Coaxial Thermocouples

  View of fast coaxial thermocouples Copyright: © HGD View of fast coaxial thermocouples

Coaxial Thermocouples for Fast Heat Transfer Measurement

Coaxial thermocouples are used as fast heat transfer gauges under high thermal and mechanical loading. These probes have been developed at the Shock Wave Laboratory and are designed for the determination of high heat fluxes under harsh conditions. For lower heat fluxes the more sensitive thin film heat transfer gauges are recommended which are also manufactured at the Shock Wave Laboratory.

  Thermocouple mounted in a model surface Copyright: © HGD Thermocouple mounted in a model surface

Typical design of the fast coaxial thermocouples

The fast coaxial thermocouples consist of a small tube and a pin which are separated by a 10 μm thick electrical isolation. Tube and pin represent the two active legs of the thermocouple. By grinding the head of the probe, a junction between the two metals is achieved. The thin grinding burr represents the thermally active part which, due to its small mass, yields a fast response time of the probe in the order of a few microseconds.

  Response time of a fast coaxial thermocouple by dipping it into hot water Copyright: © HGD Response time of a fast coaxial thermocouple by dipping it into hot water

Fast heat flux measurement

A temperature change at the thermocouple junction leads to a change of the thermo-electric voltage which can be measured directly. Moreover, by the grinding process the thermocouples can be adjusted to any surface geometry, i.e. there are no steps or gaps due to the installation of the probe.

The temperature coefficient and the thermal properties of the thermocouple are determined by static and dynamic calibration. Considering short measuring times, the classic equations of the heat transfer into a semi-infinite slab can be used to derive the heat flux.

 

Dimensions

Diameter: 0.47 mm; 0.92 mm; 1.47 mm; 1.9 mm; 3.6 mm; 4.8 mm (optional)
Length: 10 - 80 mm

Material

Type E: chromel, constantan
Type K: chromel, alumel