Abstract: Heat transfer between a surface and an environment, such as a steam/air mixture in a pressure vessel, is determined by directing heat transferred to the surface to a location away from the environment by use of a heat pipe. The heat content of the working fluid of the heat pipe is determined by causing a fluid to flow through a jacket encircling the condensation part of the heat pipe and measuring the mass flow of the fluid and the temperature change. The method is particularly useful in determining heat transfer coefficients for use in safety analysis for pressurized water reactor power stations.