OMICRON Magazine

Magazine | Issue 2 2019 Precise measurement results can only be obtained from correctly calibrated test sets, yet the accuracy of these devices alters over time in a process that is unique to each one. Therefore, regular calibration and necessary adjustments for test sets according to their application areas are absolutely essential. The service offered by OMICRON Calibrations takes this a step further: We have established an internationally recognized and state- accredited ISO/IEC 17025 calibration lab that meets the highest international standards. It is our firm intention to offer our customers the most accurate and complete calibration process anywhere in the world. accuracy – what exactly does that mean? Misunderstandings can occur easily when talking about the accuracy of measuring instruments. Accuracy is not simply a matter of what’s quoted in the specifications as it is heavily influenced by time factors. If a device is freshly calibrated (adjusted) before delivery, then it only retains its highest accuracy for a certain period of time. In order to ensure that our devices are still working properly after one or two years, the tolerance limits we use are very conservative. In determining them, we take into account the fact that accuracy will be affected by factors such as natural component aging and the drift that occurs through the normal use of the device. What we consider “normal use”, is actually the regular use of the device in condi- tions where there is no significant mechanical or thermal loading resulting from vibrations or continual fluctuations in temperature and humidity. Conditions such as these will unsurprisingly result in a more rapid drift in measurement accuracy. Staff in professional calibration labs tend to use the term “measurement uncertainty” rather than any vaguely defined concept of “accuracy”. Measurement uncertainty is deter- mined for each testing assignment by consid- ering all the variables that might influence it, such as the test set-up, connections, measure- ment devices, the test object, and how they all interact. To provide a reproducible assessment of measurement uncertainty, all influencing factors must be determined, documented, and collated in a statistically correct manner: › Specifications for the measuring instruments used, including the effect of temperature on the result › Effect of temperature or humidity on the item being measured › Noise or errors from quantification of the measurement (digital transformer) › Interactions between tester and measurement instrument › Effect of cable connections, screening measures, or different grounding points › Calibration results for the measurement equipment and its historical development Therefore, measurement uncertainty is a verifiable measure of accuracy that can be used when carrying out a measurement. Calibration, verification, and adjustment – what’s the difference? The expectations of what calibration can and cannot do are not always clear. In situa- tions where calibration is performed by the manufacturer or in the factory, it is normally assumed that the device will also have been verified and adjusted so that any deviation is again reduced to a minimum. However, by definition, a conventional calibration in line with ISO 17025 only includes the measure- ment of deliberately selected measuring points under defined, reproducible ambient condi- tions with a specified measurement uncertain- ty. On the other hand, a comparison, assess- ment, or interpretation of the measurement 25