The relationship between platinum resistance and temperature is well understood and depending on the purity of platinum used the temperature coefficient differs.
The most common type of RTD sensor is known as the Pt100, this is because the sensor has a resistance of 100 ohms at 0°C. There are RTDs with base resistance of 500 or 1000 ohms available on the market, but they are less widespread.
The most widely used platinum is one defined by the IEC 60751 publication (International Electrotechnical Committee). IEC standard platinum has a temperature coefficient, or a nominal α value, of 0.003851 ohms/ohms /°C or °C¯¹.
α = R₁₀₀-R₀
This means that for every degree change in temperature there is a resistance change of 0.3851 ohms. So at 0°C the resistance is 100 ohms, at 100°C resistance = 138.51 ohms.
JIS standard platinum has a temperature coefficient, or nominal α value, of 0.003916°C¯¹. So a 100 ohm platinum RTD will increase in resistance by 0.3916 ohms for every 1°C of temperature increase. So at 0°C the resistance is 100 ohms, at 100°C resistance = 139.16 ohms
The linear relationship of IEC standard Pt100 is shown in figure 1.
Fig. 1 Resistance Temperature Curve for 100Pt Platinum RTD (α = 0.003851 ºC-1)
RTDs can be manufactured to different tolerances. Higher tolerances give temperature readings that have a much smaller margin of error relative to those with lower tolerances.
A standard tolerance classification system has been defined and is used industry wide. The IEC system is seen as the industry standard, although there are other tolerance classes that exist.
The IEC has industry standard classes of W0.1, W0.15, W0.3 and W0.6.
These classes translate as follows in terms of resistance and temperature tolerances:
For example a W0.15 (Class A) RTD giving a reading of 0°C has a margin of error of ±0.15°C or ±0.06Ω. So a W0.15 (Class A) RTD could show a resistance of between 99.94 and 100.06 at 0°C.
An RTD giving a resistance value of 99.77 at 0°C would be classed as a W0.6 (Class C) tolerant RTD.
Fig. 2 Tolerances for RTDs as defined by IEC 60751
|IEC Standard||DIN4370||Temperature Range ºC||Tolerance Ω at 0ºC||Tolerance ºC|
|W0.03||1/10 DIN||-100 to 350||100±0.012 Ω||±0.03 °C|
|W0.1||1/3 DIN||-100 to 350||100±0.04 Ω||±0.10 °C|
|W0.15||Class A (1/2 DIN)||-100 to 450||100±0.06 Ω||±0.15 °C|
|W0.3||Class B (DIN)||-196 to 660||100±0.12 Ω||±0.30 °C|
|W0.6||Class C||-196 to 660||100±0.23 Ω||±0.60 °C|