Measuring Inside Diameter with Vernier Calipers
It is very important for you to use a more careful touch when you measure an inside diameter with a vernier caliper, however, the measurement principle are the same as the outside diameter. You should carefully move the jaw in a small circle in all directions if you first do the work and a slight misalignment could cause a plus error. Then the movement of the instrument will feel more free again when you are closer to the line of measurement; after that open the adjustment until you pick up the feel again. Finally, you need to move the jaw again and the circle will be smaller at this time, make another adjustment when you approach the line of measurement. Continue this process until the jaws are squarely on the measured and reference points of the part feature. This technique is called centralizing, and the best way to learn it is to practice.