Abstract: An adapter (1) is proposed for measuring a physical state variable of a medium in a system (15) by means of a sensor. The adapter (1) has a descender tube closed towards the medium (2) which can be inserted into the medium through an opening (5) and contains the sensor and a flange plate (3) having a central recess and that can be sealingly connected with a connecting flange (5A). The descender tube (2) is sealingly fitted in the flange plate (3). The sealing fitting of the descender tube (2) in the flange plate (3) results from a temperature difference being established between the descender tube (2) and the flange plate (3), wherein the descender tube (2) is inserted into the flange plate (3), and then temperature equilibrium established between the descender tube (2) and the flange plate (3) in order to obtain a sealing press-fit connection between the descender tube (2) and the flange plate (3). The production of such an adapter is less complex and faster than with welding.