Temperature measuring devices with a thermocouple, called electric thermometers, are needed where local temperature measurements and instrument readings with the aid of expansion type thermometers are not possible and the measured variable must be transmitted electrically to a measuring and control station. If temperature measuring devices are intended to be installed into closed systems, for example piping and vessels, these devices must safeguard absolute tightness and yet exhibit high temperature sensitivity, that means extremely short response times.
For satisfying these requirements, it is known to use so-called sheathed thermocouples. These industrial thermocouples consist of two spaced conductors of dissimilar metals, a metal sheath surrounding the pair of conductors and closed at one end and containing a filler material which insulates the two metal conductors from each other. The two ends at one side of the metal conductors are joined by soldering to form the measuring junction. The measuring junction is located at the closed end of the metal sheath. It may contact the metal sheath or may be insulated from the sheath. In order to achieve a very short response time, i.e. a high temperature sensitivity, the metal sheath must be very thin, i.e. in the order of 0.03 to 0.3 mm. The two free ends of the metal conductors are carried through an insulating closure at the open end of the sheath and fixed in a coupling terminal for the connection of extension wires.
Where such thermocouples are intended for use on pressurized equipment, they will be provided with a screwed pressure joint. This pressure joint is of the multipiece type and comprises a ground finished high carbon steel ring that surrounds the sheath. The leakproof seat of the thermocouple is achieved through a slight deformation of the sheath.
Temperature measuring devices with screwed pressure joints as described above are suitable at most for use at pressures up to approximately 500 bar. At more elevated pressure levels, the screwed pressure joint is no longer leakproof and the sheath of the thermocouple is subject to deformation which makes the insulation ineffective. Trials have, therefore, been made by surrounding the sheath with a protective tube which is also closed at one end. This protective tube is welded to the screwed pressure joint. However, this design has proved to be too sluggish for a number of complex control procedures, i.e. the response time is too long. Measuring junction and process fluid are separated by the wall of the closed end protective tube.