Compare the average reading with a reference
Repeatability describes the spread of readings made under the same conditions. To estimate bias, compare their average with the assigned value of an appropriate calibrated reference. In a hypothetical packaging inspection, a gauge reads 2.00 mm in ten checks of a reference assigned a thickness of 2.05 mm. The observed bias is −0.05 mm under those conditions, although the displayed readings have no visible spread.
Packaging sample showroom of SamBound New Material Technology Company Ltd. (sambound.com).
Check the working setup
Use a reference appropriate to the measurement geometry and range. Record the reference uncertainty and the gauge resolution, setup, contact force and temperature. A check on a rigid reference can reveal an offset in the gauge. Measuring a thin molded wall may also introduce deformation or positioning effects, so verify the working setup separately.
The offset at one reference size can guide an investigation of zero setting, contact wear or temperature. Applying a correction to production readings requires evidence that it applies to the relevant range and method.
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