Influence of protection tube on thermocouple effect length

Abstract

Thermocouples are often equipped with ceramic protection tubes. The existence of the tube changes the heat transfer of thermocouple wires. It will change the bead temperature, i.e., the measurement temperature. The one-dimensional axial heat transfer program for the thermocouple with the protection tube is developed. The accuracy of the program is validated by three-dimensional CFD simulations. The influence of the protection tube on the bead temperature and the effect length is studied with the one-dimensional program. The existence of the tube lowers the bead temperature. The smaller the wire protruding length from the tube, the larger the bead temperature drop. The effect of the tube thermal properties such as the thermal conductivity and emissivity on the thermocouple temperature is studied. The properties have strong effect on the tube temperature profile but their effect on the bead temperature is slight. An analytical method is developed to calculate the effect length of the thermocouple with the protection tube. The error of the method is less than 12% compared with the numerical result for the tested cases. The tube existence increases the effect length. Increasing the wire protruding length from the tube, the tube influence on the effect length decreases.

Keywords

• Thermocouple
• Gas temperature measurement
• Bead temperature
• Protection tube
• Effect length