About calibration calculations
Calibration is comparison: known inputs applied, measured outputs recorded, results judged against tolerance. Adjustment is a separate act that follows only when the comparison fails. Keeping those two ideas distinct is what makes calibration data useful, because the most valuable record in the whole exercise is the as-found reading — how wrong the instrument was before anyone touched it.
As-found history is what reveals drift rate, and drift rate is what calibration intervals should actually be based on. An instrument adjusted without recording its as-found state has surrendered that information permanently.
The tools here handle the arithmetic that surrounds the procedure: translating accuracy specifications between percent of span, percent of reading and engineering units; confirming the reference standard is good enough through the test uncertainty ratio; generating test point tables with acceptance limits; and evaluating hysteresis and repeatability from up and down readings.
How these tools fit together
- Interpret the tolerance. Translate the accuracy specification into the units you will actually measure in, at the operating point that matters.
- Verify the standard. Confirm the reference is meaningfully better than the tolerance being checked — conventionally a four to one ratio.
- Generate the test table. Produce test points with ideal outputs and acceptance limits before going to the field, with columns for as-found and as-left readings.
- Evaluate the results. Assess error at each point, plus hysteresis between up and down runs and repeatability between repeated runs.
Frequently asked questions
What is a test uncertainty ratio?
The ratio between the tolerance being verified and the uncertainty of the standard used to verify it. The traditional criterion is four to one; below that, acceptance limits should be guard-banded or the increased false-accept risk consciously accepted.
What is the difference between hysteresis and repeatability?
Hysteresis is systematic and direction-dependent: the difference between readings taken approaching a point from below versus from above. Repeatability is random scatter between repeated readings taken the same way in the same direction.
All calculators are provided for reference and education. Verify independently before use in safety-critical work — see our disclaimer.