TUR stands for test uncertainty ratio: the span of the tolerance divided by twice the 95 % expanded uncertainty of the calibration process, which for symmetric limits equals the tolerance half-width over U. A TUR of 4:1 or better keeps false-accept risk low enough that many quality systems accept results without further risk analysis. Unlike the test accuracy ratio, it counts the full process uncertainty, including the UUT's resolution and repeatability.
TUR at the 1 mV point is only 2.3 to 1, so either guard band it or send it to a lab with a better standard.
A healthy TAR can hide a poor TUR once the UUT's own resolution and repeatability enter the budget.
Also heard as
- test uncertainty ratio
- uncertainty ratio
Do not mix it up with
- TAR: TAR compares the tolerance with the standard's accuracy spec only; TUR uses the full expanded uncertainty of the measurement.
Related terms
TAR
Test accuracy ratio, the UUT tolerance divided by the accuracy spec of the standard used to test it.
Four to one
The rule of thumb that measurement uncertainty should be no more than a quarter of the tolerance being checked.
Expanded uncertainty
Combined standard uncertainty multiplied by a coverage factor to give an interval with a stated coverage probability.
Guard band
A margin that pulls acceptance limits inside the tolerance limits to cut the risk of passing nonconforming items.
PFA
Probability of false accept, the chance that a nonconforming item is measured and passed as good.