Intrumentation- Introduction To Measurement System

Instrumentation and Measurement Techniques •Introduction •Instrumentation System •Errors •Transducers •Other Measurements: flow, level, pressure, temperature, miscellaneous

Objectives •Introduction to general elements of measurement system •Introduction to general functions of instruments

Specific Objective •

Define the basic concept of instrumentation : – Measurement – Calibration – Primary and Secondary Standards

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Describe the measuring system by block diagrams representing: – Transducer – Signal Conditioner – Display unit /recorder

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Explain various classification of the above objective Explain instrumentation system performance with reference to : – Static performance – Dynamic performance

Measurement System A measurement system converts the unknown quantity under measurement to a numerical unit using instrument. Number + measured unit 6.8 kg/(ms)2 Measurement can be defined as a comparison with a standard therefore calibration and routine maintenance are prerequisites of accurate measurement. It is multidiscipline subject which embraces physics, thermodynamics, mechanics , fluids chemistry and electrotechnology.

Instrument • 1) Instrumentation is the application of instruments for monitoring, sensing and measurement .Some of its applications are • a) product testing • b) monitoring- health, safety, costing • c) control system • d) R&D

Instrument • 2) The advantage of electronic instrument over mechanical instruments, • a) rapid response • b) flexibility of electrical signal –amplitude, distance

Calibration Calibration can be performed in two ways : • a) varying one input quantity with all other parameters kept constant and observing the output b) marking or graduating • an output scale as the primary quantity is varied through its full range . Two type of standards are used primary standard and secondary standard (working standard)

Terms •Measurement - Comparison between an unknown quantity and a predefined standard •Measurand- the unknown quantity to be measured •Instrument - physical device uses to determine measurand numerically.

Transducer • a) Transducer- an element which converts physical quantity being measured. • b) Signal conditioner – modify the tranduced signal in a form that can be recorded. • c) Recorder /display – to record or display the measured quantity. • Example: Bourdon gauge - Bourdon tube (transducer), Links and gear(Signal conditioner), Graduated scale and pointer (Display )

Transducer •A device to which change or converts physical quantity in a more easily measurable quantity Transducer

(Input) Sensor

(Output) Actuator

Transducer Basic Requirement • • • • •

Ruggedness Linearity Repeatability High Signal to Noise Ratio High stability and reliability

Sensor A device which senses and detects the physical quantity of measurand and converts to electrical form. Example of sensors: Mechanical

: Bourdon tube pressure meter.

Electrical

: Potentiometer

Optical

: Photon counter

Chemical

: Thermocouples

*All sensors are transducers but not all transducers are sensors

Actuator A device that senses and detects the electrical quantity and converts to physical form. Example of actuator: • Valve in heat exchanger system • Motor speed control where the motor is driving the conveyor belt • Magnetic relays that turn on/off of the fans • Compressor in a control air conditioning

Variable Conversion Element A device that converts analogue signals to digital form or vice versa. Example of converters: ADC -- Analogue to digital converter DAC -- Digital to analogue converter

Variable manipulation element A device that manipulate the signal presented to it while preserving the original information. Example: Amplifier

: Signal gain

Buffer

: Unity conversion

Attenuator

: Signal reduce

Signal conditioning • Operation performed on the signal to bring it to the desired form. • Include variable conversion & variable manipulation element. Example: Filter, amplifier, comparator circuit, differential circuit, ADC, multiplexer and etc.

Telemetry Transmission of data from remote sources to serve specific purposes. Example: Remote control, coaxial cable, wireless transmission, optical fiber cable and etc...

Data output element To convey the measured quantity for further action. Display, recording and control. Display

: Monitor(CRT), LCD

Recording

: Magnetic tapes, printer

Control

: Data managing system

Essential Devices These following devices must always present for all instruments: Power supply

: To supply power(current) to instruments

Pulse generator

: To generate clock to digital system

Protection device

: To supervise current flow.

Shielding system

: To protect instrument from interfere by external disturbance.

Function of Instruments & Measurement Systems Indicating instruments

:Meter display, digital display

Example: speedometer in car, pressure gauge Recording function

: Data Keeping

Example: Printer, magnetic disc Controlling function

:Temperature, position, speed, liquid level, flow control.

Performance Indication An ideal measuring system is one where the output signal has a linear relationship with the measurand. Error is the difference between the indicated value and the true value. Measuring and control system performance can be examined in two ways , • Static performance –when steady or constant input signals are applied • Dynamic performance – when changing input signals are applied

Static Performance Indication • Static sensitivity is defined as the ratio of the change in output to the corresponding change in input under static or steady state conditions, . For a system having static sensitivities of K1 ,K2,K3 ……, the overall system sensitivity is given by

Dynamic Performance Indication • Dynamic performance of both measuring and control system is specified by response to certain standard test inputs • Step input- abrupt change from one steady value to another ,will give the transient response • Ramp input –which varies linearly with time, will give the ramp response • Sine wave input – will give the frequency response

Indicated value − True value x 100% True value

Error Calculation • Accuracy can be stated in terms of errors introduced • Percentage error = Indicated value – True value . X . 100% Maximum scale value Precision is used to specify the closeness of output result when a measuring device is subjected to the same input on a number of occasions

EXAMPLE A measuring system consist of a transducer , an amplifier and a recorder, with an individual sensitivities as follows : • Transducer sensitivity 0.2 mV /0 C • Amplifier gain 2.0 V/mV • Recorder sensitivity 5.0 mm /V • Determine the overall system sensitivity • K= K1 x K2x K3 • = 0.2 mV/0C x 2.0V/mVx 5.0 mm/V • =2.0 mm/0 C

EXAMPLE A 0 to 10 bar pressure gauge was found to have an error of ± 0.15 bar when calibrated by the manufacturer .Calculate (a) the percentage error of the gauge and (b) the possible error as a percentage of the indicated value when reading of 2.0 bars was obtained in a test . Percentage error = 0.15/10. x 100 = ± 1.5% Possible error = ± 0.15% ∴ error at 2.0 bars = 0.15/10. x 100 =± 7.5% • The gauge is therefore more unreliable at the lower end of its range, and alternative gauge with a more suitable range should be used .

EXAMPLE • For a general measuring system where the errors in the transducer, signal conditioner , and recorder are ± 2 % , ± 3%,and ±4 % respectively, calculate the maximum possible system error and the probable or root- sum –square error. • Maximum possible error = ± (2+3+4)% =±9% • Root –sum-square error = ± √(22+32+42)% • = √ 29% =± 5.4% • Thus the error is possibly as large as ± 9% but probably not larger than ± 5.4%.

Transducer • • • • • • • • • • •

Resistance transducers Resistance strain gauge Resistance temperature transducer Photo-emisive cell Capacitive transducer Inductive transducer Linear variable differential transformer Piezos-electric transducer Electromagnetic transducers thermoelectric transducer Photoelectric cell