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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 http://www.rosemount.com 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.

Carmel


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

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.

Rgds,
Dav


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.

Steve
www.mtl-inst.com


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.
ubtrefkou_5432@yahoo.com


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.

Regards
Roy


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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Posted by Sam on 9 September, 2014 - 4:30 am
There still seems to be confusion is some areas so I will do a summary of all the questions asked so far and that will hopefully clarify things for everyone. In the old days we used two wire RTD's and would run these two wires all the way to the electro-mechanical temperature indicator that might have been situated 100 meters away from the RTD. We then realized that the indication was not accurate since the resistance of the two wire runs is added to the resistance of the RTD, so looking at the indication we had to do a manual calculation in order to get an accurate temperature indication. Due to this manual calculation that needed to be done every time it was soon realized that by just adding a third wire the electronic indicator will be able to do this simple calculations itself. The calculation was simple since all you needed to know was the resistance of one leg and you can then program the electronics to deduct twice this resistance from the total input resistance and you will be left with only the resistance of the RTD. So the third leg was used for that, just to measure the resistance of the one leg since this extra third leg's resistance will be identical to the one of the legs connected to the RTD. Problem solved.

Someone then later suggested that there might be a slight resistance difference between the measured third leg and the leg actually connected to the RTD so the fourth leg was introduced to further eliminate and get a better average for the resistance of one of the legs connected to the RTD. So there you have it, the four wire RTD.

Now take into consideration that this was in the old days. With today's technology of having a RTD connected to a transmitter that is situated in the field there is really no need to even go with a three wire RTD since there is hardly and line compensation that needs to be done on a distance of a hundred or four hundred millimetre length of RTD wire. The compensation will be something like 0,01 Ohms that must compensated for which is so small that it can be disregarded. The norm however is still to use three and even four wire RTD's on these transmitters since it is a matter of the design engineers saying, better safe than sorry but it is really just wasting money. Their answer to that is that the price difference is so small one might as well use the three and four wire RTD's.

Also just for information, as a standard it is never a good idea to do long direct runs of RTD wires and the best practice is to get your transmitter as close to your RTD as possible in the field, and run the 4-20mA output to your control system, PLC or indicator.

To answer one of the other questions with regards to not using the fourth leg or not using the third leg. On your transmitter there is a little drawing and it shows you how to connect either a two, three or four wire RTD. If not it might have numbers in which case you will need the manual of the transmitter to show you where you need to connect each leg. Depending on what you want to use you need to set the software to the same on the transmitter. If you cannot access the software of the transmitter but you are sure that it is set for a four wire RTD and you only have a three wire RTD, put a bridge piece in on the transmitter's terminals where the fourth leg would have been connected. The transmitter will then see your three wire RTD as a four wire. If you have a four wire RTD and you want to connect to a transmitter that is set for a three wire RTD, just cut or tie back the fourth leg of the RTD and just don't connect it.

Like explained above your transmitter will still work if you don't do it like above but your compensation will not be as good. Above is just the ideal to get full compensation.

What PT100 stand for have been explain by Sandy already and I cannot say more than that or give a better explanation.


Posted by san on 21 September, 2014 - 7:15 am
For a 10m cable, which is the best options?

2 wire/3 Wire/4 wire RTD.

I am going to test the temperature profile in a laminator used for PV module manufacturing. I am using 10m cable (class 5 EN 60228 standard) to connect the leads of PT100 RTDīs placed on a test laminate to data logger.

Could any one please let me know, is it a good idea to use 3/4 wire RTD to compensate resistance of 10m cable (& connecting string between RTD lead and cable).

thanking you.


Posted by d on 21 September, 2014 - 10:47 am
3 wire is more than adequate at that distance. your vendor can better inform you as it depends somewhat on the RTD particulars.

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