Temperature measurement platinum resistance thermomete

Temperature Measurement and Control. Temperature-sensing devices are usually thermocouples placed as close to the sample as possible. Thermocouples are inexpensive, rugged, and fairly linear in their response to temperature changes. Platinum resistance thermometers are also used in this application. The emf generated by the thermocouple may be used to drive one axis of an x-y recorder, or a feedback circuit to the heater may be used to obtain a programmed linear heating rate. In the latter case, the time axis of a strip-chart recorder is proportional to temperature. Instruments that depend upon a linear increase in power to the heater often have severely nonlinear temperature increases because of heat losses to the environment. 

The principle of the power compensated DSC [37] is different from the heat flux DSC described previously. The heaters beneath the pans aim to minimise the difference in temperature between a specimen and an inert material. When a transition occurs in the specimen, the reference heater will aim to compensate for this and keep the reference pan at a similar temperature. Individual platinum resistance thermometers measure the temperature of each pan, and the power required to maintain this... 

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. 

Electrical resistance thermometers, the most widely used of which is Callendar s platinum resistance thermometer. This is probably the most convenient and accurate apparatus for measuring temperatures between the boiling-point of liquid air (—190° C.) and the melting-point of platinum (1,500° C.). Lead has recently been applied at very low temperatures. 

These fixed points are used to calibrate a different kind of thermometer that is easier to use than a gas thermometer. Over the temperature range from 13.8033 to 1234.93 °A (or K), which is the temperature interval most commonly encountered, the thermometer used for ITS-90 is a platinum resistance thermometer. In this thermometer, the resistance of a specially wound coil of platinum wire is measured and related to temperature. More specifically, temperatures are expressed in terms of W(T9o), the ratio of the resistance R(Ttriple point of water R (273.16 K), as given in equation (1.11)... 

An instrument for measuring temperatures, in the rubber industry the term is usually applied to an instrument for determining the surface temperature of mill and calender rolls, moulds, etc. The instrument is usually based on thermocouples or, where higher accuracy is required, platinum resistance thermometers. Infrared (IR) techniques are now used which have the advantage of non contact but require careful calibration for the emissivity of the surface. 

SAQ 1.1 A temperature is measured with the same platinum-resistance thermometer used in Worked Example 1.1, and a resistance R = 11.4 x 10 4 2 determined. What is the temperature ... 

The temperature coefficient of conductance is approximately 1-2 % per °C in aqueous 2> as well as nonaqueous solutions 27). This is due mainly to thetemper-ature coefficient of change in the solvent viscosity. Therefore temperature variations must be held well within 0.005 °C for precise data. In addition, the absolute temperature of the bath should be known to better than 0.01 °C by measurement with an accurate thermometer such as a calibrated platinum resistance thermometer. The thermostat bath medium should consist of a low dielectric constant material such as light paraffin oil. It has been shown 4) that errors of up to 0.5 % can be caused by use of water as a bath medium, probably because of capacitative leakage of current. 

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 calorimetric thermometer measures temperature changes within the calorimeter bucket. It must be able to provide excellent resolution and repeatability. High single-point accuracy is not required since it is the change in temperature that is important in fuel calorimetry. Mercurial thermometers, platinum resistance thermometers, quartz oscillators, and thermistor systems have all been successfully used as calorimelric thermometers. 

The International Practical Temperature Scale of 1968 (IPTS-68) is currently the internationally accepted method of measuring temperature reproducibly. A standard platinum resistance thermometer is the transfer medium that is used over most of the range of practical thermometry. 

RTDs are constructed of a resistive material with leads attached and placed into a protective sheath. Platinum resistance thermometers are the international standard for temperature measurements between the triple point of H2 at 13.81 K (24.86°R) and the freezing point of antimony at 630.75°C (1,167.35°F). The RTD elements include platinum, nickel of various purities, 70% nickel/30% iron (Balco), and copper, listed in order of decreasing temperature range. Their features and relative performance characteristics in comparison with other sensors are tabulated in Table 3.169. 

Barometric pressure is detected by bellows against a reference, which is provided by a sealed-in vacuum. Exposing the instrument to direct sunlight, radiant heaters, or to direct drafts, such as from open windows or doors, should be avoided. Air temperature is measured with a platinum resistance thermometer. It is placed in a long tube that has been painted white on the outside to reflect solar radiation. A small blower pulls ambient air into the tube and across the thermometer. 

Although there axe a myriad of devices used to measure the temperature of an object, thermal analysis instruments predominantly use thermocouples, platinum resistance thermometers, and thermistors. Thus, only these items, with special emphasis on thermocouples, will be discussed. 

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. 

Platinum Resistance Thermometer. Platinum resistance thermometers are based on the electrical resistance of Pt. This resistance for Pt wires wound on a mica support and enclosed in a glass or silica vessel can be manufactured to be either 25.5 Q at 0°C or 2.5 Q at 0°C. The electrical resistance, typically measured in a Wheatstone or Mueller bridge, increases by about 0.1 Q or 0.01 Q, respectively, per degree centigrade (0.4%/°C). Empirical equations convert Pt resistance and its small nonlinearities to temperature ... 

Practical difficulties arise in making very precise determinations of temperature on the thermodynamic scale the precision of the more refined thermometric techniques considerably exceeds the accuracy with which the experimental thermometer scale may be related to the thermodynamic scale. For this reason, a scale known as the International Temperature Scale has been devised, with several fixed points and with interpolation formulas based on practical thermometers (e.g., the platinum resistance thermometer between 13.803 K and 1234.93 K). This scale is intended to correspond as closely as possible to the thermodynamic scale but to permit more precision in the measurement of temperatures. Further details about this scale are given in Chapter XVII. 

If the vapor pressure of liquid nitrogen is used for the temperature measurement, one allows N2 gas to condense in chamber B and the N2 pressures can be read directly on a pressure gauge. If a copper-Constantan thermocouple or a platinum resistance thermometer is used, it must be well calibrated, since accurate absolute temperatures are needed. If chamber B is not used, all further instractions concerning it may be disregarded. 

The ITS-90 scale extends from 0.65 K to the highest temperature measurable with the Planck radiation law (—6000 K). Several defining ranges and subranges are used, and some of these overlap. Below —25 K, the measurements are based on vapor pressure or gas thermometry. Between 13.8 K and 1235 K, Tg is determined with a platinum resistance thermometer, and this is by far the most important standard thermometer used in physical chemistry. Above 1235 K, an optical pyrometer is the standard measrrremerrt instmment. The procedtrres used for different ranges are sttmmarized below. 

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