Sir Williams Siemens, in the year 1871, was probably the 1st to introduce temperature measurement using the resistance/temperature characteristics of various materials and largely laid the foundation for modern resistance thermometry. The resistance of the material is given by the given formula:
R = (ρ × L)/A
- R is for Resistance (measured in ohms)
- ρ stands for Resistivity (measured in ohm/cm)
- L is for Length (measured in cm)
- A stands for Cross-sectional area (measured in cm )
Sir William Siemens proposed using platinum metal as an element inside the resistance thermometers. Platinum is particularly suitable for this very purpose because it has the ability to withstand high-temperature while still maintaining great stability. As a precious noble metal, it has very limited susceptibility to any sort of contamination. The classic RTD design i.e. the design of the resistance-temperature-detector with platinum was given by C.H. Meyers in the year 1932. Another lab standard replaced Meyers’ design. This was the birdcage element, which was proposed by Evans & Burns. Resistance changes with time & temperature due to voltages were minimized, and the birdcage became the ultimate lab standard. However, due to the unsupported construction & subsequent susceptibility to any kind of vibration, this configuration proved to be very delicate for industrial environments. There were then the partially supported versions of RTD that offered a compromise between the birdcage’s approach & the sealed helix. One of these approaches makes use of a platinum helix threaded via a ceramic cylinder & secured with a glass alloy. These devices maintain great stability in moderate vibration applications.
RTD resistance & temperature characteristics are stable, repeatable, and have an almost linear positive temperature coefficient from 200°C-800°C. Such characteristics make an RTD the de facto industry standard. The temperature is calculated by measuring the resistance & then utilizing the RTD.s .R vs T. characteristics for extrapolating the temperature. Superior sensitivity & superior stability of such devices compared to thermocouples provide them significant pluses in the low & medium temperatures. Additionally, resistive devices generally simplify control & readout electronics. Resistance thermometers might be known as the RTD or the PRT (i.e. the platinum-resistance-thermometer), or the SPRT (i.e. the standard-platinum-resistance-thermometer). Chemical stability as well as availability in the purest form, and greatly reproducible electrical characteristics has made platinum the chosen metal for RTD, made from either the IEC/DIN grade platinum or from the reference-grade one. The difference is in metal’s purity. The IEC/DIN standard one is pure platinum, which is contaminated intentionally with some other platinum.
Some common RTD materials are copper, platinum, nickel, Balco (which is basically an alloy made of seventy percent of nickel & thirty percent of iron). The benefit of all these metals is that they could be produced in very high purity and are therefore available with very reproducible temperature/resistance properties. Such metals can even be drawn into the fine diameter wires needed in resistance thermometry.
The max temp rating of an RTD is based on 2 main factors. The 1st is the elemental material. Platinum RTD could be utilized up to 650degree Celsius (i.e. 1202 degree F). The temperature rating of other materials is much lower and varies from one material to another. The 2nd factor that determines the temperature rating is the probe construction. All of these different styles use construction aspects that make them ideal for use in all of these styles. No single style fits all types.