Self-validating thermocouple

Scientists at Britains National Physical Laboratory have developed a self-validating thermocouple technology which incorporates metal-carbon eutectic alloy fixed points. This means that the thermocouple can self-validate, or check that it is measuring the temperature correctly.

Current high-temperature (above 1100°C) thermocouples become less and less reliable as the temperature increases - in extreme cases, at very high temperatures (>1750°C) they can be measuring the temperature incorrectly by as much as tens of degrees.

When these thermocouples are used in precision manufacturing facilities, such as heat-treatment furnaces for high-performance turbine components, even a 5°C error is unacceptable and could lead to substantial waste, because the components produced would not be suitable for use.

In the new thermocouple technology, temperature fixed points comprising metal-carbon eutectic alloys form an integral part of the thermocouple. A graphite crucible, containing the fixed-point material, forms the measuring junction.

The thermocouple wires are attached to either end of the cylindrical graphite crucible, so the graphite crucible provides the electrical connection between the two thermoelements. The high electrical and thermal conductivity of the graphite ensure the good electrical and thermal response of the thermocouple.

The melting and freezing plateaux of the eutectic alloy are observable as a 'hesitation' in the thermocouple output as the temperature of the measuring junction passes through the fixed-point temperature.

The 'hesitation' is the melting temperature of the fixed point, so the temperature is known and the thermocouple is calibrated at this point.