Resistance thermometer, metallic platinum

Metal resistance thermometer Coil of fine platinum wire Low High -100 to 700... 

For most purposes, the platinum resistance thermometer is still the first choice among metallic resistance thermometers. For measurements extending either over a broad temperature range from 1 to 300 K or for a narrower temperature range below 30 K, semiconductor and diode thermometers are the principal competitors to PRTs and are often to be preferred. 

For temperatures above about 20 K, the metallic resistance thermometers are more sensitive than the nonmetallic resistance thermometers. Temperatures above 20 K can be measured routinely with an industrial-type platinum resistance thermometer with an accuracy of better than 100 mK with time responses somewhat better than 1 s. Accuracy at the millidegree level requires a precision capsule type platinum resistance thermometer and careful calibration. 

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. 

Resistance thermometers are made of a pure metal, such as platinum, nickel, or copper. The electrical resistance of such a material is almost linearly dependent on temperature. Resistance thermometers are stable, having a small drift. A widely used and the best-known resistance probe is the IW-100 probe, which is platinum, having a resistance of 100 ohms at the temperature of 0 °C. Other resistance values for PT probes are available. The resistance versus temperature values as well as tolerances for platinum probes are standardized. The shape and size of a resistance probe can vary considerably, resulting in changes in probe dynamics. 

High purity platinum wire is used in resistance thermometers because the temperature coefficient of resistance of pure platinum is linear over a wide temperature range. The platinum resistance thermometer is the recognized instrument for the interpolation of the international practical temperature scale from—259.35 to 630.74°C. Whereas such precision measurements require very high purity platinum, for most routine industrial measurements lower purity metal can be tolerated. Conventional wire-wound devices are quite fragile and this disadvantage has led to the introduction of printed resistance thermometers, which are cheap to produce and much more durable. They can be used as an inexpensive replacement for thermocouple applications in intermediate temperature applications. 

The Nemst calorimeter is a calorimeter for the measurement of specific heat capacities at low temperatures. The sample to be measured is suspended in a glass or metal envelope that car be evacuated. The sample is heated by means of a platinum wire located in a bore inside the sample. The wire also serves as a resistance thermometer. The specific heat capacity is determined by recording the temperainre rise in the sample for a given delivery of energy. 

In 1821, Sir Humphrey Davy discovered that as temperature changed, the resistance of metals changed as well. By 1887 H.L. Callendar completed studies showing that purified platinum wires exhibited sufficient stability and reproducibility for use as thermometer standards. Further studies brought the Comitd International des Poids et Measures in 1927 to accept the Standard Platinum Resistance Thermometer (SPRT) as a calibration tool for the newly adopted practical temperature scale. 

Another approach is to use a carefully selected standard platinum alloy for which the constants in Eqs. (6) and (8) are well known. In the mid-1980s, the International Electrotechnical Commission (lEC) recommended that ITS-90 be based on the Callendar-van Dusen interpolation formula and proposed constants for a Ft resistor with = 100 O A = 3.90802 X 10 5=-5.802 X 10 C =-4.27350 X 10 (or a = 0.00385, 8 = 1.50701, /3 = 0.111). The platinum wire used in platinum resistance thermometers that conform to this proposed standard is a platinum alloy containing small amounts of several different elements (mostly noble metals) adjusted so as to achieve the required a = 0.00385 K . This alloy is now widely used in Europe and by some American manufacturers of resistance thermometers note, however, that other American firms use a wire for which a = 0.00392 K . In spite of the fact that this lEC proposal was not adopted, the Callendar-van Dusen constants given above are a guide to appropriate values, which can always be checked by calibration. 

Differential scanning calorimetry (DSC) is a calorimetric method that finds widespread use in many fields, including protein dynamics, polymers, pharmaceuticals, and inorganic materials. DSC measures energy (heat) flow into a sample and a reference substance as a function of controlled increase or decrease of temperature. In a typical power-compensated DSC (Fig. 3.2), the sample and reference are placed on metal pans in identical furnaces each containing a platinum resistance thermometer (thermocouple) and heater. During a thermal transition (e.g., when a physical change in the sample occurs),... 

A very convenient form of measuring temperatures is now at our disposal in the form of electrical thermometers., These are of twofold construction they may be either resistance or contact thermometers. In the resistance thermometers, use is made of the fact that the resistance of a metal to the passage of a current depends upon the temperature, so that if, for instance, a platinum wire is exposed to the heat of a furnace and a weak current is passed through it, that current will affect the needle of a galvanometer according to the temperature of the wire. The contact thermometer, on the other hand, depends upon the fact that if two pieces of different metals, as, for instance, an iridium and a platinum wire, are placed in contact,... 

OTHER COMMENTS Platinum metal is used in the manufacture of apparatus used in laboratories and industries, including thermocouples, acid-proof containers, chemical reaction vessels, platinum resistance thermometers, electrodes, etc. has found applications in dentistry, electroplating industry, and in the jewelry industry soluble platinum salts have been used as catalysts in the production of high octane gasoline, vinylesters, petrochemicals, and pharmaceuticals platinum metals, as well as soluble platinum salts, have been employed as oxidation catalysts in the manufacture of sulfuric acid, nitric acid from ammonia, and acetic acid soluble platinum salts have been used and reused in the reclamation of platinum ore. 

A bolometer is a type of resistance thermometer constructed of strips of metals, such as platinum or nickel, or of a semiconductor. Semiconductor bolometers are often called thermistors. These materials exhibit a relatively large change in resistance as a function of temperature, The responsive element is kept small and blackened to absorb radiant heat. Bolometers are not so extensively used as other infrared transducers for the mid-infrared region, Flowever, a germanium bolometer. operated at 1..3 K, is nearly an ideal transducer for radiation in the. 5 to 400 cm (2000 to 2.3 pm) range. 

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