Types and Classes of RTD


RTDs are available in 2-wire, 3-wire and 4-wire configurations. The 'n' in n-wire configuration stands for number of lead wires connected to the RTD. Accuracy of RTD depends upon its configuration.
Whenever we measure resistance of RTDs, we get an output fairly greater than our expected output. This happens because of the resistance of lead wires, which gets added in our measurement. This added resistance makes the controller think that the temperature is more than what it really is. Hence we need to compensate it, to get accurate readings.
  • 2-wire configuration: This type of configuration has the least accuracy. There is no wire provided for compensation. Hence, in this configuration, error is more.An alternate method to compensate lead wire resistance is by connecting a 2 or 3-wire RTD to a transmitter. A transmitter converts RTD resistance into a low current signal and then transmits it to the controller.
  • 3-wire configuration: In this configuration, 3 lead wires come out from the RTD instead of 2. Out of these, 2 wires carry the measuring current and the third wire is used as a potential lead. The resistance of the current measuring wires is ideally matched, and therefore cancelled. Resistance of the third wire is equal to the resistance of RTD at the beginning of the range.
  • 4-wire configuration: This is the most accurate method for measuring resistance of RTD. This is generally used only where long lead wires are required, since it is complicated and time consuming.In this configuration, current-potential method is used. 2 wires are used as current leads and the other 2 as potential leads. A current of known value is passed through the current leads and the potential generated across the potential leads is measured. The resistance is calculated by using Ohm's Law. i.e. Dividing measured potential by the known passed current.In this case the lead wire resistance does not come in picture as the divisor in the equation is the current, which is not affected by the increased value of resistance and the dividend is the voltage. The input impedance of the voltage measurement circuit is too high to pass any significant current into it. Since no current is flowing, the voltage across the leads does not change with change in resistance. As both, V and I, in the ohm's law are not affected by lead wire resistance, the Resistance of RTD is not affected as well.
Now, the question arises that how does this third wire help in compensating for lead wire error. The controller measures resistance through wires 2 and 3, and then this resistance is subtracted from total circuit resistance measured through wires 1 and 2. This gives the true resistance value of the RTD.


Measurement:

RTD's resistance varies according to the following equation:


RT = R0 (1 + αT)

where, Resistance at given temperature = RT                           
           Base Resistance Value Or Resistance of RTD at 0 deg.C =  R0

           Temperature in deg.C = T
           Temperature coefficient = α
           Temperature rise per deg. C increment = αT

Temperature coefficient is the ratio of change in resistance per degree change in temperature over the range 0-100 deg.C. It depends on type and purity of material used to manufacture the element. It is constant for a particular element. It is the value which defines the amount of resistance at a particular temperature

For example, if Temperature coefficient of an Pt100 RTD is  0.00385 ohm/ohm/deg. C,
i.e. α =  0.00385 and  R0 = 100
Then for every degree rise in temperature, we will get an increment of αT = 0.385 ohm in resistance. Therefore for a temperature of 200 deg. C, the resistance will be 

R200 = 100 x (1 + 0.00385 x 200)

        = 177 ohm.

The standard α values are as follows:
For Platinum: 0.003926 ohm/ohm/deg.C

For industrial RTDs it is 0.00385 ohm/ohm/deg.C
       
There are various types of RTDs depending upon the resistive element and their temperature at 0 deg.C
R0).

They are named accordingly: 
 Name of element - R0


Various RTDs are as follows:


  • Pt100
  • Pt200
  • Pt500
  • Pt1000
  • Pt3000
  • Pt6000
  • Pt9000


Classes of RTDs:


Different classes of RTDs are defined according to their tolerance and accuracy by a standard known as IEC 60751. They are as follows:

Class A:  + (0.15 + 0.002 * T)
Class B:  + (0.30 + 0.005 * T)
Class C:  + (0.60 + 0.01 * T)
Class 1/3 DIN: + (0.10 + 0.0017 * T)


Accuracy of class A is highest with +0.15 deg.C at 0 deg.C. followed by class B at +0.3 deg.C. at 0 deg.C and class C at +0.6 deg.C.



For further information about RTD:

Advantages and Limitations of RTD                    Few things to check before selecting RTD

What is RTD?

Related Articles:

Temperature Units & Conversions                                       Thermocouple