In steel processing, the applications in which ratio thermometers perform as well as, if not better than, single waveband thermometers occur when near gray body conditions prevail, such as found in the Hot Rolling process. One exception is zinc coated steel transforming from one emissivity condition to another, such as the Galvanneal process. A performance parameter for ratio devices, the "equivalent" wavelength approximation, is used to estimate errors with different waveband pairs for the two processes. Estimates of the expected temperature variations due to emissivity ratio uncertainty are compared with plant results from hot rolling of oxidized, low-carbon steel strip. The required insensitivity to emissivity ratio variations in galvannealing guided the selection of wavelength pairs, hence we changed a standard instrument design to obtain satisfactory performance. The results were subsequently validated in on-line process measurements and used for many years in production.
