OMICRON Magazine

results and a forecast of the trend in the toler- ances are additional services that a customer will have to commission separately. 1. All that is covered by calibration is the measurement of physical variables and a comparison of the measurement results from the test object with those from a suitable, traceable reference instrument under defined, reproducible conditions. 2. Verification is a comparison between the measurement results from a device and its limiting values, e. g., the published tolerances, and the interpretation of these results. This results in a statement of whether the device is still operating within its tolerance limits. 3. In the case of adjustment , the deviation between the readings from the test set and a reference device is reduced to a minimum or adjusted so it again lies within the speci- fied tolerance range. This involves making an active intervention in the test set itself. How often to calibrate? There are no specific guidelines regarding when and how often a test set should be calibrated. It is up to users to decide whether their testing equipment is still suitable for the job at hand. However, the recommendations provided by the manu- facturer will be helpful in determining how frequently calibration should take place. One important factor is the application area of the test set: for example, different require- ments will apply if it is being used in meter testing as opposed to protection technology. Calibrating test sets annually is commonplace and good practice though it is not mandated by either ISO 9001 or ISO 17025. A clear require- ment, however, is set out in the standards to the effect that test sets must perform accurately for the application in question. These functional tests can be carried out by taking comparative measurements of suitable test objects or by making comparisons with correspondingly accurate measurement instru- ments. This is also an approach that might help support the case for extending calibration intervals. However, every exten- sion that goes beyond the interval recommended by the manufacturer must be rigorously justified and documented. 26