Calibration

CALIBRATION CALIBRATION Procedures, Standard And

Introduction • In this section, the procedures and methodologies to carry out a calibration will be discussed. In addition, the approach to calculate errors in a calibration will be shown.

Objective Of Calibration • The prime objective of calibration is to ensure that measurements taken are accurate at all times taking into consideration wear and tear, degradation and abuse. It is with this in mind that we need to adopt and appropriate methodology to do a proper calibration.

Scope of Calibration In developing a calibration program, the following should be taken into consideration :  Involve all testers or those who use measuring and test equipment  Calibration must be conducted on all equipment used to verify product compliance.  Engineering measuring equipment must all be calibrated.

Hierarchy of Standards • The hierarchy of standards used in a calibration is shown as follows :

Primary Standard Reference Standard Working Standards Measuring & Test Equipment

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Primary or National standards are the ultimate reference standards for determining a measurement. As an example, a standard measurement of a meter is set by a steel rod of this length which is the primary standard. All other standards used to conduct a calibration must be checked with reference to this standard from the working to the reference standard

Standard Adequacy • Working standard used to calibrate internal measuring equipment must be traceable to either a National or Primary Standard. In addition, their accuracy must be 4 to 10 times more accurate compared to the equipment to be calibrated. For instance, portable water analyzer such as pH meter, NTU meter with the standard measurement scale, can be calibrated internally by using the standard solution that provided. • Occasionally, an equipment may be state of the art and hence, there is no reference standard available. In this case, this equipment can be calibrated with another equipment of the same type, make and model.

Traceability of Calibration • In a country, a calibration must be traceable to a National Standard held by a national body (eg. SIRIM or other government agencies of Malaysia). If this national standard is not available. The calibration must be traceable to another country. Traceability of a calibration is important as an incorrect reference standard used will not ensure that the equipment calibrated has the required accuracy. • As most companies send out their equipment for calibration, mandatory that they check the capability of the calibration agent. Those that are certified to the ISO 9000 Guide 25 will be able to provide an appropriate traceability. For others, you will need to check the reference standards used and their respective traceability.

Factors to Consider in a Calibration

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Calibration Environment Interval of Calibration Accuracy of Standard Master Out of Tolerance Equipment Subcontractor Controls Uncertainty of Measurement Handling the Storage Identification and Status of Calibration Training Documentation and Records

Calibration Environment Environment conditions which could affect a calibration are as follows. This will largely dependent on the sensitivity of an equipment to the various conditions. Consult the equipment manufacturer to determine the appropriate environmental conditions required to do a calibration. • • • • • • • •

Temperature Humidity Vibration Dust Noise Light Air Movement Magnetic Field Some examples where controlling certain environment factors is critical for the different types of measurement are as follows :

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Chemical Analyses – pH value and NTU are critical

Interval of Calibration •

Interval between one calibration to another is an important aspect to ensure the continuing accuracy of an equipment. This must be established for all measuring and test equipment, each may be set at different intervals dependent on the degree of usage and other factors such as: > Stability > Frequency of Use > Accuracy > Purpose of Use

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If you are unsure which is the best interval to choose, always follow the equipment manufacturer’s recommendations. This interval should be reviewed from time and as a rule of thumb, it should be adjusted based on the calibration history. For example, if an equipment is calibrated on a six monthly basis and it is found to be out of calibration frequently, then the calibration interval should be changed either 2 or 3 monthly.

CALIBRATION INTERVAL •

What actions and the calibration interval on the equipment would you recommend to the user for the following :

1. An instrument is found to have failed 2 times in the last 2 calibrations performed at 6 monthly intervals. 2. A newly purchased pH on-line analyzer which will be used in production daily. Calibration interval recommended by manufacturer is 3 monthly. 3. A Magnetic Flow Meter used to measure the flow of water with specification of 0.5 to 1 % accuracy. Accuracy of Magnetic Flow Meter is 0.5 to 1% with Calibration interval of yearly using the Flow Meter Verificator.

ACCURACY OF REFERENCE STANDARD

• As discussed in the previous section, the reference standard used to do a calibration must be at least 4 to 10 times more accurate than the equipment. In general, a 95% confidence level is preferred and this gives a + 2 sigma (standard deviation) or 4 sigma capability in a measurement.

OUT OF TOLERENCE EQUIPMENT •

When an equipment is found to be out of tolerance or not having the required accuracy, certain steps must be taken. Actions to be taken include decisions on whether the equipment can be used or not and what needs to be done on the affected products as follows : > Review all measurements prior to and after adjustment is made to the affected equipment. > Decide on whether the equipment can be used or has to be thrown away or used for lesser accuracy measurements. > Review previous batches of products ( 1 week or more) where measurements have been taken using this out of tolerance equipment prior to and after an adjustment has been made to correct the out of calibration situation. Decide whether they meet specification or not. > If the equipment can be used but needs correction to their readings, then the compensation or offset factors must be stated on the equipment.

Subcontractor Controls • In choosing an appropriate subcontractor to perform your calibration, it is critical that an accredited calibrating lab or agent is used (those certified to ISO 9000 Guide 25). Ask for a copy of their certification before selecting them. You will also need to ensure that the reference standards used by the calibrating lab or agent is traceable to national standard. The calibration method used should also be checked to ensure they used an appropriate or recognized procedure. Both of these requirement can be verified on their Certificate of Calibration, which is issued to you upon completion of a calibration for each set of equipment.

Uncertainty Of Measurement The uncertainty of measurement of a calibration needs to be calculated to determine all the possible sources of errors. It is given by the formula : Uncertainty of Measurement = + t (from a Students t-table) x s (standard deviation of variation). As a rule of thumb, the uncertainty of measurement must not exceed 30% of the tolerance of a measurement. If it does, it’s mean that the equipment is not suitable for use as its required accuracy is not sufficient. A measurement taken using this equipment will give a false or inaccurate value.

Handling and Storage •

The proper handling and storage of measuring equipment and their reference standards are an essential part of establishing an effective Calibration Program. Some of the guidelines to follow are as follows:

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Handling – ensure the equipment is appropriately handled during use, storage and transportation. Proper Storage – ensure equipment is stored in their respective casing or special storage container under appropriate environmental conditions as required. Proper Packing for Transportation – ensure the equipment is appropriately packed to protected them from movement and damage during transportation (eg. Use “bubble bags” or impact absorbent materials). Check that the calibration status is valid before issuing an equipment for use (especially newly purchased equipment). Ensure that the equipment is appropriately sealed with a tamper-proof sticker or marking, etc to prevent unauthorized adjustment or repair.

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Identification and Status of Calibration

• Upon completion of a calibration, each equipment must be appropriately labeled to indicate their status. Depending on the size of the equipment, it may not be possible to attach a calibration label on all instruments. For an equipment such as thermometer it may not be practical to stick such a label. Therefore, other means such as color coding can be used instead of a label to indicate their calibration status. Alternatively, the label may be attached to the casing or container.

Example of the various labels or identifications which can be used are as follows:

• Passed Calibration

ID#: BY:

DATE: DUE:

• Tamper Proof Label These labels are used to prevent tampering on the equipment after it is calibrated. An example is as follows :

Calibration Seal

VOID If Broken

PBA

• “DO NOT USE” Label This label is used to indicate that a specific equipment cannot be used either because it is damaged or needs repair or malfunctioned or its calibration is out of tolerance.

PBA

Perbadanan Bekalan Air Pulau Pinang Sdn. Bhd.

DO NOT USE • “No Calibration Required” Label This label is used to indicate those equipment which do not require any calibration. These are mainly those used for rough measurement and for which accuracy is not important. Some examples are process instrumentation such as chlorine analyzers and ultrasonic level sensor, etc.

PBA

Perbadanan Bekalan Air Pulau Pinang Sdn. Bhd.

CALIBRATION NOT REQUIRED

Training • Personnel appointed to perform calibration must be appropriately trained together with proper education and experience. They must understand the characteristic and features of an equipment, and its usage (ie. Required accuracy, stability, sensitivity, etc). Furthermore, the personnel must know the calibration procedures and methodologies including the calculation of uncertainty of measurement and sources of errors. Various training courses are now available and those responsible for performing calibration must attend these courses.

Documentation and Records •

Appropriate documentation and records must be maintained as part of a calibration program. Essential ones are as follows : a) Master List of Equipment A master list of equipment shall be maintained to indicate all the measuring equipment used in a company. It should state as a minimum, the following details :

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Equipment Identification (ID) Location Required Accuracy Measurement Range Calibration Interval Calibration Lab/Agent (internal or external) Date Calibrated Date due for Next Calibration Pass / Fail Calibration

b)

Calibration Plan

For each type of equipment, a calibration plan shall be developed. This plan shall outline all the steps to conduct a calibration including :     

Reference standards to be used ; Recording methodologies ; Environmental conditions for calibration ; Calculations ; Approach to handle an out of tolerance situation. The plan should also outline the recalling system to ensure each piece of equipment is calibrated either on or before their due date. A reminder system or calibration schedule is necessary. c)

Training Records

Training records shall be maintained for all personnel authorized to perform calibration.

d)

Out of Calibration Notice and Records

Notices advising the relevant department and personnel of an equipment found to be out of calibration and the corresponding actions (s) taken shall be maintained.

e)

Manufacturers’ Handbooks

Manufacturers’ handbooks or reference manuals used as part of calibration shall also be controlled and kept by the department responsible for calibration.

f)

Calibration Certificate

Calibration certificate provide by an external calibrating organization must be kept as proof of calibration and are to be maintained as part of the calibration records. Internal calibration certificate also required to be kept at the respective cost centre.

g)

Records of Usage

Records of usage are to show the persons who have used the equipment and to indicate the degree of usage and what measurements were taken. h)

Measurement Equipment History Record

A history record should be maintained for each piece of equipment. It should contain the following details :        

make, model and type or brand date of purchase department using this equipment and its location use date of first calibration adjustment made during calibration (if any) repair & service details action taken on out of tolerance situation (if any) date of disposal (when it is unusable).

Thank you Prepared By: Mohd Suhaimi Shariff