There are situations when it is necessary to measure the diameter of something, such as a pipeline, but it is not possible to measure the length of the circumference (due to the pipe insulation or temperature).
In this case, you can apply the method of calculating the diameter of the chord. For this method only calipers are needed.
If you only have a «caliper» with long jaws or do not have enough of its measuring range, it is possible to apply some kind of «spacer». Ideally, a plane-parallel end gauge is suitable… ;) It must be inserted, as shown in Fig. 2, and when calculating, from the length of the jaws subtract the height of this «spacer». Н = Н1 - Н2
The accuracy of this method depends only on the tool that you will use.