Test Uncertainty Ratio (TUR) Calculator
Check that your calibrator is good enough for the instrument under test. The test uncertainty ratio compares the tolerance of the device with the expanded uncertainty of the calibration; 4:1 or better is the usual target.
- TUR = tolerance ÷ uncertainty = 0.5 ÷ 0.1 = 5 : 1
- TAR = tolerance ÷ calibrator accuracy = 0.5 ÷ 0.08 = 6.25 : 1
TUR and TAR Formulas
TAR = Device tolerance ÷ Calibrator accuracy specification
TUR uses the full uncertainty of the calibration process (reference standard, method, environment and resolution), which is why ANSI/NCSL Z540.3 prefers it over the older TAR.
Worked example
A gauge with a tolerance of ±0.5 bar is calibrated with an expanded uncertainty of ±0.1 bar. TUR = 0.5 ÷ 0.1 = 5 : 1, which meets the 4 : 1 guideline.
Frequently Asked Questions
What is a good test uncertainty ratio?
4 : 1 or higher is the widely used guideline. Some industries accept 3 : 1, and high accuracy laboratories may aim for 10 : 1.
What is the difference between TUR and TAR?
TAR compares tolerance with the calibrator specification only. TUR compares it with the total uncertainty of the calibration, so it is the more realistic measure.
What if my TUR is below 4 : 1?
Use a more accurate standard, reduce other uncertainty sources, or apply guard banding so that readings near the tolerance limit are treated as failures.
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