 Welcome to this module on depth measurement. The depth micrometer is an accurate and reliable tool to use for depth measurement. The depths of holes, slots, shoulders, and projections can be measured accurately to within one thousandth of an inch. When using a depth micrometer, two points must be kept in mind. Depth micrometers measure from a reference plane to a point. The large base of the depth micrometer makes up the reference plane. The very small area of the measuring rod makes up the point of contact. It is important that the area in which the reference base of the depth micrometer makes contact with the workpiece is clean and free of dirt or burrs. The other aspect of the depth micrometer, which must always be kept in consideration, is that it reads in reverse from other micrometers. Depth micrometers can be purchased with a number of different length rods, allowing the measuring tool to be used over a broader range of depths. The head movement of the depth micrometer is one inch. Whenever you use the micrometer, with the existing rod or two different length rods, check the accuracy of the depth micrometer against a known standard. When using the depth micrometer, there is no need to move the micrometer around to attain the proper feel. In fact, sliding the micrometer should be avoided. The small rods will wear very quickly and the accuracy of the micrometer may be lost. For greater accuracy, take several readings at slightly different positions. When measuring the depth of a drilled hole, it is important to measure at the outside wall of the hole to obtain the depth of the full diameter portion of the hole. Hold the measuring rod next to the wall of the drilled hole to assure an accurate full diameter depth measurement. Depth micrometers come in a variety of styles. Each style is designed for a certain set of circumstances. The most common depth micrometer has a two and a half inch long base, with a one-eighth inch or five-thirty seconds inch diameter rotating rod. Some depth micrometers come with non-rotating rods. The non-rotating rod is ideal for measuring a narrow shoulder without the rod rolling off. Depth micrometers can also be purchased with very small rods. Rods as small as 0.045 inches inch in diameter are available for measuring very narrow slots, recesses, or the depths of small holes. The half-base depth micrometer will allow you to measure depths of holes and slots close to shoulders or between obstructions. You have completed this learning activity, depth measurement.