of the unit for measurement of temperature History The definition of the measurement unit for temperature has a long and complex history. As early as 1742, Anders Celsius proposed a centigrade scale of temperature, based on water, with zero at the freezing point and a value of 100 at the boiling point. BIPM(19) has compiled the history of the unit, starting from the normal scale of hydrogen of 1878 up 53

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to the current international temperature scale (ITS90 or EIT-90) of 1990. It is however interesting to note that a whole century went by until, in 1954, the 10th CGPM (General Conference on Weights and Measures) adopted the proposal made in 1854 by William Thomson Kelvin, of defining the unit for thermodynamic temperature (presently named after him) in terms of the interval between absolute zero and a single fixed point. The current definition was approved by the 13th General Conference on Weights and Measures, in 1967. Definition The kelvin (symbol K), unit of thermodynamic temperature, is the fraction 1/273,16 of the thermodynamic temperature of the triple point of water. The triple point of water is the point where it is possible to have equilibrium or coexistence of the substance - water in this case - in its three states: solid, liquid and gaseous. When speaking of temperature scales, it is common to find references to the thermodynamic tempera-

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ture, to which the international definition refers, and, also, to the practical scale of temperature. The practical scale or Celsius scale, known before as centigrade, is the most commonly used. Its zero is the freezing point of water, and the boiling point is defined as 100 oC, both measured under specified conditions. Under zero of this scale, temperatures have a negative value; which is why we commonly say that in a harsh winter, temperature may go down to minus forty degrees (Celsius degrees). On the other hand, the thermodynamic temperature scale is expressed in kelvin by definition, and has its zero at what is called absolute zero, equivalent to -273,16 oC. Thus, this scale has no negative values and its intervals are the same as those of the Celsius scale. Experts in thermometry usually express temperatures below 0 oC in kelvin, and those higher than 0 oC in Celsius degrees. They also insist on the fact that the freezing point of water, 0 oC, at normal atmospheric pressure, occurs really at 273,15 K while the triple point of water occurs at 273,16 K, equivalent to 0,01 oC.

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Standards The standard for the temperature unit is the physical embodiment of the international temperature scale ITS-90. Its purpose is to specify procedures and practical thermometers, internationally approved, that allow national laboratories to do direct realizations of the scale and to determine highly reproducible values. This direct realization is done by means of a series of sealed cells that contain a pure substance; the substance is in a state that corresponds to a given temperature which, in turn, represents a fixed definition point. The fixed definition points were originally selected so that they would correspond as closely as possible to the thermodynamic scale. The data is compiled in the legal document known as ITS-90. In October 1999(13), the 21st General Conference on Weights and Measures, invited the International Committee to work towards extending the ITS-90 below its present lower limit of 0,65K. There are many fixed points of definition for the ITS-90 scale. Some are shown in Table 3.

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TABLE 3 Some fixed points of definition for the ITS-90 scale Temperature T90/K t90/oC

Substance

State

from 3 to 5 from - 270,15 to - 268,15 He - Helium

Saturated vapor pressure

83,805 8

- 189,344 2

Ar - Argon

Triple point

234,315 6

- 38,834 4

Hg - Mercury Triple point

273,16

0,01

H2O - Water

302,914 6

29,764 6

Ga - Gallium Melting point

429,748 5

156,598 5

In - Indium

Solidification point

505,078

231,928

Sn - Tin

Solidification point

692,677

419,527

Zn - Zinc

Solidification point

933,473

660,323

Al - Aluminium Solidification point

1 234,93

961,78

Ag - Silver

Solidification point

1 337,33

1 064,18

Au - Gold

Solidification point

1 357,77

1 084,62

Cu - Copper Solidification point

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Metrology for non-metrologists

Uncertainties With the sealed cells it is possible to calibrate temperature measurement devices with a relative uncertainty of the order of 10-6. Measuring instruments The first thermometer of which we have any reference was built by the Italian scientist Galileo Galilei, around 1593. Today, there are several types of sensors to measure temperature, and all of them infer temperature through some change in a physical characteristic (42). The artifacts most commonly in use are: changeof-state devices, liquid-expansion devices, thermocouples, resistive devices and thermistors, optical and infrared radiators, bimetallic devices, pyrometers. The change-of-state devices are indicating labels, pellets and crayons, lacquers, liquid crystals, grains and cones, that change their appearance when a given temperature is reached. They are normally used for temperatures between 38 oC and 1 780 oC. The change due to the temperature is permanent 58

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so that they cannot be used over again, their response time is relatively slow and their accuracy is not very high, but they are useful for industrial applications such as soldering or in ceramic ovens. The home thermometer is the best known representative of the fluid-expansion devices. These thermometers can use mercury or an organic liquid such as alcohol, and some use a gas. They can work by partial, total or complete immersion. They can be used repeatedly, they do not require a source of energy, but the data they give cannot be directly recorded or transmitted. Thermocouples are built from two pieces, made of different metals, joined at one end, and with a voltmeter; they are accurate, robust, reliable, and their price is relatively low. Their measurement interval depends on the metals used and usually is between - 270 oC and 2 300 oC. Resistive devices (also known as RTDs) are based on the principle that a change in temperature brings about a change in the electrical resistance. When using metals, the resistance increases with a temperature increase; on the contrary, with thermistors, the electrical resistance of the ceramic semicon59

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ductor diminishes with an increase of temperature. These are stable devices but they have a drawback; because they work based on the flow of current through a sensor, a certain amount of heat is generated and can influence their accuracy. RTDs work at temperatures between -250 oC and 850 oC; thermistors between - 40 oC and 150 oC. Optical pyrometers or sensors rely on the fact that light emitted by a hot body is related to its temperature; they work between 700 oC and 4 200 oC. Infrared pyrometers or sensors measure the amount of radiation emitted by a surface; they are appropriate for temperatures around 3 000 oC. They are more expensive but both have the advantage of not having to be in direct contact with the surface whose temperature is to be measured. Bimetallic devices are based on the different thermal expansion of different metals. Two pieces of different metals are joined together; upon being heated, one piece will expand more than the other when exposed to the same change of temperature, and the generated motion is transmitted to an indicator on a temperature scale. They have the advantage of being portable and of not requiring a source of energy. 60

Standards and reference materials

Other temperature measurement devices used in metrology are the standard platinum resistance thermometer, SPRT, the constant volume gas thermometers, CVGT, the radiation thermometers.(55) Specifications and tolerances are set in accordance with the type of temperature measurement artifact, its intended use, and the temperature interval of its readings. For instance, in industry, between 0 oC and 100 oC, an accuracy of 1 oC is considered necessary; above 100 oC, the required accuracy changes to 5 oC (6).

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TIME AND FREQUENCY What do we measure The concept of time has always drawn the interest of philosophers and physicists. Aristotle and Newton, among many others, tried to define time(44) and, more recently, Hawking(17) speaks of real time and imaginary time. For practical purposes, time is a concept related to the order and duration of events; if two events do not occur simultaneously in a given space, they occur in a given order and with an interval between them(9). For primitive man, the first intimation of the flow of time must have been the daily cycle of day and night, with the visible movements of the stars. We may reasonably suppose that longer durations were later conceived through observation of lunar phases and of the seasons. History Time intervals were initially measured based on the position of celestial bodies. One of the first artifacts must have been the sun-dial, based on the observation that the length of shade changes during the 63

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day; it consists of a rod (called style or gnomon), parallel to the axis of the Earth, that projects its shade on a quadrant. It is believed to date as far back as 579 BC and is attributed to Anaximander or to Thales of Miletus. Fire clocks were used to measure time during the night, in closed rooms, or during sunless days, and were nothing more than knotted ropes, marked candles or a certain amount of oil. Later on, there were water clocks of which a very ancient model is known, with a float, built in China, but whose best representative is the clepsydra, perfected in Greece. This instrument was used by Assyrians, Egyptians, Greeks and Romans, and its use continued well into the Renaissance. It is based on the assumed regularity of the flow of water through an orifice, and the better models used different diameters at different levels. The clepsydra in turn originated the well-known and distinctive sand glass or hourglass. Mechanical clocks are believed to have their origin in China; they came to Europe around the thirteenth century. The first clock moved exclusively by weights, for which we have a description, was built in 1364 by Henri de Vick, a German watchmaker, 64

Standards and reference materials

for King Charles V of France. We owe the pendulum clock to Huygens, in 1657; he also developed the mechanisms that would make pocket watches possible. A Nuremberg locksmith, Peter Henlein, created the spiral or royal spring and by the seventeenth century, the mechanisms were mostly spring and balance. Clocks often had additional sound systems of bells, carillons or “cuckoo”. All of these gave rise to an important industry and real works of art. In 1855, E.D. Johnson built the chronometer. Already in 1780, Louis Recordon had invented the automatic chain for pocket watches but it was not until 1924 when John Harwood used it in wristwatches. In the twentieth century, electric watches and alarm clocks became very common, but the widespread use of watches really came about when battery-operated watches became available on the market; they were originally called digital watches althought there are also analog models. Presently, very accurate quartz watches are being manufactured. The high degree of accuracy for time measurements which can be obtained today is possible with atomic clocks, used in science - particularly in me65

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trology. They are stable because the frequencies produced are only very slightly influenced by external factors such as temperature, pressure or humidity.

International definition of the units for measurement of time [13th General Conference on Weights and Measures, 1967], and of frequency Formerly, the definition referred to what we might call the astronomical second, nowadays we refer to the atomic second.

The second, (symbol s), is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

where 9 192 631 770 is the frequency of the energy involved in said caesium transition; the ground state is considered to be the state where electrons are at their lowest energy level; the hyperfine levels represent the smallest energy increase that they can undergo in that state (6). 66

Standards and reference materials

The derived unit for frequency is the hertz.

The hertz (symbol Hz) is the frequency of a periodic phenomenon, the period of which is one second.

The hour (symbol h) and the minute (symbol min), are not decimal multiples of the second and thus are not SI units. However, their use is so widespread that they are considered units accepted for use with the SI (see Table 3). In some cases, it is also necessary to refer to larger time intervals such as the week, the month and the year. Standards Practical realization of the definition of the second is done using a caesium atomic clock. It is based on the fact that atoms, under diverse excitements, emit monochromatic radiations and can thus generate a period (the duration of an oscillation) which can be defined very accurately. Other standards use other sources of frequencies, such as the hydrogen maser, rubidium standards, 67

Metrology for non-metrologists

commercial caesium standards, etc. They are sufficiently accurate for most applications, and they are considered secondary standards. It is not sufficient to be able to measure time intervals accurately; there must also be a world-wide scale for comparisons and precise relations; air transportation timetables are a good example of the importance of this synchronization. This demands permanent maintenance of the same continuous temporal reference as an element in the practical realization of the standard.

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Atomic caesium clock(43) Internal energy of an atom (electrons+nucleus) assumes values which correspond to the diverse quantum states ot the atom. The atom has the possibility of carrying out a transition between one level of energy EA and another level of energy E B, with emission or absorption of radiation. Frequency ν of the radiation is determined by the relationship: h.ν = | EB - EA | where h is Planck’s constant. The transition adopted to define the second was selected not only because of its own properties (monochromatism of the radiation which implies a well-defined frequency, with slight sensitivity to external perturbations), but also due to technical reasons (among others, the transition frequency is in a domain of frequencies accessible to current electronic instruments, ease of use of caesium to obtain an atomic beam and for detection of ionization). The caesium clock uses a very precise quartz oscillator whose frequency is verified by generation of an electromagnetic radiation which illuminates a cloud of caesium atoms. If the radiation frequency is precisely 9 192 631 770 cycles per second, the caesium atoms become polarized and can be detected by a magnetic field. If the frequency deviates slightly, the number of polarized atoms diminishes and this generates a signal for correction to keep the oscillator’s frequency at its nominal value.

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Time scales (19) The International Atomic Time (TAI) scale, is calculated at the BIPM. In 1999 it was obtained from data from some two hundred atomic clocks in nearly fifty national metrology laboratories. To keep the scale unit of TAI as close as possible to the SI second, BIPM uses data from those national laboratories which maintain the best primary caesium standards. TAI is a uniform and stable scale which does not, therefore, keep in step with the slightly irregular rotation of the Earth. For public and practical purposes it is necessary to have a scale that does so in the long term. Such a scale is Coordinated Universal Time (UTC), which is identical with TAI, except that, from time to time, a leap second is added to ensure that, when averaged over a year, the Sun crosses the Greenwich meridian at noon UTC to within 0,9 second. When fractions of a second are not important, the well-known “Greenwich Meridian Time or Greenwich Mean Time, GMT” is practically equivalent to UTC. However, it is recommended not to use the term GMT but instead to always use the term UTC.

Diffusion of the scale is done through several means and may require special reception instruments.

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It can de done by: -

-

telephone access to a time service, with an accuracy of up to 50 ms, coded hourly signals (for instance, 3 MHz to 30 MHz short wave, with an accuracy of 10 ms (36), 1350 KHz modulated frequency, etc.) with accuracies of milliseconds, accuracies of 10 ns by reception of television signals using GPS, Global Positioning System, based on artificial satellites.

Uncertainties Current time standards work with relative uncertainties of the order of 10-14 and, in some cases, up to 10-15. It has also been calculated that, in a million years of use, the atomic time scale TAI will differ from the ideal scale by less than a second. More than the accuracy, that may not be constant, the most important characteristic of a UTC scale (generated at national laboratories) is its stability.

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Measuring instruments Usual measurements of time are done with diverse types of timepieces (such as clocks, watches and chronometers) with a greater or smaller accuracy according to the needs, calibrated with the UTC or TAI scales. Time interval counters and quartz oscillators are also used. For their part, measurements of frequencies require very high accuracies in applications such as digital communication and global positioning systems (GPS).

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ELECTRICITY AND MAGNETISM What do we measure Some materials, known as conductors, have free electric charges that can move, such as electrons in metals and ions in salt solutions. In these materials, in the presence of an electric field, a stable flow is produced in the direction of the field; such a flow is an electric current. Ohm’s Law relates the three basic elements of electricity with the equation: E = IR where E is the electric potential, commonly called voltage, I is the electric current, and R is the resistance. Based on this law, the electricity unit could have been defined through any one of these three elements. It was decided to define it in terms of electric current, leaving electric potential and resistance as derived units.

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Electric current is a property of matter that produces electric and magnetic effects. In an isolated system, it is constant and produced in packets. The smallest isolated charge is that of the electron. A simple manifestation of electric current is obtained by rubbing with a silk cloth two spheres, of amber for instance, suspended in a non-conductor material; the spheres repel each other because they have the same electric charge. If the spheres are of different materials, such as one of amber and the other of glass, they attract one another because they have different charges(1). We can visualize the behavior of electricity and the interdependence of its characteristics through the following similarity. If we have a pipe carrying water, we can characterize it by the amount of water that flows through the pipe, the pressure at which it flows, and the properties of the pipe itself. In electricity, the water pressure would be equivalent to the electric potential, expressed in volts (V); the amount of water would be the electric current expressed in amperes (A); and the friction due to the pipe material would be equivalent to the electric resistance in ohms (Ω).

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History Nearly 2 600 years ago, Thales of Miletus noted that when amber was rubbed with wool or leather, it attracted small pieces of hay or feathers. Some 250 years later, Aristotle commented on the (electric) discharges produced by a fish - a variety of eel. The poet Lucretius described 2 100 year ago the magnet stone found in the region of Magnesia. In 1 600, William Gilbert made a clear distinction between electric and magnetic phenomena; the first machine to produce electricity through friction was built 63 years later. Nowadays, the phenomenon is well known although complex, and closely related to quantum mechanics. Appendix 4 lists some of the scientists who have contributed to the development of knowledge on electricity.

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International definition of the units for measurement of electricity and magnetism [9th General Conference on Weights and Measures, 1948]

The ampere (symbol A) is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newtons per metre of length.

The main derived units are the volt and the ohm.

The volt (symbol V) is the potential difference between two points of a conducting wire carrying a constant current of one ampere, when the power dissipated between these points is equal to one watt.

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The ohm (symbol Ω) is the electric resistance between two points of a conductor when a constant potential difference of one volt, applied to these points, produces in the conductor a current of one ampere, the conductor not being the seat of any electromotive force.

Standards The principles and devices used in a standard reflect scientific development and the technical facilities available. Formerly, electric current balances were used for the ampere, but they had a high uncertainty. Presently, better results are obtained using the ohm and the quantized volt, and Ohm’s Law. Practical realization of the unit is done with a system that is itself a standard. The Josephson effect is used for the reference unit of the volt and the Hall effect for the resistance. The work carried out in the realization procedure is complex and requires specialized apparatus and instruments as well as highly qualified personnel.

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Uncertainty Measurement uncertainty of the electric potential (volt) in an array of Josephson junctions is of a few parts in 1010 and for the resistance standard with the Hall effect of a few parts in109. The high reliability in the transportation of the Josephson and quantized Hall systems has given as a result that national laboratories can have internationally comparable standard systems. Measuring instruments With current technology it is possible to build analog and digital devices to measure electric current. As in all scientific work, use of computers facilitates, speeds up and gives higher certainty of results. Measurement work uses extensively digital processing and knowledge of quantum mechanics, making this high technology work even if the results are in popular use in apparatus such as ampere meters, voltmeters and resistance meters. We must also make a distinction between measurements of high resolution/low uncertainty - standards and reference systems - and those for practical applications. 78

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LIGHT (PHOTOMETRY AND RADIOMETRY) What do we measure The different forms of radiant energy include cosmic rays, gamma rays, X rays, ultraviolet rays, light visible to man, infrared, microwave, electric and radio waves (hertzian). In the case of photometry, we are primarily interested in the phenomenon called light, one of the forms of radiant energy, that is energy as electromagnetic waves, emitted as photons, and with a set frequency and wavelength. From the point of view of the spectrum visible to man, light for him has been mostly sunlight and its substitutes through the centuries: fire, torches, candles, and all the lamps: oil, kerosene, gas; electric lighting in the form of incandescent carbon filament, tungsten filament, sodium, neon, fluorescent, vapor of mercury, etc. History The study of light goes way back into history. Four centuries before Christ, Euclides worked on his 79

Metrology for non-metrologists

Optica, however the mechanism of vision was not identified until the beginning of the seventh century. Other researchers have studied intensively this phenomenon: Ibn al-Haitham in the eleventh century, Galileus in 1610, Kepler in 1611 with his Dioptrics, Descartes in 1637 when he discovered the law governing refraction, Newton in 1704 with his treaty on Opticks(53). Later on, Huygens, Fresnel, Maxwell, Michelson and many more have contributed to this field of study. For practical purposes, photometry tries to express the visual impression of an “average observer”. Different people have different visual perceptions; for this reason, the International Commission on Light did a series of measurements in a large number of persons, in order to be able to somehow define this “average observer”. Because human visual response varies with the wavelength, and the human eye does not perceive infrared and ultraviolet radiation, work is done on measuring physical quantities - in this case the energy characteristics of radiation - and this is the field of radiometry. Thus, although photometry and radiometry are two different fields, they are very closely related. 80

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International definition of the unit for measurement of light History The unit and its standard have an uneven history(31). The candela was originally defined in the eighteenth century; it was based on burning elements and thus had a very low reproducibility. It was later modified (Carcel 1800, Hefner 1884) but working conditions still were a critical factor. In 1880, Violle suggested using a piece of platinum at a temperature corresponding to the transition point between the solid and the liquid states. There were problems derived from the purity requirements of platinum and Blondel suggested in 1896 the use of a black body that would keep a constant high temperature; in 1930, Burgess placed the platinum in a thorium crucible inside an induction furnace. Due to the difficulties for realization of the photometric unit, several congresses modified the Violle candle, in 1884, 1889, 1909, 1921, 1933, 1937, 1938 and 1954 when the candela was recognized as the sixth base unit, after the metre, the kilogram, the second, the

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ampere and the kelvin - up to the current definition approved by the 16th General Conference on Weights and Measures, in 1979.

International definition of the units of measurement in photometry and radiometry [16th General Conference on Weights Measures, 1979]. The candela (symbol cd) is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

The following working units are derived from the candela.

The lumen (symbol lm) is the luminous flux emitted in a unit solid angle of one steradian by a uniform point source having a luminous intensity of one candela.

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The lux (symbol lx) is the illuminance of a surface receiving a luminous flux of one lumen, uniformly distributed over one square metre of the surface.

The candela per square metre (symbol cd.m-2) is the luminance perpendicular to the plane surface of one square metre of a source of which the luminous intensity perpendicular to that surface is one candela.

In photometry, we use: luminous flux (lm), luminous efficiency (lm.W-1), luminous intensity (cd), luminance (cd.m-2), illuminance (lx). In radiometry, the units are: the energy flow rate, or heat flow rate, or power (W), the energy intensity or radiant intensity (W.sr-1), the energy luminance or radiance (W.m-2.sr-1), the energy illuminance also known as irradiance or thermal flow density (W.m-2).

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Metrology for non-metrologists

Standards Presently, maintenance of photometric and radiometric standards no longer emphasizes photometric methods, but rather radiometry based on detectors. The primary standard at the BIPM relies on a commercial cryogenic substitution electric radiometer, considered to be one of the most accurate radiometers available. In addition, when the highest degree of accuracy is not required, there are sets of silicon photodiodes that are used as working and transfer standards. Transfer to national or other standards is also done using lamps, calibrated by comparison. Uncertainties The candela standard is realized with a relative uncertainty of 3 x 10-3. Measuring instruments In photometry and radiation, the following are used: radiometers; absorption, black body, polarization, electric, and photoelectric photometers; integrators, colorimeters, spectrophotometers, spectroradiometers, and radiation measurers; also, cryogenic radiometers (detector-based radiometers) for standards. 84

Standards and reference materials

ACOUSTICS AND VIBRATION What do we measure With the exception of people deaf from birth, human beings intuitively perceive the concept of sound. For all animals, sound is an important part of the environment. For man, particularly, sounds are involved in communications with others and in the awareness of external circumstances either natural or man-made (music, the noise of machines working, warning bells and sirens, etc.) Sound can be defined as a mechanical alteration, such as a change of density, or of particle displacement or velocity, in an elastic medium such as air or water. We consider a medium to be elastic if it can go back to its original shape and size once the alteration that provoked the tension, shear or compression, has ceased. Based on this definition, we can in turn define a sound field as an elastic medium where a mechanical alteration, such as a change of density, or of particle displacement or velocity, is produced and propagated.

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Metrology for non-metrologists

In acoustics, we study and measure basic properties of sound:

-

intensity or loudness, determined by the wave amplitude

-

pitch, determined by the frequency or number of vibrations

-

timbre, determined by the additional vibrations (harmonic sounds) together with the fundamental vibration

A normal human being cannot hear sounds of a frequency lower than 16 Hz (infrasounds) or higher than 20 kHz (ultrasounds or supersonics). Quantitative measurements of sound began in the nineteenth century, but it was really in the twentieth century, particularly its last 20-30 years, when studies have been done on the nuisance and the hazards of noise on the human auditory system(29). Recently, in 1999, the CIPM created a Consultative Committee on Acoustics, Ultrasound and Vibration. 86

Standards and reference materials

The International Standardization Organization, ISO, has established several standards, strictly in the field of acoustics, that include aspects such as: standard tuning frequency, methods to calculate loudness levels, reference quantities for acoustic levels, etc.; and an even larger number of standards in related fields. For its part, the International Electrotechnical Commission, IEC, has been standardizing aspects related to microphones and their calibration, sound level meters, sound intensity, human ear simulators, etc. For metrological purposes, the most common measurements in acoustics are: the magnitude of a sound field and the strength of a sound source. In practice(29), to measure the strength of a sound field, we use the sound pressure because it is the easiest to transform from a form of energy (particle alteration in the elastic medium) to another equivalent form that is the one usually measured (for example, pascal, Pa, equivalent to newtons per square metre, N/m2). In the case of a source of sound, characterization is done by its power.

87

Metrology for non-metrologists

Both the sound pressure and the power of a source of sound are measured in relative decibels at 20 µPa and 1 pW respectively.

Definition of the units for measurements in acoustics and vibration We are familiar with watts in terms of illumination we know that we can read comfortably when using a light bulb of 100 W, while a 25 W bulb will give a very dim light. In the case of light, this is an arithmetical relation. In comparison, sensibility to sound is different. Sounds of an ordinary conversation are around 1 mW, which we can express as 1000 microwatts (µW), but soft sounds fall to fractions of 1 mW. The human ear perceives differences in intensity exponentially. Thus, if 2000 µW “sound” a certain amount louder than 1000 µW, then we need 4000 µW instead of 3000 to perceive the same amount of increase and, in turn, 8000 µ W are necessary for the perception of the same increase starting from 4000 µW. The ratios 2000/1000, 4000/2000, 8000/

88

Standards and reference materials

4000, are all equal, thought the differences between values are not, and it is by ratios that the ear judges. When a sound has 10 times the power of a second sound, the ratio is 10, whose logarithm is 1. In this case, we say that the difference in sound intensity is one bel (so called after Alexander Graham Bell). Similarly, if a sound is 100 times stronger than another, it is 2 bels stronger; if it is 1000 times stronger, it is 3 bels stronger. This type of unit reflects the logarithmic way the ear works. Because the bel turns out to be too large a unit for usual measurement needs, we use the decibel. Thus, a sound will be one decibel stronger than another when it is 1,26 times stronger, because 0,1 is nearly the logarithm base 10 of 1,26. This non-dimensional derived unit “one” has been used to express logarithmic values such as the logarithmic decrease, the pressure level or the power level, in acoustics and electrotechnics. We use the name bel (symbol B) and its commonly used sub-multiple the decibel (symbol dB) when using logarithms of base ten; we speak of the neper

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Metrology for non-metrologists

(symbol Np) when using natural or neperian logarithms. Acceptance of these units is still under study by the CGPM.

Standards The basic quantity for all measurements in acoustics is the sound pressure. There is no practical way to obtain a reference source that would generate a sound pressure of one pascal, and work continues to find a way of generating or measuring a sound field in such a way it can be used as a reference standard. Up to the present, accuracy of measurements relies on the use of accurately calibrated microphones. For measurement purposes, the acoustic signal is converted to an electric signal, using a condenser or electrostatic microphone. In this type of microphone, a diaphragm acts as one of the plates of the capacitor; the vibration produces changes in

90

Standards and reference materials

the capacitance and these, in turn, produce changes in the output voltage. The International Electrotechnical Commission, IEC, has a set of specifications for standard microphones, both for laboratory and for field work. Calibration is done using sound calibrators with a reference sound source. The IEC has established specifications for calibration using the reciprocity technique(29), based on condensing microphones. This technique was selected for its uncertainty level and has been internationally approved for the realization of the primary reference standard; it is being refined through studies and world-wide intercomparisons.

Uncertainties The minimum sound pressure difference that the human ear can perceive is 1 dB (one decibel). However, for many applications, such as those having to do with the determination of noise and, particularly, that of aircraft, certification requires measurements on the order of 0,1 dB; thus, pri-

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Metrology for non-metrologists

mary references for measurement must have an uncertainty of around 0,05 dB. Measuring instruments Other measuring instruments are used in addition to microphones. To determine pressure in continuous sounds, an exponential-averaging meter is used, and the values are expressed in decibels as a sound pressure level. For discrete sounds, it is an integratingaveraging sound level meter, and the value is expressed in decibels as an equivalent continuous sound pressure level. Intensity of sound is a measure of the magnitude and direction of the flow of sound energy. It is usually measured using two microphones and the sound intensity level is expressed in decibels relative to 10-12 Wm-2. With measurement of sound intensity, it is possible to determine the power of a source without the need for specialized environments, but the method is not yet widespread.

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Standards and reference materials

IONIZING RADIATION What do we measure We call ionizing radiations those highly penetrating electromagnetic radiations of extremely short wavelength, at least as energetic as X-rays, whose radiation is strong enough to remove or add electrons from matter, thus producing ions. Among ionizing radiations we have: those that produce charged particles such as α and β radiations and the proton radiations; those that produce noncharged particles such as γ rays and x-rays (both liberate photons) and the neutron radiations. These radiations may be natural or artificially produced in particle accelerators such as cyclotrons, betatrons, synchrotrons or linear accelerators. History X-rays were discovered by Wilhelm Konrad Röntgen en 1895. In 1896, Antoine Henri Becquerel (whose name has been given to the radioactive material disintegration unit) discovered radioactivity in an uranium salt. Pierre and Marie Curie 93

Metrology for non-metrologists

showed that all uranium salts were radioactive, as well as thorium salts, and they also discovered the radioactive elements polonium and radium, present in the mineral called pitchblende. Radioactive emissions are not homogeneous and Ernest Rutherford, in 1899, classified them according to their charges and their penetrating power, and gave them the names of alpha, beta and gamma radiations.

Definition of the measurements units for ionizing radiation The nucleus of a radionuclide can be transformed or disintegrated spontaneously (see Appendix 5). Activity is characterized by the average number of disintegrations per second, and is measured with a unit called the becquerel. Another important measurement is the absorbed dose, the quantity of energy imparted by ionizing radiation to a unit mass of matter, and it can be considered the fundamental unit in dosimetry. The SI does not have base units for ionizing radiation, but it recognizes the becquerel and the gray

94

Standards and reference materials

as derived units; in their simplest form they can be stated as follows.

The becquerel (symbol Bq) is the activity of a radioactive source in which one disintegration is produced per second.

The gray (symbol Gy) is the dose of ionizing radiation uniformly absorbed by a unit mass of matter, at a rate of 1 joule per kilogram of its mass.

Standards Because of the variety of emitted particles and of the alterations suffered by the radioactive sources, there is no single primary standard for the becquerel. Primary references are set up as a blend of instruments and measurement methods, specific to each type of radionuclide. As an example, for α emissions, (plutonium 239 and plutonium 240 for instance) a silicon detector 95

Metrology for non-metrologists

is used as a counter in a defined solid angle. A counter in a sodium iodide crystal well is used for γ emissions (iodine 123 or iridium 192, for instance). Similarly, there is no single standard for the gray in dosimetry. In this case, the methods are based on colorimetry, ionometry (with highly sensitive instruments that can be used for all radiations), Fricke dosimetry, thermoluminescence (in radioprotection and radiotherapy), electronic paramagnetic resonance (for industrial radiations).

Uncertainties Uncertainties in measurements, both for the becquerel and the gray, are on the order of 10 -2 to 10-3.

Measuring instruments As for the setting up of standards, measurement instruments are detectors, counters, dosimeters, calibrators for α and γ rays, ionization chambers, calorimeters, extrapolation chambers (variable ionization), etc. 96

Standards and reference materials

CHEMISTRY What do we measure Stoichiometry is the branch of chemistry and chemical engineering that deals with the quantities of substances that enter into, and are produced by chemical reactions. Every chemical reaction has its characteristic proportions; they are determined from chemical formulas, equations, atomic weights and molecular weights, and from determination of what and how much is used and produced - that is, the amount of matter involved. All of stoichiometry is based essentially on the evaluation of the number of moles of substances, as a precise indicator of the amount of substance. In chemistry, particularly in analytical chemistry, the amount of matter in a given sample is crucial information. It is also an important element in other aspects such as concentration of solutions, the determination of pH, etc. In chemical industry, it is necessary to know the amount of substances used in the diverse reactions and in the products obtained from them.

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Metrology for non-metrologists

History Chemistry can be said to have been “studied” since the most remote ages. Metal work (copper, gold and silver, bronze, iron), ceramics, enamels, pigments, etc., all involve chemical processes to some degree and this requires a certain amount of knowledge, even if at first this knowledge was eminently practical. Around 1460, there was in Florence, Italy, a manuscript with fourteen treatises, allegedly written by a possibly fictitious person, Hermes Trismegistus, and known as the Corpus hermeticum. It is believed to date back to years 100-300 although, in view of some references to Egypt, some believe part of its contents to go as far back as 2500 BC; this would make it the earliest known recorded knowledge of chemistry. Empedocles (500 BC), whom we know through Aristotle, believed there are four elements in nature: fire, earth, air, and water. The first “atomic” theory (matter is made up of atoms, infinitely small and indivisible) of which we have notice is due to Leucippus (around 475 BC) and his disciple Democritus. It was taken up by Epicurus (341 BC), 98

Standards and reference materials

the Latin poet Lucretius (De rerum natura), and in his time, by Aristotle who taught that all matter is composed of mixtures of these four elements and that they are not permanent but can change one into another. This led to the belief that it was possible to transmute bodies, such as base metals, into others, such as gold. The first “chemist” may well have been Jabir alHayian (also known as Jabir or Geber) of the court of Harun al-Rashid (circa year 786), who studied Greek documents and to whom a large amount of written texts is attributed. He is believed to have mastered the techniques of practical chemistry known at the time. He based his work on sulfur, mercury and salt, and wrote instructions on how to carry out the manipulations. There is material on chemistry written during the eighth century in Northern Africa and, with the availability in Europe of translations from the Arab in the twelfth and thirteenth centuries, there exist records of the work performed by the alchemists. In the fourteenth century one of them, Nicolas Flamel, is reputed to have been able to succeed in producing gold; what we know he did produce was 99

Metrology for non-metrologists

a gold fever, during the fifteenth and sixteenth centuries: a fever for alchemy, that hermetic and esoteric world, to which scientists such as Robert Boyle, John Locke and Isaac Newton were not immune. But alchemy, more than a simple search for the production of gold, began to be a science. It seriously studied chemical reactions, contributed to many discoveries and new knowledge (for instance: distillation of aqua vitae or ethanol, preparation of aqua regia, nitric acid, sulfuric acid, many salts), and generated countless controversies. As an example, Robert Boyle in his book The Sceptical Chymist (1661) refuted the chemical theories based on the four elements and on the alchemist trio (mercury, sulfur and salt), arguing they could not explain the results of experiments. For our purposes, possibly the most important chemist must have been Antoine Laurent Lavoisier who, in 1789, published his Traité élémentaire de chimie. Lavoisier always insisted on the fact that measurements were important in chemistry; qualitative observation was not sufficient, it was necessary to work quantitatively. 100

Standards and reference materials

Later, in 1811, Amadeo Avogadro stated the principle known by his name: equal volumes of gas at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties. This number, the Avogadro number, is 6,023 x 1023; it is the number of molecules of any gas present in a volume of 22,41 liters and is the same for the lightest gas (hydrogen) as for a heavy gas such as carbon dioxide or bromine. Avogadro’s number is one of the fundamental constants of chemistry.

International definition of the unit for measurement in chemistry [14th General Conference on Weights and Measures, 1971]. History Formerly, the mole was defined as the molecular weight of a substance expressed in grams. Presently, and although this is not obvious from the way the unit is expressed, the term is applied to an amount of 6,023 x 1023 (Avogadro’ number) of chemical entities; thus, we can speak of a mole of atoms, a mole of ions, of radicals, of electrons, of quanta. Nowadays, when the

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Metrology for non-metrologists

mole is used, the elementary entities must be specified and these may be atoms, molecules, ions, electrons, other particles or specified groups of such particles.

The mole (symbol mol) is the amount of substance of a system which contains as many elementary entities as there are atoms in 0,012 kilogram of carbon 12.

The recent adoption of a derived unit, by the 21st General Conference on Weights and Measures in 1999(13), is based on a recommendation for the use of SI units in medicine and biochemistry due to the importance of avoiding the results of clinical measurement being given in various local units.

The katal (symbol kat) is the mole per second unit, for use in medicine and biochemistry for the expression of catalytic activity.

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Standards and reference materials

Standards and reference materials There is as yet no unique primary standard realization for the mole although work is being done towards having reliable standards. Working standards in chemistry consist of a set of methods, called primary, together with chemically pure substances, with a defined titer, and in a known matrix, the reference materials. The Consultative Committee on the Amount of Substance (Comité Consultatif sur la Quantité de Matière - CCQM) of the CIPM has recommended several methods as having a high potential for their recognition as primary methods; among them we have: Primary methods of direct measurement: Electrochemistry: - coulometric titration - pH measurements - electrolytic conductivity

103

Methods of classical analytical chemistry: - gravimetry - titrimetry Primary correlation methods of measurement: - isotopic dilution with mass spectrometry - nuclear magnetic resonance - differential calorimetry As to the materials to be used, let us recall here the definitions for reference material and for certified reference material. Reference material: material or substance one or more of whose property values are sufficiently homogeneous and well established to be used for the calibration of an apparatus, the assessment of a measurement method, or for assigning values to materials. Certified reference material: Reference material, accompanied by a certificate, one or more of whose property values are certified by a procedure which establishes traceability to an accurate realization of the unit in which the property values are expressed, and for

104

which each certified value is accompanied by an uncertainty at a stated level of confidence. An example of these is the usual certified reference materials used in laboratories to calibrate instruments, and to verify methods and reagents.

Uncertainties In chemistry, uncertainties of results vary according to the element to be quantified and its concentration. However, we can speak of levels from 10-3 to 10-4.

Measuring instruments All determinations involve analytical techniques and instruments for those methods considered to be primary and they have already been mentioned.

105

106

REFERENCES 1. Alonso, Marcelo Física, Curso elemental Tomo I, Mecánica; Tomo II, Hidromecánica-Calor; Tomo IV, Electromagnetismo, física atómica La Habana Cuba, Cultural S.A., 1953 2. Alvaro Medeiros de Farias Theisen Fundamentos da Metrologia Industrial Porto Alegre, 1997 3. Audoin, C. Caesium Beam Frequency Standards: Classical and Optically Pumped Metrologia, 1992, 29, 113-134 4. Benson, Harris University Physics New York, John Wiley & Sons, 1991 5. Campos, J. et al Realization of the candela from a partial filtering V(() detector traceable to a cryogenic radiometer Metrologia, 1995/96, 32, 675-679

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6. CENAM personal communications 7. Cerruti, L. The Mole, Amedeo Avogadro and others Metrologia, 1994, 31, 159-166 8. Clare, J.F. Realization of a photometric scale based on cryogenic radiometry Metrologia, 1998, 35, 251-254 9. Concise Encyclopedia of the Sciences. New York, Facts on File,1978 10. Cooter, I.L. Electrical standards and measurements Electro-Technology, 79, 53, Jan 1967. 11. De Bièvre P. and H.S. Peiser The reliability of values of molar mass, the factor that relates measurements expressed in two SI base units (mass and amount of substance) Metrologia, 1997, 34, 49-59 12. Dictionary of Physics, volume 1 McGraw-Hill, 1993 108

References

13. Draft resolutions 21st. CGPM (information obtained from the BIPM site on the Internet) 14. Edwards, C.S., et al A 633 nm iodine-stabilized diode-laser frequency standard Metrologia, 1999, 36, 41-45 15. Gardner, J.L. et al New basis for the Australian realization of the candela Metrologia, 1988, 35, 235-239 16. Guide for the Use of the International System of Units (SI) Barry N. Taylor NIST Special Publication 811 US Department of Commerce, NIST, 1995 17. Hawking, Stephen ¿Se vislumbra el final de la física teórica? in Agujeros negros y pequeños universos. México, Editorial Planeta, 1994

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18. Hopkins, Robert A. The International (SI) Metric System and How it Works 3rd. revised ed., Tarzana, CA, AMJ Publishing Co., 1975 19. http://www.bipm.fr 20. http://www.cenam.mx 21. http://www.euromet.org 22. http://www.ibpinet.com.br/sim 23. http://www.lcie.fr 24. http://www.nist.gov 25. http://www.oiml.org 26. http://www.ptb.de 27. International Comparison Final Report Field: Acoustics; EUROMET Metrologia, 1997, 34, 197-198 28. ISO Guide to the expression of uncertainty in measurement Geneva, ISO, 1993 110

References

29. Jarvis, Duncan Sound measurements Metrologia, 1999, 36, 249-255 30. José Dajes Castro Sistema Internacional de Unidades de Medida Lima, Perú; INDECOPI, 1999 31. Matamoros García, Carlos H. La candela, principios y usos Mexico, CENAM, 1999 32. McGlashan M.L. Amount of substance and the mole Metrologia, 1994/95, 31, 447-455 33. McGlashan, M.L. Entitic quantities, molar quantities and relations between them Metrologia, 1997, 34, 7-11 34. National Bureau of Standards Standard cells, their construction, maintenance and characteristics US Department of Commerce, 1965

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35. NBSIR 75-926 The National Measurement System for Length and Related Dimensional Measurements. Part I. US Dept. of Commerce, National Bureau of Standards, 1976. 36. NIST Time and Frequency FAQ (information obtained from the NIST site on the Internet) 37. Norma Centroamericana ICAITI 4010 Guía para el uso del sistema internacional de unidades; contiene factores de conversión para pasar de unidades de otros sistemas a unidades SI Guatemala, ICAITI, n.d. 38. Ohno, Y. Detector-based luminous-flux calibration using the Absolute Integrating-Sphere Method Metrologia, 1998, 35, 473-478 39. Ohno, Y. and J.K. Jackson Characterization of modified FEL quartz-halogen lamps for photometric standards Metrologia, 1995/96, 32, 693-696

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References

40. OIML D 2 (1999) Legal units of measurement France, OIML, 1999 41. OIML R111 Weights of classes E1, E2, F1, F2, M1, M2, M3. France, OIML, 1994 42. Omega Engineering Inc http://www.omega.com/techref/measureguide.html 43. Padrões de Unidades de Medidas, Referências Metrológicas da França e do Brasil Rio de Janeiro, INMETRO, 1999 44. Park, David The How and the Why - An essay on the origin and development of physical theory N.J., Princeton University Press, 1988 45. Samaan N.D. and F. Abdullah Computer-aided Modelling of Pressure Balances Metrologia, 1993/94, 30, 641-644 46. SI Guide, International System of Units Geneva, ISO, 1998

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47. Simpson, D.I. Computerized Techniques for Calibrating Pressure Balances Metrologia, 1993/94, 30, 655-658 48. Tarbeyev Yu. V., and E.T. Frantsuz Measuring Procedure to realize the Ampere by the Superconducting Mass Levitation Method Metrologia, 1992, 29, 313-314 49. The Boeing Co., Primary Standards Unit A Precision Electrical Measurement Course August 1962 50. UNEP (United Nations Environmental Programme) Radiation; doses, effects, risks Nairobi, Kenya, 1985 51. US Department of Commerce, National Bureau of Standards Definition of Ampere and Magnetic Constant Precision Measurement and Calibration NBS Special Publication 300, vol.3, 1972

114

References

52. US Department of Commerce, National Bureau of Standards Frequency and time Precision Measurement and Calibration NBS Special Publication 300, vol.5, 1972 53. Vasco Ronchi Optics, the science of vision N.Y., University Press, 1957 54. VIM, (Vocabulaire International des Termes Fondamentaux et Généraux de Métrologie) International Vocabulary of Basic and General Terms in Metrology Genève, ISO, 1993 55. Working Group 1, Comité Consultatif de Thermométrie On the International Temperature Scale of 1990 (ITS-90), Part I: Some definitions. Short Communication. Metrologia 1997, 34, 427-429

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Appendix 1 Fundamental physical constants and their relationship to the base units

R RKK h e2

4x10

2x10

e

-6

k

-8 2e h 4x10

RK-90 KJ-90 R (10 -9 )

-8

?

KkJ

-8

2x10

-7

3x10

K

A

4x10

-9

4x10

(10 -10 )

?

-6

R

8x10

-15

-8

3x10

mol

S exact

NA 8x10 -8 -4 10

c -8

m

cd

8x10

-12

10

h

? kg -3

10

-12

G

8x10

R

m 12 c ?

me

-8

-8

8x10

8x10

Reproduced by authorization of the BIPM

116

Appendix 1

c

speed of light in vacuum

h

Planck’s constant

α

fine structure constant

R

gas constant

k

Boltzmann’s constant

Kj

Josephson’s constant

Kj-90

conventional value of Josephson’s constant

Rk

von Klitzing’s constant

Rk-90

conventional value of von Klitzing’s constant

e

elementary charge

NA

Avogadro’s number

G

gravitational constant

m12

C

mass of carbon 12

me

electron mass

R∞

Rydberg’s constant

117

newton pascal joule watt coulomb

force

118

pressure, stress

energy, work, quantity of heat

power, radiant flux

electric charge, quantity of electricity

C

W

J

Pa

N

Hz

sr(c)

steradian(a)

solid angle hertz

rad

radian (a)

plane angle

frequency

Name

SI derived unit

Derived quantity

J/s

N·m

N/m2

Expression in terms of other SI units

Some SI derived units

Appendix 2

s·A

m2·kg·s-3

m2·kg·s-2

m-1·kg·s-2

m·kg·s-2

s-1

m2·m-2 = 1

(b)

Expression in terms of SI base units m·m-1 = 1 (b)

Metrology for non-metrologists

degree Celsius(d) lumen lux becquerel

Celsius temperature

luminous flux

illuminance

activity (referred to a radionuclide)

weber

magnetic flux

henry

siemens

electric conductance

inductance

ohm

electric resistance

tesla

farad

capacitance

magnetic flux density

volt

electric potential difference, electromotive force

119 Bq

lx

lm

°C

H

T

Wb

m2·m-4·cd = m-2 ·cd

lm/m2

s-1

m2·m-2·cd = cd

K

m2· kg·s-2·A-2

kg·s-2·A-1

m2· kg·s-2·A-1

m-2·kg-1 ·s3·A2

m2·kg·s-3·A-2

m-2·kg-1 ·s4·A2

m2·kg·s-3·A-1

cd·sr (c)

Wb/A

Wb/m2

V·s

A/V

V/A

Ω S

C/V

W/A

F

V

Appendix 2

sievert

gray

Sv

Gy

J/kg

J/kg

m2·s-2

m2·s-2

(a) The radian and steradian may be used with advantage in expressions for derived units to distinguish between quantities of different nature but with the same dimension. (b) In practice, the symbols rad and sr are used where appropriate, but the derived unit “1” is generally omitted. (c) In photometry, the name steradian and the symbol sr are usually retained in expressions for units. (d) This unit may be used in combination with SI prefixes, e.g. millidegree Celsius, m°C.

organ equivalent dose

dose equivalent, personal dose equivalent,

absorbed dose, specific energy (imparted), kerma, index of absorbed dose

Metrology for non-metrologists

120

121 mili micro nano pico

-6

-9

-12

0,001 0,000 001

0,000 000 001

0,000 000 000 001

kilo

-3

1 000

= 10

= 10

= 10

= 10

= 10

mega

3

giga

Symbol

1 000 000

= 10

= 109

Prefix

6

1 000 000 000

Factor

p

one billionth

one thousand millionth

one millionth

µ n

one thousandth

one thousand

one million

one thousand millions

m

k

M

G

Factor in words

Most common multiples and submultiples for use with SI

Appendix 3

Appendix 3

Metrology for non-metrologists

Appendix 4 Scientists who have worked with electricity

Epicurus (342-270 BC). First hypothesis on magnetism Roger Bacon (1214-1294). Magnetic repulsion Peter Peregrinus (1269). First text on magnetism Cornelius Gema (sixteenth century). Attraction between lines of force G. Cardan (1501-1576). Differences between electric and magnetic phenomena Nicolo Cabeo (1585-1650). Repulsion William Gilbert (1540-1603). First electroscope S. Gray (1696-1736). Classification of bodies that can be electrified by rubbing F. Hauskbee (seventeenth century). Superficial distribution of electricity

122

Appendix 4

C. Du Fay (1698-1739). Electric fluids, good and bad conductors Benjamin Franklin (1706-1790). Lightning as an electric phenomenon H. Cavendish (1731-1810). Dielectric constant, electric capacity and potential Charles A. Coulomb (1736-1806). Quantitative law of attraction between two poles Georg Simon Ohm (1789-1854). Fundamental law of electricity Alessandro Volta (1745-1827). Voltaic pile G. Green (1793-1841). Concept of potential in electrostatics J. Karl F. Gauss (1777-1855). Terrestrial magnetism M. Faraday (1791-1867). Dielectric constant and magnetic lines of force Wilhelm Weber (1804-1890). Permanent dipoles

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Hans C. Oersted. (1770-1851). Magnetism is a manifestation of electricity in motion M. Ampere (1775-1836). Two parallel currents are attracted if they flow in the same sense, and repelled if in opposite senses Pierre S. Laplace (1749-1836). Jean B. Biot (1774-1862). J. Henry. (1797-1878). Induced electromotive force James Clerk Maxwell (1831-1879). Light is an electromagnetic phenomenon H. R. Hertz (1857-1894). Verification that the speed of electromagnetic waves equals the speed of light Ernest Rutherford (1871-1937). Theory of the nuclear atom Edwin H. Hall (1855-1938). When a magnetic field is applied perpendicular to a current-carrying strip, a potential difference appears across the width of the strip

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Leon N. Cooper (in 1956). In the ground state, at 0 K, with no external field and no current flowing, all the electrons form Cooper pairs in which two electrons have opposite momenta and spin Briand David Josephson (1940- ). Cooper pairs can tunnel through a thin (1 nm) insulating barrier separating two superconductors; since each pair carries a charge of 2e, a supercurrent is created in the absence of any applied potential difference

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Appendix 5 Radionuclides - basic concepts(50)

Atoms of the same element always have the same number of protons in their nuclei but they can have different numbers of neutrons. Those with different numbers of neutrons belong to different varieties of the same element and are called its isotopes. They are usually distinguished by the total number of particles in their nuclei. Thus, uranium-238 has 92 protons and 146 neutrons while uranium-235 has the same 92 protons but only 143 neutrons. Atoms thus characterized are called nuclides. Some nuclides, the minority, are stable or produce such low radiation that they can be considered stable. In those that are unstable, every transformation frees energy and there are many variations for these sequences of transformation or decay as it is called. The whole transformation process is called radioactivity and the unstable nuclides are called radionuclides. The average number of transforma-

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Appendix 5

tions that take place each second in an amount of radioactive material is called its activity. The activity is measured in becquerels, after the man who discovered the phenomenon. The transformations may be natural or man-made by, for instance, neutron bombardment of stable nuclides. The diverse forms of radiation are emitted with different energies and penetrating power. Alpha (α) radiations can be stopped by a sheet of paper and they barely penetrate the outer layers of the skin; however, they are extremely damaging when they get into the body through an open wound, or when they are eaten or breathed in. Beta (β) radiations can penetrate through a couple of centimetres of living tissue and gamma (γ ) radiations are stopped only by thick slabs of lead or concrete. It is the energy of α radiation that does the damage. The amount of radiation deposited in living tissues is called the absorbed dose and it is measured with a unit called the gray.

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On the other hand, a dose of α radiation is more damaging (20 times more so) than the same dose of β or γ radiation; for this reason, the damage potential is taken into account in what is known as the dose equivalent which is measured in sieverts.

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Temperature Time and frequency Electricity and magnetism Photometry and radiometry Acoustics and vibrations Ionizing radiation Chemistry References

53 63 73 79 85 93 97 107

Appendix 1 Fundamental physical constants and their relationship to base units 116 Appendix 2 Some derived SI units

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Appendix 3 Multiples and submultiples of common use with SI

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Appendix 4 Scientists in the field of electricity

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Appendix 5 Radionuclides - basic concepts

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Acknowledgements

ACKNOWLEDGEMENTS Several people and organizations have helped to make this publication possible. The Organization of the American States, OAS, and the German Cooperation for Development, GTZ, were the first to believe that such a document could be of use. For the help received, the authors wish to thank the Bureau International des Poids et Mesures (BIPM); Dr. Gérard Geneves at the Laboratoire Central des Industries Eléctriques du Bureau National de Métrologie in France (BNM-LCIE); Dr. Duncan Jarvis, Acoustical and Noise Standards, of the National Physical Laboratory of the United Kingdom (NPL, UK); Dr. Hans-Jürgen von Martens, “Acceleration” section of the Physikalisch-Technische Bundesanstalt (PTB) in Germany, Ing. Lester Hernández from COGUANOR in Guatemala. For its part, the National Institute of Standards and Technology in the USA, NIST, through its Director for International Affairs, Dr. Steve Carpenter, made freely available copies of its specialised publications. The authors are particularly indebted to the scientific excellence of the Centro Nacional de Metrología de México (CENAM) and the contribution of its Director, Dr. Héctor Nava Jaimes, and the professional staff

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of that organization; all of them shared fully and unreservedly their knowledge and their work practices. Several changes in this second edition were suggested by CENAM personnel. We are particularly indebted for their valuable comments to Dr. Ismael Castelazo Sinencio, Director of Tecnological Services and M.Sc. Rubén Lazos, Scientific Coordinator, both at CENAM, and to Dr. Luis Mussio, Head of Metrology at the Laboratorio Tecnológico del Uruguay (LATU). The authors would also like to thank Mr.Hermon Edmonson and Mr. Allan Foreman of the Bureau of Standards of Jamaica for their help with the English version of this publication. Responsibility for the contents of this publication lies solely with the authors.

July 2002

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Presentation

PRESENTATION The aim of this book is to make available, to those who are not themselves metrologists, a scientifically and technically sound document as an introduction to the main aspects of Metrology in the hope that it will help them understand its importance. A study of history shows that the progress of nations has always been related to their progress in measurements. Metrology is the science of measurements and measurements are a permanent and integral part of our daily lives, a fact we often disregard. Metrology blends tradition and change; measurement systems reflect a people’s traditions, but at the same time we are always seeking new standards and ways of measuring as part of our progress and evolution. Thanks to our measurement instruments and apparatus, tests and assays can be done to establish if a product or service conforms to existing quality standards and this, in turn, gives an assurance of quality of those products and services offered to consumers. Because they facilitate and regulate commercial transactions, correct measurements are fundamental for governments, for enterprises and for the population at large. Very often, the amounts and characteristics of a product are the result of a contract between the client (consumer) and the provider (manufacturer); measurements contribute to this process and thus influence

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quality of life for the population by protecting the consumer, helping to preserve the environment, and contributing to a rational use of natural resources. Metrology related activities in a given country are usually the responsibility of one or several bodies, autonomous or governmental and, according to their scope and their field of application, they are characterized as Scientific, Legal or Industrial Metrology. Scientific metrology is responsible for the research needed to produce standards with a sound scientific basis, and it promotes their acceptance and international equivalence. Legal and industrial metrology relate to the national use of the standards in commerce and in industry. The field of Legal metrology relates to commercial transactions; it seeks to ensure, at all levels, that the client who buys something is effectively receiving the amount agreed upon. For its part, the aim of Industrial metrology is to promote competitiveness in manufacturing and service industries, through permanent improvement of the measurements that influence quality. As a result of the current dynamics of world commerce, metrology has become even more important, with a stronger emphasis on the relationship between metrology and quality, measurements and quality control, calibration, laboratory accreditation, traceability, and certification. Metrology is the basic core for these func-

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tions and, when carried out coherently, it can bring order and contribute to the final aim of improving and guaranteeing quality in products and services. In every country, Metrology plays a singular role, related to Government, Enterprises and Population, a relationship known as the GEP model. From the Government point of view, this model is essential to fully understand the purpose of the infrastructure required to support the establishment of policies and regulations for manufacturing products and for services, both those produced locally and those imported from other countries. Government must also be aware that the measurement capabilities of a country are a measure of its level of technological development in several fields, including manufacturing and services (such as health, education, etc.), and that they directly influence competitiveness of enterprises. Internationally, enterprises, and not governments, are the ones who compete, and one of the pillars of international competitiveness is quality; it must be recognized that metrology is a necessary (albeit insufficient by itself) condition for quality. The capacity of an enterprise to innovate is one of the factors of the competitiveness of the enterprise. Innovation may be applied to production or management processes, to products, to services, or to any other function of the enterprise. Permanent improvement of quality

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requires continuous improvement of activities; continuous improvement requires procedures that use measurement parameters, so that the newly implemented procedures may be compared with what had been used before. Measurements are, therefore, an integral part of the innovation process. Change is the only constant in an environment of continuous improvement. For enterprises, the purpose of continuous improvement is generally to win markets and expand production facilities that, in turn, will open new avenues for growth and the creation of new jobs. Metrology is essential to support the control of the products being manufactured and their impact on the well being of the Population. Communities consume national and foreign products, and Metrology is called upon to determine that these products are in accordance with health and safety standards and specifications. Metrology’s relationship with population is twofold: with its influence on the development of enterprises, it indirectly contributes to the creation of new jobs, but it also helps to protect people by watching over the contents, the quality and the safety of consumer products as well as the impact of these products on the environment. Worldwide open commerce has meant a growing interdependence among nations. More and more often, countries find themselves signing bilateral or regional agreements and treaties. These involve different sectors (industry, commerce, health, defense, environment, etc.) and enterprises are then faced with operational

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international rules that apply to manufacture, buying of raw materials, marketing, etc. If we add to this the fact that consumers are ever more influenced by global patterns of consumption, it is easy to see how essential it is to have a technical infrastructure that can act as the framework for global coordination and order. A primary requirement for this order is the adoption and recognition of an international system of measurement units. The first serious formal step for an international order on measurements was the international Metre Convention or Treaty of the Meter (May 20, 1875), which gave birth to the BIPM (Bureau International des Poids et Mesures - International Office of Weights and Measures). In October 1995, the 20th General Conference on Weights and Measures (Conférence Générale des Poids et Mesures - CGPM) asked the International Committee on Weights and Measures (Comité International des Poids et Mesures - CIPM) to carry out a study on international needs related to Metrology, so as to be able to direct and establish the respective roles of BIPM, the National Institutes of Metrology and the Regional Metrology Organizations. In the Western Hemisphere, the national metrology organizations of 34 countries are associated in the InterAmerican Metrology System, known as SIM. SIM coordinates its functions based on an organization of five sub-regions that correspond to the five main economic and commercial groups of the Western Hemisphere. These metrology groups are: NORAMET (North

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America), CAMET (Central America), CARIMET (the Caribbean), ANDIMET (Andean Group), and SURAMET (South America). Because of the recognized importance of Metrology and because of the importance of its being better understood by different groups of specialists, this publication is addressed, as its title clearly shows, to those whose specialty is not Metrology. The first chapter is a general introduction, the second tries to explain what is measured and why, the third strives to underline the importance of this field of endeavor with a very brief description of some applications, the fourth chapter details the measurement standards and the reference materials currently used for the main units of the International System of Units (SI). Our hope is that reading this publication will help to gain easily a better understanding of current Metrology.

Oscar Harasic Regional Coordinator of the Project Inter-American System for Metrology, Standardization, Accreditation and Quality, Organization of the American States, OAS.

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