Calibration: a quality driver!

July 5th, 2018, Published in Articles: EngineerIT

From left: Eugene Golovins, NMISA/DCLF Electrical; Lloyd Townsend, WIKA Instruments; and Petrus Klopper, Chairman SAIMC Tshwane branch.

Is calibration a quality driver or a necessary evil? This was the subject of an interesting presentation by Lloyd Townsend of Wika Instruments. The company hosted the SAIMC Tshwane branch July technical evening, and the presentation was followed by an enlightening debate. Almost 500 years ago, the Egyptians already knew that calibration is a quality driver. The workers calibrated their yardsticks by comparing them with a “royal cubit” (approximately 52,36 cm) made of stone and thus managed to achieve, for example, side lengths on the Cheops pyramid of 230,33 m which differ from each other by only about 0,05%.

Calibration is often viewed as a secondary task in plant operation but as every measuring instrument is subject to ageing as a result of mechanical, chemical or thermal stress and thus delivering values that change over time, regular calibration is advisable. Regular calibration provides information on the instrument’s metrological characteristics. They either confirm the measured value or they flag up a change.

In the process of calibration, the displayed value of the measuring instrument is compared with the measuring result of a different device which is known to function correctly and accurately and which itself has been made to coincide directly or indirectly with a national (or international) reference instrument.

“One talks about verification when the calibration has been carried out or supervised by an official body,” Towsend said. “Both of these variants are purely intended for determining the quality of the displayed values. No intervention to the measuring instrument itself is allowed. With adjustment, it is understood that there is an intervention to the measuring device in order to minimise a detected measuring deviation. Typically, adjustment is followed by a further calibration in order to check and document the final state of the measuring instrument.”

Another important aspect is that measuring results must be “traceable” to a national or international standard via a chain of comparative measurements. To this end, the displayed values of the measuring instrument used or a measurement standard are compared over one or several stages to this standard. At each of these stages, calibration with a standard that was previously calibrated with a higher-ranking standard is carried out. In accordance with the ranking order of the standards, from the working standard or factory standard and the reference standard to the national standard, the calibration bodies have a calibration hierarchy. This ranges from the in-house calibration laboratory to the accredited calibration laboratory and to the national metrological institute.

After the presentation, an enlightening discussion took place about the “ten times rule”, referring to each level of calibration needing to be ten times better. It was questioned whether two or just one certificate showing the first calibration test and the results after adjustment should be issued.