Thermometers working standard

Transfer standard thermometer Working standard thermometer... 

FIGURE 16.8 PRT design typical used as transfer or working standard thermometer. (Courtesy ofRosemount, Inc)... 

Transfer or working-standard thermocouples (including connecting wires—see Fig. 16.17) are individually calibrated by comparison calibration against a defining standard thermometer (such as an SPRT) or another transfer standard thermometer (usually a thermocouple). 

The defining standard thermometers are thermometers specified for ITS-90. They should be calibrated by NIST at regular time intervals, and are then used to calibrate transfer or working standard thermometers. Transfer standard thermometers serve as intermediate standards to reduce the use and drift of the defining standard thermometers. 

The national laboratories of several countries, including the National Institute of Standards and Technology in the United States, maintain stable secondary thermometers (e.g., platinum resistance thermometers and thermocouples) that have been calibrated according to the ITS-90. These secondary thermometers are used as working standards to calibrate other laboratory and commercial temperature-measuring devices. 

Working-grade thermometers, conventionally called industrial resistance thermometers, are generally smaller than the SPRT element and may be as small as 2.5 mm in diameter and 10 mm in length. These are available in various 0°C resistances, eg, 100, 200, and 500 Q. They are available as unsheathed elements or in a wide variety of sheaths and enclosures, both standard and custom. They are relatively inexpensive. They are usually made to be interchangeable, without relying on iadividual cahbration, within limits of 0.25 K or closer upon special order. A typical tolerance statement for a precision-class industrial resistance thermometer is... 

Whereas it is no longer an iaterpolation standard of the scale, the thermoelectric principle is one of the most common ways to transduce temperature, although it is challenged ia some disciplines by small iadustrial platinum resistance thermometers (PRTs) and thermistors. Thermocouple junctions can be made very small and ia almost infinite variety, and for base metal thermocouples the component materials are very cheap. Properties of various types of working thermocouple are shown in Table 3 additional properties are given in Reference 5. 

The most satisfactory and the most accurate method for the calibration of a thermometer is to compare it directly with short-scale thermometers, each of limited range, which have been standardised at the National Physical Laboratory.f The comparison should be made as far as possible under the same conditions as the thermometer is to be used. For experiments demanding the highest precision (e.gf., in research work), it is a good plan to have two similar sets of short-scale thermometers, one of which has been standardised at the N.P.L. the unstandardised set is t The Bureau of Standards in the U.S.A., etc. 

The standard galvanometer is sufficiently sensitive to work satisfactorily with a full scale range of 10 millivolts which gives a very open scale particularly for base-metal couples, where Ho in. of scale corresponds to 2H°C. This recorder is also used with resistance thermometers in which case the electrical circuit takes the form of a Wheatstone bridge, and the scale can be opened until the entire range corresponds to so little as 2°C. The zero of the scale can be adjusted to correspond to any fixed electromotive force so that the scale may be put within any range of temperature desired. The great adaptability of the instrument is readily apparent. 

The mercury thermometer is sufficiently accurate for practical work in the determination of temperature changes in the canning of food materials if it is properly calibrated and standardized. 

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