Calibration · Glossary

What is Test Uncertainty Ratio?

Symbol: TURtolerance ÷ calibration uncertaintyCalibration

Test uncertainty ratio compares the tolerance of the instrument under test with the uncertainty of the calibration; 4:1 is a common minimum.

Test Uncertainty Ratio Explained

A high TUR means the calibration process adds little doubt to the pass or fail decision. Below about 4:1, guard bands or better references are needed.

TAR, the older test accuracy ratio, compares accuracies instead of uncertainties.

Test Uncertainty Ratio Formula

TUR = Tolerance ÷ U₉₅

Tolerance	half width of the allowed error (±)
U₉₅	expanded calibration uncertainty at 95 %

Example: Tolerance ± 0.25 %, calibration uncertainty ± 0.05 %: TUR = 5:1.

Calculators for Test Uncertainty Ratio

CalculatorTest Uncertainty Ratio CalculatorFree online calculator with formula and worked example.CalculatorGuard Band CalculatorFree online calculator with formula and worked example.

Free Downloads

Free downloadMeasurement Uncertainty Budget Excel SheetFree download in Excel and PDF.

Frequently Asked Questions

What is Test Uncertainty Ratio?

Test uncertainty ratio compares the tolerance of the instrument under test with the uncertainty of the calibration; 4:1 is a common minimum.

Where does 4:1 come from?

It is a long standing industry rule, written into ANSI/NCSL Z540.3 for the case without risk calculation.

What if TUR is only 2:1?

Use guard bands to reduce the risk of passing bad instruments, or a better reference.

Learn more on the blog

Calibration traceability: 6 proven links for trusted results

A practical guide on instrumentationblog.in with more detail on test uncertainty ratio and how it is applied on real plants.

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