Effects of thermocouple electrical insulation on the measurement of surface temperature

• Investigating the impact of thermocouple insulation on temperature measurement.
• Regardless of the insulation, a large diameter thermocouple has the greatest impact.
• Measurement using an 80 μm thermocouple is unaffected by insulation even above 800 °C.
• Insulation has an impact above 250 °C for 200 μm diameter wires.
• There is no specific critical diameter of thermocouple wire(s).

Analytical, numerical and experimental analyses have been performed to investigate the effects of thermocouple wire electrical insulation on the temperature measurement of a reference surface. Two diameters of type K thermocouple, 80 μm and 200 μm, with different exposed wire lengths (0 mm, 5 mm, 10 mm, 15 mm and 20 mm) were used to measure various surface temperatures (4 °C, 8 °C, 15 °C, 25 °C and 35 °C). Measurements were made with the thermocouple in direct contact with the surface, with wires extending vertically and exposed to natural convection. Analytical results of the thermocouple wire with insulation confirm that there is no specific value for the critical radius and the rate of heat flux around the thermocouple wire continuously increases with the wire diameter even when this is larger than the critical radius. Numerical simulation using COMSOL Multiphysics software also confirms that there is negligible thermal effect from the electrical insulation. Moreover, the experimental results agree well with those obtained by both the analytical and numerical methods and further confirm that the diameter of the thermocouple has an impact on the temperature measurement.