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) degrees Celsius to (961.78) degrees Celsius. 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 degrees Celsius, making it somewhat doubtful at much higher temperatures. Above 300 degrees Celsius, Nickel however has a non-linear relationship that tends to limit its range of temperature. 

The resistance at zero degrees Celsius 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 zero degrees Celsius. Therefore, the name Pt100 Platinum RTD happens to be suitable in the range of temperature from -200 degrees Celsius to 850 degrees Celsius. 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 zero degrees Celsius – hundred degrees Celsius. α equals to (R100 –R0)/(R0xΔT) where:

• R100 happens to be the sensor resistance at hundred degrees Celsius
• R0 happens to be the sensor resistance at zero degrees Celsius 
• 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 zero degrees Celsius 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.