Thermometer, accuracy industrial

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. 

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 SPRTs are devices of superb accuracy and resolution, but they are fragile and can easily be broken. They can also be put out of calibration by strain, induced by even slight mechanical shock or vibration. The principal use of SPRTs in science and industry is to maintain the calibrations of working thermometers. 

Bimetal thermometers are made in ranges from + 1000°F (538°C) down to — 300°F (— 184°C) to and lower. However, at low temperatures, the rate of deflection drops off quite rapidly. Because of its long-term instability at high temperatures, the maximum temperature for continuous use is about 800°F (427°C). However, special bimetal thermometers can be obtained for continuous use up to 1200°F (649°C). Good bimetal thermometers retain their accuracy indefinitely. Usually industrial bimetal thermometers read with an accuracy of 1% at any point on the scale. The speed ot response of bimetal thermometers is generally about the same as that for liquid-in-glass thermometers in similar ranges. 

If somewhat better accuracy and/or a sensor smaller (with a shorter response time) than the mercury-in-glass thermometer is required, then an industrial-grade PRT may be used. Normally, one can expect such PRTs to have an uncertainty of 0.02°C, plus whatever statistical uncertainty is present, if the PRTs are calibrated periodically (Mangum and Evans, 1982 Connolly, 1982).Some industrial PRTs are capable of giving better results than this and some much worse (Mangum and Evans, 1982). A few can be stable and reproducible to 0.005°C, but that represents a small percentage of PRTs and they must be specially selected. 

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...

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... 

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. 

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