Research in high-pressure devices, such as the diamond anvil cell and the large volume press, requires knowledge of the pressure and temperature in the sample. Here, a large volume press and an internal resistive heater were used to generate high load and heat to various combinations of intimately mixed powders of materials. X-ray diffraction and custom software were used to jointly estimate the pressures and temperatures in the samples and establish calibrants forin situexperiments at extreme conditions.