Thermometer, accuracy resistance

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. 

Most low-temperature engineering temperature measurements are made with metallic resistance thermometers, nonmetallic resistance thermometers, or thermocouples. In the selection of a thermometer for a specific application one must consider such factors as absolute accuracy, reproducibility, sensitivity, heat capacity, self-heating, heat conduction, stability, simplicity and convenience of operation, ruggedness, and cost. Other characteristics may be of importance in certain applications. 

Industrial resistance thermometers are also the subject of a number of national and international standards, which describe both cahbration constants and classes of accuracy and interchangeabihty. lEC pubhcation 751 was revised in 1976 to conform to ITS-90, and national standards will be revised to conform to this document. lEC 751 uses the fixed-poiat values of ITS-90 with the simpler algorithm of IPTS-48 ... 

As normally used in the process industries, the sensitivity and percentage of span accuracy of these thermometers are generally the equal of those of other temperature-measuring instruments. Sensitivity and absolute accuracy are not the equal of those of short-span electrical instruments used in connection with resistance-thermometer bulbs. Also, the maximum temperature is somewhat limited. 

The resistance must be measured with extreme care and accuracy, since a small error in measuring the resistance will cause a much larger error in determining the temperature rise. When the temperature of the winding is to be determined by the resistance, the temperature of the winding before the test, measured either by thermometer or by ETD, may be considered as the cold temperature for the resistance measured. The machine must be left cold for at least 12 to 24 hours, depending upon the size of the machine, to obtain a stable reading. 

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. 

The most interesting liquids for low-temperature thermometry are 3He and 4He, especially for the calibration of resistance thermometers in the range from 0.5 to 4.2 K. Vapour pressure of H2 is also interesting to realize vapour pressure-fixed points included in ITS-90. The measure of He vapour pressure has been carried out with great accuracy [42,43] to establish the ITS-90 (see Section 8.3). There are several experimental precautions to be observed in order to obtain reliable measurements [2],... 

The contact resistance may change when the thermometer is moved from a position to another. Hence the accuracy of resistance temperature measurements below about 25 mK... 

Resistance temperature detector. In the low-medium temperature range, thermometers based on resistometry are often used. A reference high-accuracy thermometer is the platinum resistance temperature detector which may be used as an interpolation standard in the temperature range from the triple point... 

Some researchers, aware of the temperature problem, elect to use electrically heated wires for heat sources. The same wires can be used as resistance thermometers with satisfactory accuracy. A drawback, however, is that tests cannot be made in the transition region of boiling, because of instability. In addition, unless the wires are quite small the currents needed become very great. For example, if a -in.-diam. copper tube with a 0.03-in. wall is intended for use with water near the critical AT, a current of about 8,000 amp. is required. 

Two main principles of temperature measurement use thermocouples and the so-called resistance thermometer. In chemical plants both methods were applied because they are easy to fit and to maintain.The accuracy of the measurement is influenced by, for example, radiation, which must be taken into account. Thermocouples can be inserted into the pressure system using special sealing techniques, or they may be mounted within a protective tube which is introduced into the pressurized volume. Thermocouple-wires are usually protected with an isulating input in closed-end capillaries with outer diameters of at least 0.5 mm. Thermocouples are technically well tested for pressures up to 6 kbar and temperatures to approx. 800°C. Above these ranges the exact measurement is negatively influenced by several parameters, and the deviations must be taken into account. The accuracy of the temperature measurement devices is normally better than 1 °C. 

Both methods cover a wide range (thermocouples, 250°C-1000°C and resistance thermometers, 230°C-2800°C, with an accuracy of better than 0.2%). The response-time of such devices is short, and the small dimensions are favourable for use under high pressure. 

For certain liquids, tile temperature of a boiling solution of the unknown may be compared with that of boiling water at the same pressure, For a given solution, the boiling-point elevation may be calibrated in terms of specific gravity at standard temperature. Usually two resistance thermometers are used. The system finds use in the control of evaporators to determine the endpoint of evaporation, Good accuracy is achieved in the determination of one dissolved component, or of mixtures of fixed composition. 

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. 

The provisional scale adopted by the Bureau of Standards may be expressed ia terms of the following fixed points. On the basis of the true thermodynamic scale these standard temperatures are known to an accuracy of possibly 0.5° at 500°C., and 3° at 1,200°C. On the basis of the platinum resistance thermometer scale defined as above, the temperatures below 1,000°C. can be determined with possibly 10 times this precision. The accuracy with which the platinum point is known on the thermodynamic scale is probably 10°C., and the accuracy of the tungsten point may be estimated as 50°C. 

Variable area flow meter, 0-60 Nm /h, accuracy 1.6% of full scale Variable area flow meter, 0-5 Nm /h, accuracy 1.6% of full scale Resistance thermometer, 0-100 °C, accuracy 0.5% of full scale Pressure range 0-500 kPa, accuracy 0.5% of full scale Testo Hygrotest 600/650 humidity temperature accuracy of up to... 

Platinum resistance thermometers that are carefully constructed and also carefully operated have been used as international standards from -259°C up to 1095°C. In general, resistance thermometers predict considerably more accurate temperatures than thermocouples. Resistance thermometers are often accurate to <0.01°C they tend to retain their accuracy after extended use. In general, they are more expensive, more fragile, and larger than thermocouples. 

Since temperature measurements are required over such a wide range and diversity of situations, a large number of different types of thermometers with varying levels of accuracy and convenience have been developed over the years. Those most frequently used are based on the expansion of a gas, liquid or solid on changes in electrical resistance on the thermoelectric effect on changes in the thermal radiation of a system on changes in the thermal (Johnson) noise of electrical resistors on changes... 

In addition to these ready-to-use thermometers, there also are available high-resolution, low-excitation power ac resistance bridges that can measure RTD resistances with moderately high accuracy (Berger et al., 1974). Such instruments cost about 2500. Since such instruments measure only a resistance value, the resistance—temperature (R vs. T) relationships for the RTD must be known accurately to allow the temperature value to be accurately known. An example of this type of instrument is the Model LR-110, available from Linear Research, Inc., San Diego, California. A number of handbooks provide a further discussion of more modern bridges. (Handbooks). 

The above rule has been found to be of sufficient accuracy for freely supported or loosely wound platinum it does not hold, however, for platinum fused into quartz (Heraeus resistance thermometer), and it is subject to small inaccuracies when the wire is embedded in a solid varnish. 

---