Dial caliper calibration
These instructions apply to mechanical as well as digital calipers.
Both the inside jaws and the outside jaws need to be calibrated, as well as the depth rod and the step measurement, if these are used. Calipers should be frequently checked for accuracy. They are more susceptible to damage than other tools.
To check for wear in the jaws, do this: clean them and close them. Then hold them up to the light and if they're worn you'll see light shining through the gaps. You can continue to use the calipers if you measure with the unworn surfaces. For total reliability, however, you'll have to send the calipers for servicing. The surfaces can be ground flat again.
For the outside jaws it's a simple matter of inserting a series of gage blocks between them and recording the caliper readings. They must not deviate by more than one graduation (.001") over the first 4" of range. From 4" to 8" the error may be .0015" (one and one-half graduation). From 8" to 12" the error can be .002". Accuracy may vary among different models and the manufacturer's specs should be consulted for this information. Take readings at 1-inch intervals. Three gage blocks (see below) of 1", 2" and 3" sizes will be all you need.
To calibrate the inside jaws you may use a set of ring gages. Do not rely on very small ring gages because the inside jaws can not accurately measure small inside diameters. If you invest in just one ring, make it the 2" size. You may also set a calibrated .0001" micrometer to 1" (and higher, if possible) and then use the inside jaws to measure this distance. Since the micrometer has a discrimination ten times that of the calipers, you'll get an accurate reading.
Repeatability means the dial hand returns to the same position on different attempts to measure the same gage block. If this fails, then you'll have to have the calipers serviced