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:

• R100 happens to be the sensor resistance at 100°C
• R0 happens to be the sensor resistance at 0°C 
• ΔT happens to be the difference that is there in the temperatures

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. 

Resistance Temperature Detector (RTD)

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.