Measuring Thermoelectric Inhomogeneity in Thermocouples

摘要

ABSTRACTManufacturers of thermocouples and calibration laboratories do not perform routine and authentictesting of the thermoelectric sensitivity in thermocouples. This testing is required because of the largenumber of physical contributors that can adversely affect thermocouple homogeneity leading tosignificant deviations in electromotive force (EMF). A detailed description of a practical testingapparatus demonstrates how thermoelectric inhomogeneity can be detected. Typical liquid bath anddry well calibrations of thermocouples provide unreliable and potentially misleading results withoutthe benefit of inhomogeneity testing. Additionally, thermoelectric inhomogeneity, the spatial nonuniformityof thermoelectric sensitivity, is present in all thermocouples, including elementalthermocouples such as Gold versus Platinum, and the error due to this condition cannot be corrected bycalibration. Data are presented for sensors intentionally constructed with regions of inhomogeneity andfor sensors used in specific applications such as gas burn off stacks. As background information thecommon causes and potential effects of thermoelectric sensitivity are reviewed.