Calibration Technology Download PDF
Introduction:
Every measuring instrument is subject to ageing as a result of mechanical, chemical or thermal stress and thus delivers measured values that change over time. This cannot be prevented, but it can be detected in good time by calibration. The Egyptians already knew this almost 5000 years ago. The workers calibrated their yardsticks by comparing them with a “royal cubit” (approx. 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 per cent.
Definition:
In the process of calibration, the displayed value of the measuring instrument is compared with the measuring result of a different measuring 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 (standard) (Fig. 1). One talks about verification when the calibration has been carried out or supervised by an official body. 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 following the intervention.
Period of validity:
In contrast to verification, which will lose its validity after a period of time set by law, the validity period of a calibration is subject to practical specifications such as manufacturer’s instructions, requirements of a quality assurance standard or in-house and customerspecific regulations. Calibration must also be carried out when the measuring instrument is used for the preparation of products which are subject to official supervision such as drugs or foodstuffs. As part of a survey of 100 management executives of international companies, the Nielsen Research Company established in 2008 that due to faulty calibrations, manufacturing companies were losing more than 1.7 million dollars annually on average, and those companies with a turnover of more than a billion dollars as much as 4 million dollars.
Trends:
Increasing demands from users on their measurement technology directly result in continuously higher demands on the reference instruments. The further development associated with this does not of course happen arbitrarily, but is often based on challenges that at first sight are not related at all to calibration. This is proven, for example, by the ever increasing air traffic over Europe: In order to accommodate more aircraft in the airways, the vertical minimum distance between individual aeroplanes at flying heights of between 29,000 and 41,000 feet has been cut in half from 2000 feet originally to 1000 feet (approx. 300 m). The resulting airspace may only be used by aeroplanes whose equipment allows a sufficiently accurate display and a sufficiently accurate observance of the flying height. Thus, the altimeter, which functions on the relationship between height and atmospheric pressure, must be able to measure with an accuracy that is clearly below 0.01 per cent. This is why aeroplane and sensor manufacturers, or the relevant service providers, demand higher-quality reference instruments for the calibration of pressure measuring instruments. At the same time, it must be possible to process calibrations cost-effectively and efficiently, which, given the prescribed test procedures, can only be achieved by a higher degree of automation.