Self-Calibrating Thermocouples with Miniature Fixed-Point Cells for Use in Power Plants

Project Number 247

Using integrated miniature fixed-point cells, a measuring uncertainty of < 1 K can be reached under operating conditions in the superheated steam range of power plants by a periodic recalibration of the thermocouples, with operating times of > 20,000 h. The fixed-point materials used for a temperature range from 500 °C to 650 °C are technically pure metals and binary alloys.

The aim is to reduce the necessary safe distance between the controlled steam temperature and the maximum permissible temperatures for pipelines and plumbing controls by a considerably lower uncertainty, thus permitting a higher efficiency ratio and lower amounts of emissions to be reached.

The basic principle of self-calibrating thermocouples consists in integrating a suitable fixed-point material encapsulated in a miniature fixed-point cell into a thermocouple. When the temperature around the melting or also freezing temperature TFP of the fixed-point material changes, the thermoelectric voltage undergoes characteristic temporal variations due to phase transformations. From these characteristic temporal changes, a calibration value U (TFP) for the thermocouple or also for the entire measuring sequence can be obtained at the fixed-point temperature.

To make this principle usable on an industrial scale for the automatic in-situ recalibration of thermocouples under operating conditions such as in power plants, the following preconditions had to be fulfilled: long-term stability of the miniature fixed-point cells, availability of small fixed-point cells and fixed-point materials with well reproducible phase transformation temperatures at a small distance to the normal operating temperature of the thermocouple to guarantee an as low as possible measuring uncertainty after a single-point calibration.

In co-operation with several large power plants, test probes which fulfilled these requirements were developed and tested under practical application conditions. At present, this novel measuring system is being introduced in industrial practice.

The project was realised from June 2003 to July 2005 by the Institute for Process Measuring and Sensor Technology at the Technical University of Ilmenau by Dr. Frank Bernhard. The budget is sponsored by the Arbeitsgemeinschaft industrieller Forschungsvereinigungen (AiF). A working group of the Working Panel "Control and Instrumentation and IT"  technically supported the investigations.