Deviation of reading
The deviation of reading of a dial gauges is usually determined using a dial gauge calibrator. A simple design of a dial gauge calibrator consists of a calibrated micrometer head attached to a fixed vertical column. The dial gauge under test is clamped and held rigidly opposite, and in line with the axis of the micrometer head.
A series of readings is taken at suitable intervals through the range of the gauge. If the gauge has only a limited range with two or three turns of its pointer a tenth of the range is a suitable interval. In the case of gauges with longer scales a few interval in each revolution is tested in order to keep the number of readings within reasonable bounds.
Repeatability of reading
The repeatability of reading a dial gauge could be determined in one of two ways. The dial gauge is firmly clamped in a suitable rigid fixture over a flat steel base and a cylinder is rolled under the contact point a number of times from various directions. The test is repeated at two or three points along the range of the gauge.
Alternatively the contact point is allowed to test on a flat surface below and the plunger is lowered on to the surface both slowly and abruptly. In both cases the largest difference between the readings is noted as the repeatability.
Stickiness or backlash of a dial gauge is revealed by the test for discrimination. The dial gauge is mounted in a rigid fixture with the end if its plunger in contact with the surface of a slightly eccentric precision mandrel mounted between centres . A sensitive indicator is used beforehand to determine the amount of eccentricity in the mandrel.