The quantity U = k × u_c, the half-width of the interval y ± U that a certificate quotes at a stated coverage probability. Calibration certificates almost always report it with k = 2, which corresponds to about 95 % coverage for a normal distribution. It is the number that gets compared with tolerances, used in TUR calculations and checked against a lab's CMC.
Expanded uncertainty at 100 bar is 0.015 bar, k equals 2, put that on the cert and in the TUR calc.
When reading an outside cert, find the k value before using the number; a U quoted at k = 3 is not comparable to one at k = 2.
Also heard as
- U
- U95
Related terms
Coverage factor
The multiplier applied to combined standard uncertainty to obtain expanded uncertainty, usually k = 2.
Combined standard uncertainty
The total standard uncertainty of a result, found by combining all input uncertainties through the measurement model.
Coverage interval
The interval around a result that contains the measurand's plausible values with a stated probability.
Coverage probability
The probability that the coverage interval contains the measurand's value, usually stated as 95 percent.
TUR
Test uncertainty ratio, the tolerance being checked divided by the expanded uncertainty of the measurement process.