The idea of using certain TAR/TUR (for example 4 to 1) is to make sure that the reference standard is good enough for the purpose. the accuracy specification of the reference standard should be 4 times better (or smaller) than the one of DUT. We commonly hear about using a TAR ratio of 4 to 1, which means that the reference standard is 4 times more accurate than the device under test (DUT). The ratio is the accuracy (or uncertainty) of the device under test compared to the one of the reference standard. Test Accuracy Ratio (TAR) and Test Uncertainty Ratio (TUR) are sometimes used to indicate the difference between the device under test and the reference standard used. Metrological Traceability in Calibration – Are you traceable? More information on the metrological traceability can be found in the below blog post: If the traceability chain is broken at any point, any measurement below that cannot be considered reliable.
The national calibration centers will make sure that the traceability in that country is at the proper level, using the International Calibration Laboratories or International comparisons. The reference calibrator should be calibrated with an even higher-level standard or sent out to an accredited or national calibration center for calibration. The portable process calibrator you used, should have been calibrated using a more accurate reference calibrator. So, for example, you may calibrate your process measurement instrument with a portable process calibrator. The traceability should be an unbroken chain of calibrations, so that the highest-level calibration has been done in a National calibration center, or equivalent. This traceability means that the reference standard must have also been calibrated using an even higher-level standard. It was mentioned that the reference standard that is used in calibration must be traceable.
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