An Improved Radiation Pyrometer

Abstract

In designing a new radiation pyrometer for industrial use above the lower limit of visible radiation, consideration of ambient temperature effects, which is of much importance in modern industrial practice, was carried out by aid of mathematical analysis. In one hypothetical case, with a very sensitive type of thermopile losing heat from its hot junction by radiation alone, it is shown that an increase in operating ambient temperature of 180°F (from 80°F to 260°F) results in a drop in output voltage of 38 percent for a constant furnace temperature of 1300°F. The corresponding error in reading is 315°F. At a furnace temperature of 3000°F, the decrease in voltage is 17 percent and the error in reading is 387°F. Under similar conditions, in another hypothetical case of a less sensitive thermopile constructed to lose much of its heat by thermal conduction, this 180°F increase in operating ambient temperature would result in a decrease in output voltage of from 12.2 percent to 12.5 percent for any furnace temperature between the two values mentioned. The error in reading would range from 58°F to 162°F over the stated span of furnace temperatures. The constancy of ratio of e.m.f.'s shown for the higher conduction factor makes it possible to provide ambient temperature compensation over a wide range of furnace temperatures with a nickel shunt compensator. The pyrometer here described, designed along these lines, shows ambient temperature errors of less than three degrees F in the furnace temperature being measured for ambient temperature variations from 50° to 200°F; transient errors would usually be one degree or less for most cases; the response is complete within 3.5°F in four seconds and within half a degree or less in six seconds. The calibration, with varying ratios of distance-to-target versus target diameter up to over 20 to 1, is unusually constant. A thermopile of ten units insures ample e.m.f. All of the thermopile junctions are spot-welded and the construction is arranged throughout to withstand high ambient temperatures.