The Calibration of Copper-Constantan Thermoelements

Abstract

HE practical usefulness of a thermoelement will often depend •*• on whether its readings can readily be interpreted in terms of the standard scale of temperature, that is, on the ease and accuracy with which it can be calibrated. The present work was undertaken for two reasons, first, to obtain a calibration curve for a particular pair of elements which had been in use at the Geophysical Laboratory, and second, to obtain data relating to the calibration of copper-constantan thermoelements in general. The two elements in question were compared with two platinum resistance thermometers, which had been very carefully calibrated in terms of the international hydrogen scale. 1 The comparison was made at the Bureau of Standards, the thermoelements and accessory apparatus being brought over from the neighboring Geophysical Laboratory. The thermoelements were the A and B briefly described in the preceding article. They were read according to the method already published, 2 using two leakage shields and the simple potentiometer there described. The resistance thermometers were used substantially according to the method followed in the original calibration. A water-bath, which is ordinarily used for the testing of clinical thermometers, 3 was utilized for the comparison. During the progress of the work the bridge, the potentiometer and the standard cell were calibrated. The data for the resistance thermometer were most conveniently worked up in sets of four. The corresponding thermoelement readings were simply averaged. Each observation here given therefore represents the mean of four readings. The resistance thermometers were read to o°.oooi and the thermoelements to o°.ooo3, the P-337-No. 2.] CALIBRATION OF THERMOELEMENTS.

Calorimetric measurements of heats of hydration of starches

“…A 50-j unction copper-constantan thermocouple was constructed and calibrated following all the precautions suggested by White (16)(17)(18)(19). The calibration of the thermocouple gave the following equation relating temperature to potential: = 0.5149S -2.501 X 10-4E2 G, water level; if, ice and water mixture; 7, electrical leads to heater coil where E is expressed in millivolts, and in 0 C. The reference point was the ice point.…”

Reliability of Common Types of Thermocouples

“…Whether or not a thermocouple will give the same e. m. f. reading for a given temperature after having been heated to some higher temperature level is important if accurate results are desired. White (9,11) showed that the vital part of a thermocouple is that along which there is a temperature gradient, since it is that part which mainly determines the calibration and constancy. If the thermocouple is always immersed to the same extent in the hot medium, and if the temperature distribution in the temperature gradient from the hot medium to the cooler medium is always the same, there is no change in the electromotive force-temperature relationship with heating and use even if the wires are not homogeneous over this section.…”

“…In work of this nature the question naturally arises as to whether or not it is necessary to calibrate each individual thermocouple when the wires come from the same spools, how much deviation there is in the same type of thermocouple made of wires from different lots, and what effect heating the thermocouples to different temperature levels has on their electromotive force-temperature relationship. Although various investigators mention factors which tend to affect the electromotive force-temperature relationship of different types of thermocouples (1,3,(5)(6)(7)(9)(10)(11)(12), actual experimental data are very limited. Roeser and co-workers state that the guarantee limits on chromel-alumel thermocouples have been fixed by the manufacturers at ±2.8°C.…”