Influence of thermocouple bead shape and position on temperature measurement

The reading of a thermocouple represents the bead temperature. This temperature is determined by the energy balance at the bead, rather than the actual temperature of the fluid. A lot of research has been done to correct the temperature measurement errors, but the research on the bead shape and bead position is limited. In order to understand the influence of bead shape and bead position on the temperature measurement of S-type thermocouple, CFD simulations are carried out for thermocouples with different bead shapes and bead positions. The heat transfer of the beads is then analyzed. The results show that when the total bead surface area increases by 0.65 mm², the temperature changes of type 1 and type 2 are less than 0.34 K, indicating the highest stability of temperature measurement. The bead temperature of type 3 is most affected by the bead size, which decreases by 7.8 K. As for the bead position, when the bead moves to the Pt wire, the bead temperature keeps decreasing. Moving 5 mm will cause the temperature to decrease by 53.53 K. On the contrary, when the bead moves 2.5 mm towards the Pt10%Rh wire, the bead temperature will increase by 3.68 K. However, moving 5 mm will cause the temperature to decrease by 10.53 K. Therefore, it can be concluded that the bead position will significantly affect the temperature measurement results.