3 Calibration and adjustment

3.1 Introduction

A perusal of the archives of the ISO 17025 international email discussion list[5] suggests that there is substantial disagreement amongst the experts concerning the components of calibration. Much of the disagreement concerns adjustment. Some instruments and reference materials, e.g. mercury-in-glass thermometers, gauge blocks, standard resistors, cannot be adjusted if they are found to be out of spec at calibration. Many instruments, however, can be adjusted to make the indicated value equal to the standard value, within the uncertainty of the instrument. In this topic we discuss pros and cons of adjustment as part of a calibration process.

3.2 Adjustable instruments

Various circumstances may be associated with a given calibration.

• The customer might be monitoring the long-term stability of the instrument (e.g. as part of a process for determining calibration intervals) and adjustment might confound the stability analysis. In this case an acceptable outcome might simply comprise a calibration certificate with a table of indicated vs standard values or corrections.

• The customer might simply want the instrument returned `in specification' and leave any adjustment decision up to the calibration lab. ISO 17025 (section 5.10.4.3) advises that if an instrument is adjusted the lab should report `as found' or `before adjustment' values, and `as left' or `after adjustment' values on the calibration certificate. This information should allow some analysis of stability to be performed post-calibration, but might increase the costs of calibration.

• An adjustable instrument might deviate from the reference by a large proportion (e.g. 80-90%) of the instrument's specified uncertainty. This instrument is technically within specification, but if left unadjusted, might drift out of specification soon after the calibration, resulting in problems for the user that may be detected only one year later. Customers who are aware of this possibility sometimes request adjustment only if the indicated value deviates from the standard value by more than a pre-determined proportion of the tolerance. Some calibration labs have standard procedures which specify the conditions under which in-tolerance adjustments should be made.

3.3 Non-adjustable instruments

For completeness we briefly discuss non-adjustable instruments. The calibration certificate of non-adjustable items may contain various items of information depending on the calibration lab operating procedures and the customer's requests.

• A simple `in specification' or `out of specification' report may be given. In this case the user is not able to analyse the stability of the instrument or reference material. ISO 17025 accredited labs, however, are obliged to keep detailed records of all such calibrations (section 5.10.4.2), so clients should be able to obtain those details after the event.

• The calibration certificate may contain a table of indicated values vs standard values, or corrections vs indicated values, or both.

3.4 Conclusion

It is important that the customer discuss with the calibration laboratory, before work commences, the details of any adjustment procedure to be followed during the calibration.

©2002 Martin Turner B.Sc. (Eng) Ph.D.
Engineering and Measurement Consultant
12 Goodman Place, Cherrybrook, NSW 2126, Australia
Tel: 0403-007 305 (International: +61-403-007 305)
Email: mjturner at biccard.com
Disclaimer The views expressed and information provided in these documents are the opinions of the authors and do not represent specific advice on any topic.

First published: 8 Sept 02 Last modified: 2 Oct 02