Proposed standard ASTM WK46204, Test Method for Determination of Thermal Conductivity and Thermal Diffusivity of Solids and Fluids Using the Transient Plane Source or Hot Disc Method, is being developed by Subcommittee E37.05 on Thermophysical Properties. Described test can be designed to match different specimen sizes and allow for measurements to be made in gaseous and vacuum environments at range of temperatures and pressures.