Investigation of Temperature Drift with Thermocyclic Oxidation on K-Type Bare Thermocouples

Abstract

The irrevocable variations in the thermoelectric properties arise in normally used bare type-K thermocouples that are subjected to high temperatures. Over an extended period of time, these long-term modifications may manifest themselves as a systematic shift in the thermocouple's characteristics. This paper investigates the temporal instability of thermocouple output at temperatures over 800 °C. This investigation focuses on the change in temperature output that occurs in thermocouples of varying diameters after they have been subjected to high temperatures for more than 365 hours. This data was used to assess the relationship between the amount of temperature drift with different lead diameters and the degradation of the material compositions of thermocouple thermoelements. It is critical to determine the pace at which the thermocouple material degrades over time. This paper concentrates in detail on the drift analysis of thermocouples of different diameters which were tested at high temperatures with their segmental material compositions. Within the scope of this investigation, the elemental analysis of the thermocouple could be carried out using scanning electron microscopy in conjunction with an energy dispersive X-ray (SEM-EDAX) both before and after the thermocouple had degraded. The results show that the thermoelectric response decreases dramatically and the highest drift reaches to about 196.4 µV for (Ø0.51 mm), about 118.6 µV for (Ø1.29 mm) and 79.2 µV for (Ø2.05 mm) thermocouples, respectively at 800 °C after prolonged usage of 365 hours. The highest thermal drift was achieved with [90-10] vol% Ni-Cr / [95-05] vol % Ni-Al, Mn, Si composite thermocouples, which related to the increased amount of the 2-3 oxide layers that were formed in the composite of bare thermoelements.