Typically there are two types of pressure testing for thermowells. By internal and external pressure. There is a clear distinction when the thermowell is internally and externally pressure tested. All flanged thermowells are subjected to an external pressure test 1.5x the flange rating. With FSO (van Stone), Threaded, Weld-in, and socket weld thermowells the pressure is applied internal. The maximum test pressure is limited by the type of the thermowell.