from the control department...
4 wire vs. 3 wire RTD
Sensor technologies. topic
Posted by KAKIL on 3 October, 2007 - 12:00 am
Please, I want to know why there is 4 wire RTD and another type 3 wire RTD, both types pt100. What does PT100 mean?

Posted by Sandy on 3 October, 2007 - 10:37 pm
The principle of operation is to measure the resistance of a platinum element (Platinum resistance thermometer). The most common type (PT100) has a resistance of 100 ohms at 0 C and 138.4 ohms at 100 C.

For a PT100 sensor, a 1 C temperature change will cause a 0.384 ohm change in resistance. A 4 wire is the most accurate way of measurement, although 3 wire tends to be the norm.

Posted by kakil on 8 October, 2007 - 8:30 am
please can you send me the electrical drawing for the pt100 that connect with the panel

Posted by stem_p1st on 9 October, 2007 - 12:26 pm
try they have great explanation and technical drawing to help you solve that

Posted by Carmel on 8 October, 2007 - 3:53 pm
3 wire are most commonaly uesed in Process Control, 4 wire for lab applicatiions. You can have head mounted transmitters, multiplexers etc. I suggest you look up temperature transmitters some of the Major manufactures web sites they will have wiring drawings.


Posted by dav on 11 October, 2007 - 12:54 am

I have another question regarding this topic. If I have a 3 wire transmitter and a 4 wire sensor, then I just "shorted" one pair of the sensor wire to become 3 wire and connect to the transmitter. Will this affect any accuracy? Please explain to me if possible.

Thank you very very much.


Posted by Steve Yates on 14 October, 2007 - 6:30 pm
Unless the application needs the accuracy that a 4 wire device can give you, I would just not use the 4th wire. But before you do this, you might want to check you have a 4 wire device in the first place.


Posted by ubtrefkou on 14 October, 2007 - 8:12 pm
In answer to Dav:

You can not do it that easy. I can send you a page with diagrams to show you the details.

Posted by DF on 14 October, 2007 - 8:13 pm
Wont work I dont think.

The way I understand it is that the "third" wire is a temperature reference to compensate for ambient temperature and wire distance effects on the wire itself.

Within the RTD the reference wire is shorted with one of the measurement wires and the resistance measurement from the reference wire to the measurement wire is negated from the resistance across the two measurement wires. The results in a net resistance measurement of the RTD itself.

So.... if you shorted the two reference wires together you would get a radically reduced resistance which would skew your temperature reading.

I hope that made some sense.

Posted by suresh on 9 May, 2008 - 10:28 pm
Accuracy of 2 wire > 3 wire > 4 wire RTD. 3 wire or 4 wire rtds used to nullify the lead wire resistance in addition to actual RTD resistance for long distance cable runs.

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Posted by Roy Matson on 10 May, 2008 - 12:05 pm
The RTD only has two ends, two wires are connected to each end. Since these wires are quite short it will have little effect if you short two together, it's normal though just to ignore the 4th wire. You would only wire as 4 wire if you needed accuracy better than 1 decimal place.


Posted by edie on 24 April, 2014 - 3:15 pm
i have a question, some application, they connect RTD directly to control cabinet (without transmitter at field) while some application use transmitter at field..

may i know, why is it?

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Posted by Steve Myres on 24 April, 2014 - 6:40 pm
An RTD signal is very low energy, so subject to noise. Depending upon how critical the measurement is and how vulnerable to noise, a transmitter may be added to help reduce that vulnerability. For a given system, the level of electrical noise and the length of run from the sensor to the instrument will bear on that.

The transmitter adds cost and another point of failure, but can also provide improved measurement resolution. Finally, RTDs require different inputs than 4-20ma signals and a given control system might not have RTD inputs installed, especially in a small system.

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Posted by Roy Matson on 25 April, 2014 - 12:57 am
To save money

A number of RTDs wired to a multiplexed input card is a lot cheaper than separate transmitters for every point.

One drawback is you can't have local indication.

Posted by Daniel M on 26 April, 2014 - 7:53 pm
besides wire length compensation, 4 wire allow some advanced transmitters to continuously check corrosion in the terminals

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