Testing the testers

Quality auditors, when assessing an organisation to ISO 9001, expect to find that measuring equipment is within its calibration interval and that the performance of the equipment is suitable for its application.

With Quality Accreditation, the control of measuring equipment has become an important process for organisations. AS/NZS ISO 9001:2000 requires organisations to carry out measurements in a manner that provides evidence of the conformity of product and that, where necessary, measuring equipment shall be calibrated periodically against standards traceable to national standards and that the results of calibration be maintained.

Auditors also expect to see records of calibration and a process to assess previous measurement results when equipment is found not to conform to requirements.

Calibration

Calibration is the comparison of measurements performed by an instrument to those made by a reference instrument or standard, for the purpose of reporting errors in the instrument being calibrated.

When sending measuring equipment to a calibration laboratory it is important to specify any functions, ranges and points which are of importance to the user.

Measurements performed during calibration must be traceable and this is achieved when each reference instrument or standard used in a calibration is traceable by a chain of inter-comparisons to national standards.

The growing global acceptance of the ISO 9001 quality standard has led to a general increase in commercial requirements for the traceable calibration of test and measurement equipment. The purpose, with an eye to product safety and fitness for use, is to ensure that the products manufactured in one country will be acceptable in another.

Reporting

Two forms of reporting are usually offered by calibration laboratories, a full calibration report or certificate of conformance.

Calibration report

A calibration report usually provides a set of results which show the performance of the instrument being calibrated in comparison to a reference instrument or standard.

Measurements are recorded for each function, range, and points tested and the uncertainty of each measurement is reported. The report enables users to be aware of the errors of their instrument and, if necessary, apply corrections based on the results given in the report. Depending on the magnitude of errors the client may wish to have the instrument adjusted.

Calibration certificate

A calibration certificate usually states the compliance of an instrument with the manufacturer's specification.

Measurements made by the instrument being calibrated are compared to the manufacturer's specification. If the instrument is within specification, a statement that the instrument meets manufacturer's specification at the points tested is made in the conformance certificate. Any points that are outside specification, or not able to be determined, are reported as exceptions. Assessment of meeting manufacturer's specification takes into account the measurement uncertainty. A statement of conformance to manufacturer's specification may also be given in calibration reports.

Uncertainty of measurement

There is uncertainty associated with every measurement and factors such as the accuracy of the reference instrument(s) or standard(s), characteristics of the instrument under calibration and influences on the measurement system contribute to uncertainty.

The way in which calibration laboratories are expected to deal with uncertainties is set out in the ISO Document 'Guide to the Expression of Uncertainty in Measurement'. The NATA publication 'Assessment of Uncertainties of Measurement' is a good reference on measurement uncertainties and very useful in understanding the requirements of the guide.

Measurement uncertainty in calibration reports is normally quoted as a range about the measured value with a certain confidence level and coverage factor. The interpretation of uncertainty stated this way is that the true value lies within the range stated with a certain level of confidence. Calibration laboratories have standardised on 95% confidence level and this means that there is a 95% probability of the true value being within the range stated.

Coverage factor is usually quoted as 2, or close to 2, and relates to the shape of the distribution associated with the measured value. Coverage factor is provided in calibration reports to allow uncertainty calculations associated with the use of the instrument to be made.

Uncertainty is stated in a calibration report as an expanded uncertainty, and this means that the confidence level and coverage factor have been taken into account in the calculation of uncertainty.

Calibration intervals

A question that is often asked of a calibration house is: "How often do I need my instrument calibrated?"

The answer to this question is not straightforward and depends on factors such as: the impact of the instrument on quality, the environment and the way in which the instrument is used, periodic checking against other instruments and recorded history of the instrument.

Calibration laboratories operate under AS ISO/IEC 17025 (General requirements for the competence of testing and calibration laboratories) and the supplementary requirements for accreditation by NATA to ISO 17025 is a useful guide in determining calibration intervals. For digital instruments the required recalibration interval is a maximum of 12 months and this interval is frequently used for instrument calibration.

ISO 17025 requires that a calibration report or label shall not contain any recommendation on the calibration interval except where this has been agreed with the client. There are three ways in which calibration intervals are commonly reported on a calibration report:

Client agreed

This is when the calibration interval has been agreed between the client and the calibration laboratory and 12 months is typical. This is reported as 'Calibration Interval'.

Client specified

At times the client will specify the calibration interval required for a particular instrument and this will be based on the client's understanding of the use, performance and history of the instrument.

This is reported as 'Client Specified Calibration Interval'.

Manufacturers recommended

Manufacturers often recommend calibration intervals for their instruments.

These recommendations are based on their knowledge of the design of the instrument and the performance history of a significant number of the same instruments over a period of time. Generally manufacturers recommend an extended calibration interval once they have gained confidence in the instrument's performance.

This is reported as 'Manufacturer's Recommended Calibrate Interval'.

The supplementary requirements give maximum checking intervals for test equipment and for digital instruments this is three to six months. Periodic checking of the performance of an instrument in relation to another instrument(s) is recommended in helping to ensure that performance is maintained between calibrations.