A computerized model of vacuum thermocouple performance

Heat-flow equations for a simplified vacuum thermocouple (VTC) were computerized using a Crank-Nicholson two-step finite-difference scheme. Comparisons were made between the computer outputs and measured values for both 500-mA and 5-mA VTCs. Curve-shape studies, radiation losses, conduction losses through all imperfect vacuum, and thermal conductivity considerations are discussed. The steady-state case demonstrated strong dependence of thermal conductivity on temperature. The output performance characteristics of the 500-mA VTC are controlled principally by the thermal conductivity of the heater, whereas those of the 5-mA units are dominated by thermal conductivity of the thermocouple wire. Calculations show that, even for very low emissivity coefficients, radiation losses are substantial in the 5-mA type, which are commonly used for AC-DC transfer standards.<>