Development of Fixed-point Cells for Self-calibrating Thermocouples for Hot Steam Temperatures about 700 °C

Project Number 286

During the last years, in the course of various VGB research projects self-calibrating thermocouples have been developed based upon miniature fixed-point cells for more exact determination of hot steam temperatures. In consequence, the 5 K safety interval to design temperature required for conventional temperature measurement could be reduced to 1 K.

In the meantime, industrial measuring equipment based upon this technique is under permanent utilisation in several power stations of VGB member companies. Devices of this kind are also being used at three measuring points in the temperature range 535 to 542 °C in the COMTES700-test plant in the Scholven Power Plant.

Two more measuring points in the 700 °C temperature range have been prepared for application of thermocouples. However, no suitable material combination from fixed-point substance and container material could be found to date.

The research project aims at development of new miniature fixed-point cells for hot steam temperatures around 700 °C. The test results could serve as comparative basis for all other temperatures measured in parallel.

Since the fixed-point cells are exposed to increased mechanical stress, new fixed-point materials adjusted to the increased temperatures and fitting crucible materials must be investigated. In particular, the construction type to be found must accommodate the varying expansion characteristics of fixed-point material and crucible ceramic material. In order to enable their assessment during the field test, conditions must be created to be able to calibrate them with a measuring tolerance of < 0.3 K.

The resulting measuring options can withstand considerably higher thermal-mechanical stress. At a measuring tolerance of 1 K, considerable savings in fuel- and service life consumption can be expected in the next-generation power plants.