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

RTD is a temperature sensitive metal. The resistance of the RTD metal isgradually 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 onlyconnected to the transmitter by using 2 wires, 3 wires means its connectedby 3 wire and 4 wire means its connected by 4 wires. So what it should be, 2wires, 3 wires, or 4 wires RTD?

In so many articles, the author of those articles is explaining the differentbetween 2 wires, 3 wires, and 4 wires RTD by using an RTD as one ofWheatstone bridge leg. The calculation is complex and it couldnt give a briefoverview why it should be 2 wires, or others. This article try to represent theeffect 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 equationderivation.

2 wires RTD

As explain above, the 2 wires RTD only have 2 wires connected to thetransmitter. See figure 1 for the illustration. In that circuit (figure 1), thechanged of RTD resistance is measured by the transmitter with the help of

constant current source. The voltage drop is measured to get the changed ofRTD resistant. But this RTD resistant measurement through the voltage dropisnt valid since there are also RL2 and RL1 (from the wire) in the voltagedrop equation, see below derivation.

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

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V = Is (RL2 + RTD + RL1)

3 wires RTD

To compensate the resistance of the wires of the RTD to the transmitter, weshould use another wire to subtract it from the voltage drop equation. Again,by using an additional current source, we will compensate the resistance ofwires (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 sothat the above equation can be:

V1 = Is2.RTD

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

4 wires RTD

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

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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 RTDtype while 4 wires RTD is the best RTD type. 3 wires RTD have a medium orgood performance compare with 2 wires or 4 wires RTD. In oil & gasindustries, 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 thatneed a very accurate temperature measurement. As a default, we couldspecify in the temperature transmitter or temperature element datasheetthat the RTD type is 3 wires. It should be have a good performance for mostoil & gas industries applications.