First of all, check the surface of the plug gauge before use, there should be no rust, scratches, black spots, etc.; The marking of the plug gauge should be correct and clear.
Secondly, the standard conditions of the plug gauge measurement are: the temperature is 20°C, and the force measurement is 0. It is difficult to meet this requirement in actual use. In order to reduce the measurement error, try to use the plug gauge and the equipment to be measured under isothermal conditions. Use as little force as possible. Do not push the plug gauge into the hole or push it sideways.