Direct Measurement of Gas Temperatures by Radiation Thermometry near 4.3 μm

Abstract

An infra‐red thermometer can measure gas temperature if its operating waveband is coincident with an absorption band in the target gas. Of particular interest are thermometers operating at wavelengths on the long‐wavelength edge of the strong 4.3 μm carbon dioxide absorption band. These are used to monitor gas temperatures in industrial boilers and incinerators and have potential for use in a variety of combustion plants. If the gas path is optically thick (i.e. the thermometer does not “see through” to a back wall) and is of uniform temperature, then the thermometer will read the gas temperature directly. The presence of an optically thick condition depends on the absorption strength, path length, gas concentration, temperature and pressure. So‐called band models can be used to analyze the situation. They can estimate the “penetration” of the thermometer into the gas. They can also estimate the thermometer reading if the gas temperature is non‐uniform and/or if the optically thick condition is not well met. This paper develops such a model based on data published by NASA and verified by laboratory measurements in a tube furnace. The model is then extended to allow some estimation of the effect of particulates. Calculations presented are for a particular thermometer spectral response, but data is referenced to allow extension to other CO2 band instruments. The model aims to allow straightforward assessment of the applicability of these instruments in industrial situations.