Platinum resistance thermometers PRTs

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. 

Temperature The level of the temperature measurement (4 K, 20 K, 77 K, or higher) is the first issue to be considered. The second issue is the range needed (e.g., a few degrees around 90 K or 1 to 400 K). If the temperature level is that of air separation or liquefact-ing of natural gas (LNG), then the favorite choice is the platinum resistance thermometer (PRT). Platinum, as with all pure metals, has an electrical resistance that goes to zero as the absolute temperature decreases to zero. Accordingly, the lower useful limit of platinum is about 20 K, or liquid hydrogen temperatures. Below 20 K, semiconductor thermometers (germanium-, carbon-, or silicon-based) are preferred. Semiconductors have just the opposite resistance-temperature dependence of metals—their resistance increases as the temperature is lowered, as fewer valence electrons can be promoted into the conduction band at lower temperatures. Thus, semiconductors are usually chosen for temperatures from about 1 to 20 K. 

Fig. 8. Diagram of a two-vessel crystalliser. C, Single crystal Hi, H2, and H3, heating elements M, magnetic stirrer P, pump PC, polycrystalline material T1 and T2, thermometers, typically Hg in glass or platinum resistance thermometers (PRT), T1 is used to control heaters T3 and T4, miniature thermometers, i.e. thermister or PRT VI, crystallisation vessel V2, dissolution vessel.

TABLE 4 Comparison of performance characteristics for thermocouples, platinum resistance thermometers (PRTs), and thermistors... 

Although not directly related to the AOL system, an airborne expendable bathythermograph (AXBT) launch tube, receiver, and microprocessor-based data acquisition system were also built to provide valuable subsurface temperature data in the WCR boundaries. The AXBT data system multiplexes the temperature data into the AOL system HP computer for simultaneous recording with the laser data. The specially adapted radio receiver/data acquisition system is configured to handle standard AN/SSQ-36 (U.S. Navy) AXBTs. Surface temperature data, acquired by a Barnes platinum resistance thermometer (PRT)-5 IR radiometer, are also recorded by the AOL computer system. 

Resistive materials used in thermometry include platinum, copper, nickel, rhodium-iron, and certain semiconductors known as thermistors. Sensors made from platinum wires are called platinum resistance thermometers (PRTs) and, though expensive, are widely used. They have excellent stability and the potential for high-precision measurement. The temperature range of operation is from -260 to 1000°C. Other resistance thermometers are less expensive than PRTs and are useful in certain situations. Copper has a fairly linear resistance-temperature relationship, but its upper temperature limit is only about 150°C, and because of its low resistance, special measurements may be required. Nickel has an upper temperature limit of about 300°C, but it oxidizes easily at high temperature and is quite nonlinear. Rhodium-iron resistors are used in cryogenic temperature measurements below the range of platinum resistors [11]. Generally, these materials (except thermistors) have a positive temperature coefficient of resistance—the resistance increases with temperature. 

RTD Platinum resistance thermometer PRT SPRT IPRT Definition... 

Platinum Resistance Thermometers (PRTs) PRTs (sometimes seen as Resistance Temperature Detectors, or RTDs) are the thermometers of choice in high-quality blackbodies. They are reliable and accurate, although they require recalibration every 2 or 3 years. They are generally used with dedicated blackbody controllers that measure the PRT resistance, convert it to temperature, and adjust heaters to maintain the desired temperature. These controllers are programmable to accept calibration constants they convert the measured PRT resistance to temperature using the Callendar-Van Dusen equation ... 

Platinum Resistance Thermometers (PRTs) The resistance of pure metals decreases as the temperature decreases. If the material is mounted so that it is not stressed or strained during temperature cycling, the resistance is very repeatable and can be used as an accurate temperature indicator. PRTs are the most common thermometers of this type. Because the resistance is small, it is important to eliminate the lead resistance from the measurements by using the four-lead method shown in the inset to Figure 12.8 to avoid the IR drop in the leads. 

FIGURE 18.4 Differential scanning calorimeter (power compensation type). The platinum resistance thermometer (PRT) sensor detects a small error signal between the programmed temperature and the sample temperature and calls for more or less power to each heater to keep both holders on program. The difference in power required between the sample and the reference is amplified and recorded. (Courtesy of PerkinElmer Corporation, Norwalk, CT.)... 

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