Instrumentation & Automation
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Dmitry Bebyakin, the ILIM metrologist

Pipe diameter by the chord

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.

  1. We apply it, as shown in Fig. 1;
  2. Measure the length L;
  3. Measure the height of the caliper jaws Н;
  4. Calculate the diameter by the formula D = (L2 ⁄ 4H) + H or
  5. Calculate the radius by the formula r = (L2 + 4H2) ⁄ 8H

Calculating the diameter of a pipe along a chord using a caliper
Pic. 1

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

Calculating the diameter of a pipe along a chord using a caliper and spacer
Pic. 2

The accuracy of this method depends only on the tool that you will use.