ACT has developed a pressure controlled heat pipe (PCHP) calibrator suitable for government calibration institutions. The purpose of the calibration product is to calibrate up to 5 thermometers to a reference thermometer. The PCHP calibrator (shown in Figure 1) features a sodium heat pipe with the ability to operate between 600°C and 1,000°C and maintain a temperature stability of +/-5mK over a 2 hour period (some temperature data are shown in Figure 2). The heat pipe features 6 thermowells located within the vapor space of the heat pipe, maintaining a high degree of temperature uniformity between the thermometers. The transient stability of the heat pipe is maintained by actively controlling the pressure of the heat pipe’s vapor space, ensuring a stable saturation temperature within the heat pipe. The pressure is manipulated by changing the effective volume of the heat pipe, simply by moving a sealed bellows. ACT developed the control schemes, data logging features, and all mechanical features of the turnkey system.