What is the function of a feeler gauge?

The important point in feer gauges ( also called thickness gages by some of the manufacturers ) is the thickness of the blades. Therefore in order to calibrate a feeler gauge you need to measure the thickness of the gauge at various locations along the length of the device. The standard used in the calibration should satisfy a TUR ( Test Uncertainty Ratio ) of at least 4:1 in order to limit the FAR ( False Accept Risk ) to nearly 2%. Such devices do not require periodic calibration. However, they should be standardized before every use by a standard in order to assure reliable measurements.