Rhodium platinum thermocouples, temperature range

Calibration at fixed points is a complex process. Standard platinum resistance thermometers and standard platinum-rhodium-platinum thermocouples are calibrated at fixed points for use as primary standards. It is recommended that calibration be done by the NBS or other qualified laboratory. The narrow-band optical pyrometer is another primary standard its range over the fi%ezing point of gold is obtained through extrapolation. Ordinary calibration of temperaturemeasuring instruments is effected by comparison of their readings with those of primary or secondary standards at temperatures other than fixed points. Comparators are used to produce those temperatures. 

Several features of the early model (Fig. 6) have been modified in the present-day, high-temperature version of this calorimeter (Fig. 7) (37). Depending upon the temperature range envisaged, the block is made of refractory steel, alumina, or beryllium oxide and is machined to house the calorimeter itself. The thermoelectric pile (about 50 platinum to platinum-rhodium thermocouples) is affixed in the grooves of an alumina plate (A), which is permanently cemented to two cylindrical tubes of alumina (B). Cylindrical containers of platinum (C) ensure the uniformity of the temperature distribution within the calorimeter cells. 

The built-in thermocouples can vary depending on the temperature range platinum — platinum/rhodium (10% Rh) 25 - 1600 °C nickel — chrom/nickel 25 — 800 °C... 

Temperature measurements. Temperatures in the range of 800 to 1300°F are commonly measured with Chromel-Alumel or platinum-platinum-rhodium thermocouple.s. The accuracy and life of a thermocouple in the temperature range of interest arc functions of the wire size and, in general, the largest possible thermocouple should be used. Either beaded thermocouples or the newer, magnesium oxide-insulated thermocouples may be used. 

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