Degradation and residual life assessment of thermocouples with damaged sheaths in corrosive environments

Abstract Mineral-Insulated Metal-Sheathed thermocouples exhibit extended lifespan and resilience in harsh environments, owing to their protective sheath. However, these thermocouples are prone to failure in a highly acidic medium such as nuclear fuel reprocessing plants. This paper investigates the degradation phenomena and its effect on measurement accuracy when a thermocouple with damage to its protective sheath is immersed in a corrosive medium. The objective is to assess the thermocouple's reliability by micro machining a small hole to simulate sheath damage, facilitating direct contact between the corrosive medium and the sensing materials of a K-type thermocouple. The thermocouples with sheath defects were subjected to aging in different concentrations of acidic medium at a constant temperature, leading to eventual failure. The time-to-failure data of thermocouples at each concentration is analyzed using the Weibull distribution. This study establishes a thermocouple accelerated life-stress relationship using the Inverse Power Law under a ruptured sheath condition. The study also explores the correlation between the residual life and the position of simulated ruptures. ANOVA is employed to test the hypotheses regarding the influence of rupture position on thermocouple performance. It is shown that the location of damage is statistically significant in determining the thermocouple's residual life under damaged sheath condition.