The ASTM International standard test method C518-21 covers the measurement of steady state thermal transmission through flat slab specimens using a heat flow apparatus. The heat flow meter apparatus is widely used due to its simplicity, speed, and applicability to various test specimens. To ensure accuracy, calibration should be done with materials that have similar thermal properties and thicknesses as the test specimen. ASTM International. Designation C518-21. Standard test method for steady state thermal transmission properties by means of a heat flow apparatus. Number 1, Scope 1.1. This test method covers the measurement of steady state thermal transmission through flat slab specimens using a heat flow apparatus. 1.2. The heat flow meter apparatus is used widely because it is relatively simple in concept, rapid, and applicable to a wide range of test specimens. The precision and bias of the heat flow meter apparatus can be excellent provided calibration is carried out within the range of heat flows expected. This means calibration shall be carried out with similar types of materials of similar thermal conductance and similar thicknesses, mean temperatures, and temperature gradients as expected for the test specimen.