Resistance temperature sensors or RTDs are temperature sensors that utilize the resistance ratio /temperature ratio of material(s) for measuring body temperatures. RTDs tend to have greater accuracy as well as stability in comparison to thermocouples, typically in the range below 600°C.
RTDs consist of an implementation material, usually platinum, copper, or nickel that exhibit resistance at certain temperatures. If one wishes to measure temperatures with much higher accuracy, then RTD happens to be an apt solution because of having fine linear features in a much wider temperature range.
Ratio of Resistance to Temperature
The ratio of resistance to temperature is depicted as a measure of the progression of component resistance per degree of temperature alteration. Such a relative alteration in the resistance is known as the temperature-coefficient of resistance (α) & remains almost constant in the sensor’s range of temperature. Platinum is quite a recommended material for such sensors because unlike other components, the resistance-to-temperature relationship herein must be repeated over a wide temperature range. Such a linear temperature range varies from -272.5°C to 961.78°C. This is even recommended for the chemical inertness possessed; making it the most apt for utilization in any of environments whatsoever.
Sensors that have been produced for the ITS-90 (i.e. the International-Temperature-Scale-Standard) have been the platinum ones. Copper also has quite a decent direct relationship of resistance with the temperature, but it tends to oxidize above 150°C, making it somewhat doubtful at much higher temperatures. Above 300°C, Nickel however has a non-linear relationship that tends to limit its range of temperature.
The resistance at 0°C is known as R0, and this happens to be an important limit that is to be characterized. The RTD element that’s usually utilized is platinum that has a resistance of 100 Ω at 0°C. Therefore, the name Pt100 Platinum RTD happens to be suitable in the range of temperature from -200°C to 850°C. Typically, industrial TTA devices tend to be utilized in the temperature range up to four hundred degrees Celsius.
Direct resistance rating as a function of temperature is 0°C – 100°C. α equals to (R100 –R0)/(R0xΔT) where:
Pure Platinum has α= 0.003926 Ω/(Ω• degrees Celsius) between zero degrees Celsius and hundred degrees Celsius.
Be this as it may, the value given by IEC 60751 & ASTM E-1137 is α equal to 0.00385 Ω/(Ω• degrees Celsius). The estimate of α is driven by a cycle that is known as doping, wherein, the contaminants have been incorporated into the subatomic lattice of Platinum in a well-controlled manner.
Components with resistances of 200 Ω, 500 Ω & 1000 Ω at 0°C are available in addition. Such RTDs are individually named as PT200, PT500 & PT1000. These types of temperature coefficients are also equivalent to the PT100, but tend to provide a greater resistance alteration per degree Celsius, that, in a way, tends to provide much greater resolution.