They have very reproducible results at low temperatures. Carbon resistor elements are cheap and widely used. While these types are the ones most widely used in industry, other more exotic shapes are used for example, carbon resistors are used at ultra-low temperatures (−273 ☌ to −173 ☌). The three main categories of RTD sensors are thin-film, wire-wound, and coiled elements. These electrically heated and well-stirred baths use silicone oils and molten salts as the medium for the various calibration temperatures. This method might be more cost-effective, since several sensors can be calibrated simultaneously with automated equipment. Unlike fixed-point calibrations, comparisons can be made at any temperature between −100 ☌ and 500 ☌ (−148 ☏ to 932 ☏). The thermometers being calibrated are compared to calibrated thermometers by means of a bath whose temperature is uniformly stable. Comparison calibrations is commonly used with secondary SPRTs and industrial RTDs. The ice point is designated as a secondary standard because its accuracy is ☐.005 ☌ (☐.009 ☏), compared to ☐.001 ☌ (☐.0018 ☏) for primary fixed points. The equipment is inexpensive, easy to use, and can accommodate several sensors at once. A common fixed-point calibration method for industrial-grade probes is the ice bath. Fixed-point calibrations provide extremely accurate calibrations (within ☐.001 ☌). These cells allow the user to reproduce actual conditions of the ITS-90 temperature scale. It uses the triple point, freezing point or melting point of pure substances such as water, zinc, tin, and argon to generate a known and repeatable temperature. Fixed point calibration is used for the highest-accuracy calibrations by national metrology laboratories. Two common calibration methods are the fixed-point method and the comparison method. Although RTDs are considered to be linear in operation, it must be proven that they are accurate with regard to the temperatures with which they will actually be used (see details in Comparison calibration option). This is necessary to meet calibration requirements. To characterize the R vs T relationship of any RTD over a temperature range that represents the planned range of use, calibration must be performed at temperatures other than 0 ☌ and 100 ☌. These different α values for platinum are achieved by doping – carefully introducing impurities, which become embedded in the lattice structure of the platinum and result in a different R vs. It is still possible to find older probes that are made with platinum that have α = 0.003916 Ω/(Ω Before these standards were widely adopted, several different α values were used. Conversely, two widely recognized standards for industrial RTDs IEC 60751 and ASTM E-1137 specify α = 0.00385 Ω/(Ω
The relative change in resistance ( temperature coefficient of resistance) varies only slightly over the useful range of the sensor. The R vs T relationship is defined as the amount of resistance change of the sensor per degree of temperature change.
Resistance/temperature relationship of metals Ĭommon RTD sensing elements constructed of platinum, copper or nickel have a repeatable resistance versus temperature relationship ( R vs T) and operating temperature range. 11 Temperature-dependent resistances for various popular resistance thermometers.10 Standard resistance thermometer data.1 Resistance/temperature relationship of metals.