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Test uncertainty ratio calculator
Result
TUR 4 : 1
At or above 4:1. Simple acceptance is defensible; state the rule on the certificate.
The method
What the ratio tells you.
How is TUR calculated?
Tolerance span — upper limit minus lower limit — divided by twice the expanded uncertainty at k = 2. For a symmetric tolerance of ±1 % on 100 V, the span is 2 V; with U = 0.25 V the ratio is 2 ÷ 0.5 = 4. Stated the other way, the half-tolerance of 1 V over U of 0.25 V is the same 4.
What happens below 4:1?
A reading just inside the limit has a real probability of a true value outside it. Under simple acceptance (ILAC-G8:09/2019 §4.2.1) that risk is shared with the customer and should be stated. Under a guard band (ILAC-G8:09/2019 §4.2.2) the acceptance limits are pulled in by U, the laboratory carries the risk, and the customer receives a stronger statement — at the cost of instruments that would have passed now failing.
Where does the uncertainty come from?
From the budget: the reference standard's certificate, the resolution of the unit under test, repeatability, drift, environment. The uncertainty budget calculator evaluates it; this page takes the result.
What does the certificate have to say?
ISO/IEC 17025 §7.8.6 requires the decision rule to be documented and stated on the certificate whenever a pass or fail is declared. The ratio is how a laboratory chooses the rule; the rule is what the certificate prints. Guidance is ILAC-G8:09/2019.
A test uncertainty ratio of 4:1 means the measurement is four times better than the tolerance it is checking. Below it, a pass under simple acceptance carries a risk the certificate should state.
About this calculator
- What is a test uncertainty ratio?
- The ratio of the tolerance of the instrument under test to the expanded uncertainty of the measurement used to test it: the tolerance span divided by twice U, which for a symmetric tolerance is the same as the half-tolerance divided by U. A TUR of 4:1 means the measurement is four times better than what it is checking.
- Why does 4:1 matter?
- It is the customary threshold below which simple acceptance carries a real probability of a false accept near the limit. Above 4:1 the uncertainty is small enough relative to the tolerance that a reading inside the limit is very probably a true value inside it. Below it, a laboratory should either guard band or state the shared risk plainly.
- TUR or TAR?
- TAR, the test accuracy ratio, compares the tolerance with the reference standard's stated accuracy alone. TUR compares it with the whole measurement uncertainty, which includes the standard, the resolution of the unit under test, repeatability and the rest. TUR is the honest one and the one ILAC-G8 and ANSI/NCSL Z540.3 mean.
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