Abstract
A substrate surface thermocouple was developed for thermal spraying. The substrate used for the study is a porous 430 stainless steel disk, though the thermocouple concept can be applied with other materials. Type N thermocouple wires are cemented in holes through the substrate, and then a copper coating is deposited across the surface to electrically connect the wire tips to complete the thermocouple circuit. The copper also promotes temperature equalization between the wire tips and the surrounding substrate surface to increase accuracy. Using finite element analysis (FEA), it was determined that the optimum thickness of the copper layer is 38 µm. With this thickness, the thermocouple should be able to measure peak-to-peak surface temperature swings due to a passing plasma jet within +/-3% when the copper thickness is uniform and all physical properties of the coating and substrate system are well-known. However, a number of assumptions were used for the FEA, so a detailed uncertainty analysis was performed. This analysis found that the expected accuracy window of the thermocouple is +19%/-10% as implemented for measuring surface temperature swings. For measuring average temperatures, the thermocouple is very accurate, because large heat fluxes into the substrate occur only when the plasma torch is directly in front of the substrate. Experimental measurements of surface temperatures with the optimized thermocouple are presented.
