2 Wires, 3 Wires, or 4 Wires RTD (Resistance Temperature Detector)?

RTD is a temperature sensitive metal. The resistance of the RTD metal is gradually changed with the changed of temperature. In applications there are 2 wires, 3 wires, and 4 wires RTD. 2 wires RTD means that the RTD only connected to the transmitter by using 2 wires, 3 wires means its connected by 3 wire and 4 wire means its connected by 4 wires. So what it should be, 2 wires, 3 wires, or 4 wires RTD?

In so many articles, the author of those articles is explaining the different between 2 wires, 3 wires, and 4 wires RTD by using an RTD as one of Wheatstone bridge leg. The calculation is complex and it couldn’t give a brief overview why it should be 2 wires, or others. This article try to represent the effect of using 2 wires, 3 wires, and 4 wires RTD in a different and a simple way. Hopefully it could give a brief understanding without complex equation derivation.

2 wires RTD

 

As explain above, the 2 wires RTD only have 2 wires connected to the transmitter. See figure 1 for the illustration. In that circuit (figure 1), the changed of RTD resistance is measured by the transmitter with the help of constant current source. The voltage drop is measured to get the changed of RTD resistant. But this RTD resistant measurement through the voltage drop isn’t valid since there are also RL2 and RL1 (from the wire) in the voltage drop equation, see below derivation.

V = Is.RL2 + Is.RTD + Is.RL1

V = Is (RL2 + RTD + RL1)

3 wires RTD

 

To compensate the resistance of the wires of the RTD to the transmitter, we should use another wire to subtract it from the voltage drop equation. Again, by using an additional current source, we will compensate the resistance of wires (see figure 2). From the figure 2 we can derive the following equation:

V = V1 + V2

V1 = V – V2

V = Is2.RL2 + Is2.RTD + Is2.RL1 + Is1.RL1

V2 = Is1.RL3 + Is1.RL1 + Is2.RL1

Then, from V and V2 we can get V1,

V1 = Is2.RTD + Is2.RL2 – Is1.RL3

To get a pure resistance change of RTD, we should use same RL2 and RL3 so that the above equation can be:

 V1 = Is2.RTD

 Even though it will always have different value between RL2 & RL3, this method of compensation (by using 3 wires RTD) is providing good accuracy and widely used in oil & gas industries.

 4 wires RTD

 

 For best compensation of wire resistance used by RTD, 4 wires are the best choices. It subtracts all the wire resistance from the voltage drop equation (see figure 3 for illustration). The resistance change of RTD will directly proportional with voltage drop of the RTD. It can be derived from the following equation:

V = V1 + V2 + V3

V1 = I1.R3 + I1.RL1 – Is.RL3

I1 = 0, then

V1 = – Is.RL3

V2 = Is.RL4 + Is.RTD + Is.RL3 – I1.RL3 – I3.RL4

I1 = I3 = 0, then

V2 = Is.RL4 + Is.RTD + Is.RL3

V3 = I3.RL2 + I3.RL4 – Is.RL4

I3 = 0, then

V3 = – Is.RL4

Then V will be,

V = – Is.RL3 + Is.RL4 + Is.RTD + Is.RL3 + – Is.RL4

V = Is.RTD

From above explanation, we can conclude that 2 wires RTD is the worst RTD type while 4 wires RTD is the best RTD type. 3 wires RTD have a medium or good performance compare with 2 wires or 4 wires RTD. In oil & gas industries, 2 wires RTD are very rarely used while 3 wires RTD is the most used one and 4 wires RTD are used only for a very special application that need a very accurate temperature measurement. As a default, we could specify in the temperature transmitter or temperature element datasheet that the RTD type is 3 wires. It should be have a good performance for most oil & gas industries applications.