RTD sensor contains a sensing element which is an electrical resistor that changes resistance with temperature. This change in resistance is well understood and is repeatable. The sensing element in an RTD usually contains either a coil of wire or a grid of conductive film which has a conductor pattern cut into it. Extension wires are attached to the sensing element so its electrical resistance can be measured from some distance away. The sensing element is then packaged so it can be placed in a position in the process where it will reach the same temperature that exists in the process.
Different materials used in the construction of RTD elements offer a different relationship between resistance and temperature. Temperature sensitive materials used in the construction of RTD element include platinum, nickel, and copper, platinum being the most commonly used. Important characteristics of an RTD element include the temperature coefficient of resistance (TCR), the nominal resistance at 0 degrees Celsius, and the tolerance classes. The most common TCR is the platinum 3850 ppm/K which means that the resistance of the sensor will increase 0.385 ohms per one degree Celsius increase in temperature. The nominal resistance of the sensor is the resistance that the sensor will have at 0 degrees Celsius.
The Platinum RTD Sensor having a nominal resistance of 100 ohms at 0 ‘C and a temperature coefficient of 0.00385 is considered as standard. It is also called as PT-100 sensor.
RTDs commonly come in two classes of accuracy. Class A devices have a tolerance of ±0.15°C at 0°C. Class B devices have a tolerance of ±0.30°C at 0°C.
At any temperature “ t ” tolerance of Class-A and Class-B sensors can be calculated using the following equations.
For Class-A sensor
Tolerance = +/-(0.15 + 0.002* |t|)
For Class-B sensor
Tolerance = +/-(0.30 + 0.005* |t|)
|Temperature||Class – A||Class – B|
|( ˚C )||( ± ˚C )||( ± Ω )||( ± ˚C )||( ± Ω )|