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.

热电偶通常配有陶瓷保护管。管子的存在改变了热电偶导线的传热。它将改变珠温，即测量温度。开发了带保护管热电偶的一维轴向传热程序。该程序的准确性通过三维 CFD 模拟得到验证。用一维程序研究了保护管对焊珠温度和效应长度的影响。管的存在降低了珠温。金属丝从管子伸出的长度越小，珠温降越大。研究了管子的热性能，如热导率和发射率对热电偶温度的影响。这些特性对管温度分布有很大影响，但对焊珠温度的影响很小。开发了一种分析方法来计算带有保护管的热电偶的效应长度。与测试案例的数值结果相比，该方法的误差小于12%。管的存在增加了效果长度。增加导线从管子伸出的长度，管子对效应长度的影响减小。