Comparative measurements have the advantages of being considerably more accurate than direct measurements. The relative size of two similar gage blocks is known with much greater accuracy than the absolute size of either. The principal reason for this is that in comparative measurement , many systematic errors are eliminated . Systematic errors refer to errors that occur consistently in all measurement regardless of repetition . Examples of comparison measurement instruments include dial indicators , test indicators , and optical comparatrs.
There are three major types of indicators. Type-A indicators have the spindle parallel to the dial face ; Type-B indicators have the spindle at right angles to the dial face;and for Type-C indicators , the measuring contact is a lever. The first two types are referred to as dial indicators and the third type is a test indicator . Indicators are available in four different classes based on the bezel diameter and discrimination . Units may be either U.S. customary or metric.