First, check the plug gauge surface before use, and there should be no rust. scratches, dark spots, etc.; signs of plug regulations should be correct and clear.
Second, the standard conditions for plug gauge measurement are: temperature is 20°C, and force measurement is 0. It is difficult to meet this requirement in practical use. In order to reduce the measurement error, try to use the plug gauge and the device under test to measure under isothermal conditions. Use a force that is as small as possible. Do not push the plug gauge into the hole or push it sideways.
Third, when measuring, the plug gauge should be inserted or pulled out along the axis of the hole, and should not be tilted; plug gauges should be inserted into the hole, and it is not allowed to turn or shake the plug gauge.