Metrology: Measurement Science and Calibration
The vocabulary that gets tested
Accuracy is closeness to the true value (bias). Precision is the repeatability of readings (spread). Resolution or discrimination is the smallest increment the device can display. Uncertainty is a quantified range around a result. A gauge can be precise and inaccurate at the same time, and a 0.0001 in. display does not make a micrometer accurate to 0.0001 in. - resolution is not accuracy.
Traceability and calibration
Traceability is an unbroken chain of comparisons back to a national standard (NIST in the US), with each link carrying a stated uncertainty. Calibration establishes the relationship between indication and true value; it does not automatically adjust anything. Verification confirms the instrument still meets specification; adjustment changes it. Records should show both as-found and as-left conditions, and calibration intervals are set by history, criticality, and drift.
Environment and technique
Standard reference temperature is 20 °C (68 °F). Steel expands roughly 6.5 µin./in. per °F, so a part measured hot reads large. Watch for Abbe error (measurement axis offset from the scale axis - calipers are vulnerable, micrometers far less), parallax, cosine error, and gauge-force deflection. Apply the 10:1 rule: gauge resolution should be about ten times finer than the tolerance being verified.
Worked example: GR&R rule of thumb
Express the study as a percentage of tolerance: %GR&R = 100 × GRR / tolerance. Below 10% the gauge is acceptable; 10–30% is marginal and depends on part criticality and cost; above 30% the gauge or method must be improved.