 Welcome to this module on inside measurement. The inside micrometer is the most direct measuring tool used for inspection. The total length of the inside micrometer is itself the overall length being measured. The inside micrometer is a much more accurate measuring tool for inside measurement than the caliber. The inside micrometer is almost always purchased as a set. The only way to use the inside micrometer over a range of sizes is by using extension rods. Only one micrometer head is needed, along with extension rods, to cover a broad range of measurements. Tubular style extension rods are available in the intermediate to extended length, while solid extension rods are used for ranges of up to approximately 32 inches. The smallest sizes of inside micrometers have a travel range of one quarter inch. The largest sizes of inside micrometers have a range of one inch. The most popular inside micrometer sets have a half inch range. The inside micrometer is very similar to the outside micrometer, except that it has no frame. In some cases a micrometer handle is used to reach into inaccessible places or in small areas where the inspector's fingers would get in the way. When using the inside micrometer it is necessary to rotate the head end of the micrometer in an axially direction as well as up and down, all the while adjusting the head to the proper feel. Rotating the head in this manner assures that the micrometer is accurately centralized in the part. The inside micrometer, in most cases, is removed from the part while the actual reading is being done. For this reason the barrel lock is relatively snug to retain the reading. Reading an inside micrometer is comparable to reading a standard micrometer except that extension rod and collar lengths must be added to the head measurement. The first example shows a 2-3 inch rod. This gives a measurement of 2.350. The second example is the same 2-3 inch rod with a half inch collar added, making the reading 2.850. The last example shows a 3-4 inch rod added, making this measurement 3.350. The inside micrometer must be calibrated before each use and after changing rods. The calibration must be done against a known standard, such as an inspection ring. If an inspection ring is not available, a properly calibrated outside micrometer may be used. You have completed this learning activity inside measurement.