Thermocouples working standard

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 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. 

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. 

Type T (copper-constant) thermocouples are most applicable in steam sterilizer validation work. Their working temperature range is wide and they are resistant to corrosion in moist environments. A high grade of thermocouple wire should be chosen. Premium grades of wire accurate to as close as 0.1°C at 121°C are recommended. These must then be calibrated against a temperature standard traceable to the National Bureau of Standards (NBS). 

Work is in progress on modifying the existing superconductive thermometric fixed-point device to extend the range to both lower and higher temperatures. Another thermocouple wire (SRM 1967, platinum) will soon be available. This platinum wire, referred to in the thermometry literature as Pt-67, will take the place of Pt-27, which was the standard referred to until 1973. 

Ceramics differ from some other materials (viz. metals, plastics, wood products, textiles) in a number of individual properties, but perhaps the most distinctive difference to a designer or potential user of eeramic ware is the particularity of the individual ceramic piece. Actually ceramics are not readily shaped or worked after firing, except for some simple shapes of limited sizes. Many ceramies are manufactured as standard items refractoiy bricks and shapes, crucibles, furnace tubes, insulators, thermocouple protection tube, fibre tubes etc. 

The most serious problem in all ebulliometry is the superheating of the solution before it starts boiling. This is avoided in the setup of Fig. 3.13 by the use of the Cottrell pump. This pump works on the same principle as a standard coffee percolator. The vapor developed at the bottom of the flask, at the position of the heater, carries an interrupted stream of liquid up onto the thermocouples that measure the solution temperature. On its way up, the vapor can reequilibrate constantly with the solution. The equipment shown schematically in Fig. 3.13 is capable of a precision of 0.015 millikelvin. If one uses the constant for benzene and a concentration of 0.1%, it can be estimated that this apparatus should be capable of measuring molecular masses up to 100,000. Indeed, molecular masses of this magnitude have been determined with this equipment. Unfortunately, no commercial instruments that can reach such high precision seems to be available. 

The single-crystal samples used in this work typically have an area of about 1 cm with a thickness of 1 mm. They are polished on both sides by standard metallurgical techniques to provide a surface smooth on the micron scale. The crystal can be heated resistively and the temperature of the sample is measured by a chromel-alumel thermocouple which is spot-welded to the crystal edge. In the case of iron crystals, bulk impurities such as sulfur and carbon are common, and before the sample is mounted in the UHV equipment, the crystal must be heated to about 873 K in a hydrogen furnace to help deplete the bulk of the sulfur. The cleaning procedure is continued in UHV where a combination of argon ion sputtering and chemical treatment is used. 

For general instruction on the use of thermocouples, see ASTM Manual on the Use of Thermocouples in Temperature Measurement (ASTM, 1981). The National Bureau of Standards publishes a complete set of thermocouple tables (Powell et al., 1974), and White (1987) provides a table of output voltage versus temperature for copper-constantan and two gold-iron thermocouple materials for cryogenic work. 

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