The inserts should be drilled in accordance with the diameter of the test items. In general, the bore diameter, taking the thermal expansion of the test item into account, should be selected to be as small as possible. Air gap between the thermometer and the bore will act as resistances to heat transfer and will significantly affect the heat transfer through heat conduction. As a consequence, the test bore diameter should be a maximum of 0.5mm larger than  the external diameter of the thermometer.