The primary difference in graduations between an external micrometer and a depth micrometer lies in the direction of the scale and its position relative to the measurement.
Key Differences Explained
While both instruments measure distance based on the same principles of a precisely threaded screw, their application dictates the graduation layout.
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Direction of Reading: On a standard external micrometer, the graduations increase as the thimble is rotated to open the jaws. In contrast, on a depth micrometer, the graduations increase as the rod extends out of the micrometer head. Therefore, the readings effectively run in opposite directions. The depth micrometer reads from the end scale, effectively counting outwards.
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Position of Scale: In an external micrometer, the barrel (or sleeve) with the main scale is fixed, and the thimble rotates around it. For a depth micrometer, the barrel is often part of the base that rests on the surface from which the depth is measured. The thimble moves linearly to extend the measuring rod.
Summary
In essence, a depth micrometer's graduations are designed to measure how far a rod extends outward from a reference surface, while an external micrometer measures the distance between two points. The direction of measurement, and therefore the scale's direction, is reversed.
