Inside calipers are used to measure inside dimensions such as the inside diameter of a tube, while outside calipers are used for measuring outside dimensions such as the outside diameter of a tube or a piece of bar stock. Because the arms on calipers are made of fairly light metal, they may be sprung out of shape quite easily. They require a light touch. The accuracy achieved in making measurements with simple calipers is very much dependent on the machinist.
When taking measurements with simple calipers, adjust the jaws of the caliper to just slip over, or between, the surfaces being measured. You should feel just a slight amount of drag. Always recheck your measurement at least once to make sure the caliper setting hasn’t slipped during the measurement process. Continuted practice is required to develop the proper touch and get consistent measurements.