A Forced Air Convection Method for Non-contact Temperature Measurement of Surfaces

Abstract

Methods of non-contact temperature measurement have been examined; emphasis being placed on radiation pyrometry. Such methods are vulnerable to emissivity errors unless blackbody emitters are involved. There are however several ways of minimizing these errors and the underlying theoretical principles of the major ones have been discussed. The author has developed a non-contact temperature measuring technique employing forced air convection to avoid emissivity errors. The technique exploits the phenomenon of heat transfer between a surface and air flowing over it when a temperature differential exists between the two. Two modes of temperature measurement have been examined, one involving initially unheated air and the other pre-heated air. In the former case the surface temperature is predicted from the temperature increase of the air passing over it. The pre-heated air mode, however, involves identifying a state of zero nett heat transfer between the surface and the air when the latter temperature is adjusted until it equals that of the former. Under these conditions the surface temperature may be inferred directly from a measurement of the air temperature. This latter mode has been applied successfully to temperature measurement of surfaces of cylindrical geometry. Since a requirement for temperature measurement of plane surfaces is widespread, most of the thesis has been devoted to applying the forced air convection principle (using both pre-heated and initially unheated air modes) to plane surfaces. It has involved a detailed theoretical and experimental investigation of the flow and temperature characteristics necessary for a successful implementation of the principle. On the basis of these investigations recommendations have been made for the design of instruments to measure the temperature of plane surfaces. One such instrument employing the initially unheated air mode has been constructed and installed on a strip annealing line for measuring the temperature(up to 550°C) of moving aluminium strip.