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 = RTBase 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 – Ro
Various RTDs are as follows:
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. Original Source]]>