A temperature sensing device for a thin-walled pressure vessel is provided. This device comprises an insert segment which lies inside the pressure vessel in heat exchange relation with the fluids contained by the vessel, an outer segment disposed outside of the pressure vessel, a linking segment normally disposed in a port through the side wall of the pressure vessel for connecting the outer segment with the insert segment, and a temperature sensing device connected to the outer segment. The device also includes an o-ring disposed around the linking portion and forming a seal between the linking portion and the sidewalls of the port.