Practical Resistance Thermometer Detectors
To achieve high stability, platinum sensor elements must be, and remain, in a fully annealed condition and contamination free. Further, support and sheath materials and construction must be carefully selected and clean to avoid sensor poisoning and strain.
While below 250°C contamination is rarely a problem, above this temperature, materials of construction, insulation and so on (particularly base metals, some forms of mica and borosilicate glass) can react with, or dissolve in the platinum. So, special mounting methods are required. RTDs that are hermetically sealed also need some oxygen in the filling gas to keep the problem elements oxidised and thus relatively harmless to the sensor. As for purity of the platinum, in industrial RTDs lower a coefficient purity platinum wire is used, than in primary standard and laboratory style thermometers because the application warrants a physically more robust element and one that is more forgiving of its surroundings in terms of contamination. So pure platinum wire doped with another metal is used to get to the standard specifications of the IEC and British standards for temperature vs resistance definition, and tolerance limits up and down the temperature scale. Other general points include the need to construct the sensors such that thermoelectric voltages, generated through the use of dissimilar metals (as per thermocouples), cancel one another out. Also, the insulation resistance between the RTD itself (including its internal connection wires) and the protective sheath (if any) must be adequate (as per IEC 751). Beyond this, the coil windings need to be non-inductive, current flow must not elicit significant self-heating and DC and AC (up to 500 Hz) must be provided for. Also, it is important to ensure that a negligible amount of heat will be conducted along the sheath, internal wires and insulators.
Before we go on to describe some of the sensor styles, it is just worth pointing out that the assemblies detailed can also be used with metals other than platinum. Also, a wide range of shapes and sizes is available, the only major restrictions being those of wire support, contamination resistance and an adequate electrical resistance with appropriate insulation. For example, surface areas can be made large in proportion to the volume occupied to encourage fast response. Alternatively, the RTD can be made very small to allow for point temperature sensing. Then again, the sensor could be made long, or large, to facilitate temperature averaging over whatever length, or area, you have in mind.
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