There is a variety of RTD sensors yet the most commonly used is the Pt100. It gets its name because of the resistance value of 100 ohms at 0°C. This platinum resistance are also produced with a base resistance of 500 or 1000 ohms. As there are many types of resistance thermometer detection sensors they are defined by international standards. These are defined by IEC 60751.
Tolerances of RTD sensors can be tailored to customer specifics and thus manufactured to different tolerances. The higher the tolerance the smaller the margin of error relative to lower tolerances. A system where these tolerances are classified is helpful for the end user and helps the interchangeability of these sensors. The IEC system is seen as the standard for the industry although there are other standards and other tolerance classes.
The IEC has industry standard classes of W.0.03, W0.1, W0.15, W0.3 and W0.6. These classes correspond to class 1/10 DIN, 1/3 DIN, class B and class C.
|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|
The standard DIN EN 60751:2009 is based on IEC 60751:2008. These standards define the accuracy classes for Pt100 resistance AA, A, B and C. Before the update the most accurate of the classes was A and all other more accurate classes are defined ass non-standard and divided in to class B values.
1/3 DIN is based on Class B and a divided value. This makes this 1/3 DIN not applicable over the whole measuring range. This variation in accuracy is only defined at the points 0°C=+/- 0.3/3°C=+/-0.1°C. As mentioned before this class is not standardized.
1/10 DIN is also based on Class B and not applicable over the whole range. This accuracy is only defined at the point 0°C=+/- 0.3°C/10°C= =/-0.03°C. The Class B 1/10 DIN is not standardized.
Class A Pt100 sensors have a resistance of 100 ohms at 0°C with a tolerance of ±0.15°C.
The accuracy of Class B DIN 4370 – W0.6 IEC 60751 is described as; Class B ± (0,3+0,005 * t), ±0,3 °C (0 °C), ±0,8 °C (100 °C), defined on range -196…+600 °C (wire wound resistor) and, -50…+500 °C (thin film resistor)
As there are many types of RTD sensors there are also different accuracy classes. Class C is defined by IEC 60751:2008. In this International standard the accuracy classes of resistor sensors are described. For this class the accuracy is: Class C ± (0,6+0,01 * t). From the classes this accuracy class is the least used for measuring industrial circuits.
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