Thermoprobe: An Instrument for Determining the Temperature of Opaque, Translucent, and Transparent Surfaces in the Incandescent Temperature Range

Abstract

An instrument has been developed that is a temperature counterpart of an electrical potentiometer. The apparatus consists of a tantalum vacuum blackbody furnace or an electrically heated tantalum ribbon of accurately known spectral emissivity, as the standard temperature surface, an ``unknown'' surface, the surface temperature of which is to be determined, and a balancing probe with which the surface temperatures of standard and unknown may be compared. The probe, in essence, consists of either a needle or a U-shaped body that can be electrically heated to the desired temperature. The probe carries a thermocouple connected to a null detector near its tip. The thermoprobe is balanced by repeatedly contacting the unknown and by varying its heating current until the null detector reading is unchanged upon contacting the surface. The probe is then switched to the standard surface and the procedure is repeated varying the temperature of the standard this time. Null balance now indicates that standard and unknown have the same surface temperature. The temperature of the standard is determined by using an optical pyrometer. This method of temperature measurement is useful when determining temperatures of transparent and translucent surfaces which cannot be pyrometrically measured, and also in cases where emissivities tend to vary rapidly. The probe was tested on a metallic (Mo, Ta), a sintered oxide (ThO2), and translucent single crystal (NiO) surface. The sensitivity of the probe approached that of a high quality optical pyrometer and it was ±1, ±2, ±1.5 K for the three surfaces in the temperature range between 1100 and 1600 K, respectively.