DISCUSSION ON CALIBRATION AND MEASUREMENT
Making a measurement is simple. We have all done it. But making measurements, in which we have a quantifiable level of confidence , is not a trivial task. Achievement of a measurement that can be compared with confidence with other measurements, possibly made in a different country, is even more difficult. Confidence and trust are critical in serious measurements.
While it is feasible for small groups of individuals or organisations to audit each other, the development of mutual trust and confidence among larger groups of organisations and between nations is not feasible without some type of standard to which everyone agrees. To facilitate measurement comparisons between organisations in different countries this standard has to be international.
If a standard is to govern a worldwide activity successfully it should be unique, a genuine industry standard. Therefore there can be no alternative standards for calibration laboratories or users of calibrated measurement systems.
A calibration is either performed by an ISO 17025 accredited laboratory and hence has documented confidence intervals and is traceable, or it is not. If the system is to work there can be no grey areas. If you don’t like ISO 17025 your only recourse is to participate in the system and change it from within.
It is of interest to note that at present all the base SI units with the exception of mass are defined in terms of fundamental physical constants and hence can be reproduced in any laboratory by skilled technicians with the appropriate equipment. Many national measurement laboratories reproduce a number of the base units using these instruments and compare their realisations with the BIPM or other national labs.
Guarantees and Probabilities
No standard can guarantee that a calibrated instrument performs within specified limits or according to the calibration certificate. Immediately after a calibration in an ISO 17025 accredited lab, the instrument should perform within defined tolerances with a specified probability. To maintain that performance until the next calibration it is the user’s responsibility to ensure that the instrument is not mishandled subjected to environmental extremes, and to select an appropriate calibration interval.
When we can truly believe a measurement result and how?
If we wish to make a measurement and estimate a range of values within which the true value is likely to lie with a quantifiable level of confidence, then our instrument has to be calibrated regularly by an ISO 17025 accredited laboratory. Uncertainty estimates would enable us to estimate a range of values within which the truevalue lay, and hence facilitate the determination at a specified confidence level, of whether or not the values attained be within the limits as predetermined.