Overview of Uncertainty Analyses for Gauge Block Calibration

Gauge blocks continue to be essential reference artifacts in manufacturing because of their simple geometry, but also because they can provide very high accuracy for a reasonable cost. The calibration technique of optical interferometry can directly link gauge block central length to the distance light travels in vacuum in 1/299792458 of a second, the definition of the International System of Units (SI) of length, the metre [1]. Gauge blocks calibrated by interferometry are then used in turn as reference standards to calibrate gauge blocks by mechanical comparison techniques, subsequently propagating the definition of the metre along the chain of calibrations, each with a specified measurement uncertainty. Even though these two calibrations involve the same artifact, dominant components in the evaluations of measurement uncertainty for these two techniques are fundamentally different. This is due to the fact that the method of calibration for interferometry involves measuring light waves in air and absolute gauge block temperature, whereas mechanical comparison involves tactile probing and the important temperature measurement is actually the temperature difference between reference standard and client gauge blocks. Detailed measurement uncertainty evaluations in accordance with the ISO Guide to the Expression of Uncertainty in Measurement (GUM) [2] for calibration of gauge blocks by techniques of interferometry and mechanical comparison can be found in the literature (see below). Some of the dominant components of these uncertainty budgets are highlighted for discussion.