I have a controller that only has terminals for a 3 wire RTD. I have several 4 wire RTDs and was wondering if there is any way to use the 4 wire RTD's on a controllers that only has terminals for 3 wire. I know this is not ideal, but it is what I am presented with at the moment.
4 Wire RTD in place of a 3 Wire
- Thread starter fifty60
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Yes, you can use a 4-wire RTD. Just leave one wire disconnected and isolated, and the other three connected as per diagram for 3-wire RTD.
petersonra
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An RTD is really a 2 wire device. A 3-wire RTD has 2 wires on one side and one on the other. A 4 wire RTD has 2 wires coming from each end. You could just tie the 4th wire in where the associated lead is and it effectively becomes a 3 wire RTD.
Doesn't matter which as long as the other 3 are connected properly. Typically the leads of a 4-wire RTD are two pair of two colors. Each color pair is a voltage drop loop, while any pair of mismatched color leads is the resistance loop. Refer to your instrumentation connection diagram to determine which wire goes to which terminal. I can't advise beyond that without terminal knowledge of the instrumentation.
Any standard insulating material will suffice, such as electrical tape, heat shrink tubing...
Do not do that. It introduces errors into the temperature measurement.
mcclary's electrical
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I believe he is recommmending the exactvsame thing as you, only wording it differently.
weressl
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The three wire RTD has two 'active' wires and one for compensation. The compensation wire is actually connected parallel with one of the 'active' wires to one end of the resistor, so those wires should read a small amount of resistance at the end where you connecting to the measuring device. This is used to compensate the measurement that may be introduced by the resistance of the home-run wire. The 4 wire RTD has a compensating wire connected to BOTH end of the resistor to compensate for the resistance AND also the difference that results in miniscule resistance difference that comes from manufacturing tolerance difference between the two leads. So you just hook up the 4 wire as a 3 wire RTD and leave one of the compensation wire unconnected.
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FWIW, the three wire RTDs (and for that matter the two wire ones also) are usually sensed with a resistance bridge circuit containing known resistors. In such a circuit the additional (fourth) compensating lead is not really useful and its presence will not allow any more accurate measurement.
But using a four wire RTD, the four wires can be used as separate current and voltage leads allowing the resistance to be measured by looking directly at the measured voltage on the voltage leads while sending a known current through the current leads. In this situation, the lead resistance is simply not part of the measurement in the first place and no compensation is needed.
Bottom line: A four wire RTD detector can only be used with a four wire RTD probe, while a four wire RTD probe can also be used with a three or two wire sensing circuit.
petersonra
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no. it will make the measurement slight more accurate as it will reduce the voltage drop slightly.
petersonra
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That is just not true.
You will get varying degrees of perfection in the reading depending on what you use, but you can use any RTD 2-3-4 wire with any RTD xmtr.
In fact by running two wires from your xmtr out to each side of a 2 wire RTD you effectively convert it to a 4 wire RTD.
kwired
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Should not matter to an input device that is looking for three wire input, as the third wire is there for voltage drop compensation. I can see it possibly throw the compensation off further if you ran all four leads back to the device than if you tied the leads of one end of the resistor together near the resistor and only run three wires back to the controller.
ELA
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Leave the 4th wire disconnected and isolated as some have said. Which connection you use for the 3rd wire depends upon the RTD module being used. Check the manual on the RTD module.
In a 3-wire RTD it is "assumed" that the excitation lead resistances are equal. If you parallel connect the 4th wire that introduces additional measurement error.
In a 3-wire RTD it is "assumed" that the excitation lead resistances are equal. If you parallel connect the 4th wire that introduces additional measurement error.
petersonra
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- Northern illinois
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- engineer
A 3-wire RTD uses the single compensation loop to approximate the VD of the resistance loop's leads, i.e. both of them. By connecting the 3rd and 4th leads, you change the voltage drop on the unmeasured side of the resistance loop.
Sure it will still function, but why set it up so you have to calibrate out an error for which there is no need to introduce in the first place.... and doing so does not improve either precision or accuracy.
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ELA
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Consider what temperature accuracy is trying to be achieved?
Consider why do installation instructions emphasize using the same length wires on each side?
What is the temperature coefficient of the RTDs you are referencing with your comment?
Compare that to the resistance per foot of the connecting wire.
Then restate: it does not make a difference for ... what length of connecting wire?
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