A Simplified Method to Make Temperature Measurements of a Metal Surface using the Surface as One Component of Thermocouple 2008-01-0918
Instrumentation of an exhaust system to measure surface temperature at multiple locations usually involves welding independent thermocouples to the surface of the system. This report describes a new type of thermocouple fabricated to measure temperature at a point or temperature difference between points on a metallic object utilizing the metal as one component of the new thermocouple. AISI 316 stainless steel is used in the current study to represent automotive exhaust pipe. The other component of the thermocouple is Nickel-Chromium (Chromel, Chromega), one of the two metals used in type K thermocouples, which are generally used for exhaust temperature measurements during emission tests. Use of the new thermocouple is contingent upon an accurate calibration of its response to changes in temperature. A variety of temperature indicators (such as Model HH22 by Omega) are readily available to convert the signal from common types of thermocouples to an actual temperature, however, no such device exists for the new thermocouple developed in this study. It is also desirable to avoid the reliable, but cumbersome traditional procedure employing an ice bath to establish a known reference junction temperature. Consequently, a method is developed and introduced to convert a temperature reading using a type K temperature indicator to the actual temperature. Results show that this less complex system can reproduce the surface measurement capability of a conventionally instrumented exhaust system. Furthermore, by attaching only two Chromel wires fore and aft a catalyst system, there is a potential use for OBD of catalysts.