An Accurate Surface Temperature Measuring System

A system is described that provides accurate contact surface-temperature measurement that is free from conduction error and heat perturbation of the measured surface. The principle is one of null heat flow provided by an electronically servoed backup heater. The probe consists of three basic parts: a measuring contact thermocouple, a differential thermocouple, and a heater, mounted in tandem. The measuring junction and one junction of the differential couple contact the surface. The second differential junction is located a short distance behind the first junction so as to have a thermal path between the junctions. The backup heater is located behind the second junction. When the probe contacts a surface that is above probe temperature, the first differential junction rises in temperature above the second junction, producing an EMF which is fed into the electronic servo. This signal is amplified, and quickly energizes the electric backup heater by the amount required to reduce and maintain the differential signal at zero. Under this condition, there is no heat exchange between the probe and the surface; the measuring junction is at the surface temperature. Present temperature capability extends from ambient to approximately 2250°F. Future designs are expected to extend the upper limit to about 3000°F.