Thermometer, accuracy platinum

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 historical development of titration calorimetry has been addressed by Grime [197]. The technique is credited to have been born in 1913, when Bell and Cowell used an apparatus consisting of a 200 cm3 Dewar vessel, a platinum stirrer, a thermometer graduated to tenths of degrees, and a volumetric burette to determine the end point of the titration of citric acid with ammonia lfom a plot of the observed temperature change against the volume of ammonia added [208]. The capabilities of titration calorimetry have enormously evolved since then, and the accuracy limits of modern titration calorimeters are comparable to those obtained in conventional isoperibol (chapter 8) or heat-flow instruments (chapter 9) [195,198],... 

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... 

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. 

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. 

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. 

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. 

Platinum resistance thermometers 1. Good linearity 2. Wide temperature range 3. Very good stability even at high temperature 4. Very good precision and accuracy 1. Slow response 2. Vibration and shock fragility 3. High cost... 

Platinum resistance thermometers - can be used over an extensive temperature range, typically 14-750 K. They are capable of a reproducibility better than 0.001 K, which is better than the accuracy with which the thermodynamic scale has been established. They have been used to measure temperature differences at temperatures close to 273 K with a precision of about 10" K. To achieve such reproducibility and precision the resistor must be of pure platinum and be mounted in a strain-free condition. Resistivity changes can occur when... 

Platinum wire is the metal of choice because of its linearity, its chemical inertness, and its accuracy, which increases with purity. Other metals are also used and their characteristics are given in Table 5.5. Temperature ranges for Pt RTDs range from -260 to 660 °C, which is significantly greater than for thermistors. Temperatures as high as 850 °C are also possible but it is difficult to avoid contamination of the platinum by the thermometer metal sheath. 

Temperature is detected by a 100-ohm platinum resistance thermometer (RDF 80RB), and is measured and transmitted through the multiprogrammer to the calculator. Accuracy is +1 C over a full-scale range (-180°C to +650°C). 

Standard platinum resistance thermometers (SPRTs) are the most accurate and most delicate RTDs in existence. Their applications are largely limited to standard laboratories and other applications requiring temperature accuracies of 1 mK or better. SPRTs are the primary interpolation instrument in the very definition of temperature in the International Temperature Scale of 1990 (ITS-90) from 13.8033 K (the triple point of... 

Table 1 Temperature accuracy of industrial platinum resistance thermometers (IPRTs) according to the most common standard, which is known variously as EC, DIN, ASTM, 0.00385-type, and/or the European curve. Note that t is the temperature in degrees Celsius...

The shown calorimeter has an accuracy of +0.1% and a temperature range of 170 to 600 K. A sample of 100-300 g is placed in two sets of silver trays, one outside and one inside a cylindrical heater. In the middle of the sample, the tip of the platinum resistance thermometer can be seen. Sample trays, thermometer, and heater are enclosed in a rounded steel shell, which for ease of temperature equilibration is filled with helium of less than one pascal pressure. The shell is covered with a thin silver sheet on the outside, gold-plated to reduce radiation losses. The calorimeter is then hung in the middle of the adiabatic jacket, drawn in heavy black. This adiabatic jacket is heated by electrical heaters and cooled by a cold gas flow, as indicated by the dials of the instmments pictured on the left at the bottom. The whole assembly, calorimeter and adiabatic jacket, is placed in a sufficient vacuum to avoid convection. 

Platinum resistance thermometers monitor the temperature, and a three-term tunable PID controller, which is RS232-linked to a PC, stabilizes the v ue at the set point to an accuracy of at least 0.1 °C by regulating the secondary heat transfer. The set point temperature can be prograimned, so that precise thermal cycling is available, or manually entered for step variation. The woridng temperature range is 5 C with a thermal response 0.2°C/s. 

The Thermometer.—In carrying out a fractional distillation one must be able, not only to rea a constant or nearly constant temperature with great accuracy, but also to take readings of rapidly rising temperatures. These requirements are best fulfilled by the ordinary mercurial thermometer, which is therefore, notwithstanding its m ny drawbacks, used in preference to the air or the platinum resistance thermometer. If accurate results are to be obtained the following points must be attended to. 

For the vast majority of applications, the extreme accuracy of SPRTs cannot justify their high cost, large size, and fragile construction. Instead, manufacturers offer a wide selection of industrial platinum resistance thermometers (IPRTs), which are smaller, far more robust, and only slightly less aceurate than SPRTs. They can be either wire-wound or made of thin films evaporated on a substrate, and are most commonly provided with 100 resistance at... 

V. Z. Geller and V. G. Peredri [2.5] used a modified version of the stationary heated-filament method. A thin-wall platinum capillary was adopted as an external resistance thermometer this allowed an increase in the accuracy of determination of thermal conductivity. The absence of convection was ensured by measurements at two to four different temperature gradients in the layer (Rayleigh number did not exceed 1500). When computing, corrections were introduced to compensate for the eccentricity of the filament, heat outflow from the ends, and the change in geometric sizes of the measuring cell. In all, the corrections did not exceed 0.3-0.5%. Correction for radiative heat losses were not introduced due to the absence of infrared absorption in the spectrum of liquid Freon-21. This, however, as shown by the calculations did not noticeably distort the results of the measurements. The tests were carried out at a gap of 0.5 mm. 

For estample, platinum has a temperature coeffident of resistance a = 0.00385 ii/ (Q°C) in the range 0 to 100°C. The measuring range of commercial-purity platinum thermometers is situated between about 200 and 850° C and the accuracy of these thermometers is +0.15% of span over the full range [4]. Resistance versus temperature can usually be described by just a quadratic function, which can be linearized in measurement amplifiers. If large temperature ranges are not present, the linearization can often be omitted. The resistance is usually measured in a bridge drcuit [2]. 

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. 

A2.2.1 Use liquid-in-glass thermometers with an accuracy after correction of 0.02 C or better, calibrated by a laboratory meeting the requirements of ISO 9000 or ISO 25, and carrying certificates confirming that the calibration is traceable to a national standard. As an alternative, use thermometric devices such as platinum resistance thermometers, of equal or better accuracy, with the same certification requirements. 

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