Strategies for Dealing with Low Test Uncertainty Ratios

The accuracy of a calibration or testing scenario is often measured using the Test Uncertainty Ratio (TUR) which is the ratio of the specification to be tested to the calibration process uncertainty. As a result of improvements in the state of the art in Test, Measurement, and Diagnostic Equipment (TMDE), it is often difficult to meet TUR requirements. When a TUR requirement can't be met, what then? As a result of the transition to the use of the Probability of False Acceptance (PFA) as a primary metric for calibration testing, there are many appropriate strategies for dealing with this situation. A common approach is to simply apply a guard band to the test tolerances. However, it has been found that this is not always appropriate. In fact, there is a fairly narrow set of situations in which guard bands should be employed. This paper discusses alternatives that should be considered in addition to guard bands.