hello,

i am an student and need to connect a pt-100. now is my question what is the best way to do it? a 2 cable connection or a 3 cable connection?

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theSiemexpert,

"Cables" are generally considered to be multi-conductor groups of conductors (wires).

RTDs can be two-wire, three-wire or four-wire devices. Meaning they can generally be connected to the monitoring device using two, three or four wires. The number of wires is usually determined by the monitoring device capabilities.

More wires means a more accurate reading, so the degree of accuracy required by the process will also play a role in choosing the number of wires to use when connecting the RTD to the monitoring device.

One should ALWAYS use twisted, shielded cables (two-wire; three-wire) and pay attention to the proper grounding of the shield drain wire to prevent induced voltages from affecting the accuracy/stability of the reading. Maintaining proper segregation from other high voltage/high current wires/cables is also very important, especially in industrial environments.

www.omega.com has excellent descriptions of all manner of sensors and instruments which are very informative and helpful.

Hope this helps!

 

A 2 wire RTD will ALWAYS have some offset error due to the resistance of the copper connection wire. The longer the wire, the higher the resistance, the larger the error.

For practical purposes, that error can be eliminated with a 3 wire RTD.

Get a 3 wire RTD and use the 3rd wire. The 3rd wire has to be the same length and gauge (thickness) as the primary two wires in order for the compensation to work properly.

 

Sir i would like to ask that as u've said

> RTDs can be two-wire, three-wire or four-wire devices. Meaning they can generally be connected
> to the monitoring device using two, three or four wires. The number of wires is usually
> determined by the monitoring device capabilities.

> More wires means a more accurate reading, so the degree of accuracy required by the process
> will also play a role in choosing the number of wires to use when connecting the RTD to
> the monitoring device.

Now as we increase the no of wire the accuracy of the RTD increases. So,
2 wire RTD<3 wire RTD<4 Wire RTD...

Can u explain i detail why this is the case?

Also i earlier thought that choosing 2 wire, 3 wire or 4 wire sensors is generally related to the cost that an industry is ready to invest as 4 wire sensor will be more costlier than 3 wire and 2 wire sensor as a 4 core cable has to be laid till the controller/transmitter/DCS/PLC...

 

amrit1391,

Your thought process was flawed, sir.

Here's a link to a very good description of RTD wiring methods:

http://en.wikipedia.org/wiki/Resistance_thermometer

The section titled, 'Wiring Configuration' is very informative.

The extra leads (more than two, that is) are used to compensate for the resistance of the wires used to connect the RTD to the monitoring device/equipment. Since the monitoring device is looking at voltage or flowing through the entire circuit--the interconnecting wires AND the RTD--the resistance of the wire, if it's a long distance between the monitoring equipment and the RTD, can add a large error. So, the monitoring equipment measures the voltage developed in the interconnecting wiring and subtracts it from the entire circuit voltage thereby resulting in a more accurate temperature reading.

2-wire RTDs have no wire resistance compensation. 3-wire RTDs have the least wire resistance compensation. 4-wire RTDs have the most wire resistance compensation.

And, it's not necessarily the RTD that's the costly part of the choice of whether or not to use 2-, 3- or 4-wire RTDs for an application, it's the monitoring equipment which must have the additional sensing capability. I've seen many four-wire RTDs used in 3-wire service--they work just fine with one lead left unterminated (disconnected). I've even seen 2-wire RTDs used in 3-wire applications with excellent results.

It's just that simple. Nothing more and nothing less. I've seen 3-wire RTDs connected to a monitoring device using two-wire cables (twisted, shielded pairs) with great results. It all depends on how the wiring between the RTDs and the monitoring device is accomplished. All that's necessary is a method for measuring the wire resistance between the monitoring device and the RTD.

Please, have a read of the two links provided in this thread--they're very good resources.

And don't overthink RTDs; they're very simple devices. It's the monitoring device/equipment that's much more complicated--and expensive. And, the way the wires used to connect the two are used is also very critical.

MOST problems with RTD circuits come from a very basic misunderstanding of how RTDs "work"--or more correctly, how RTDs have to be wired to monitoring device/equipment to work properly. Failure to connect them properly will result in a LOT of wasted time and effort, and failure to use the proper shielded wiring and to properly ground (earth) the shield drain wire, and failure to isolate the interconnecting wiring from high voltage/high current wiring/cables will also result in a lot of wasted time and effort. Usually, the voltage/current impressed on RTDs by the monitoring device/equipment they are connected to is very low voltage/current, and so is very sensitive to high voltage/high current wiring run in the same conduit or cable tray or trench. I've seen a <b>LOT</b> of motor RTD problems caused by people trying to run the motor RTD wiring in the same conduit as the high voltage motor leads (440 VAC; 4160 VAC; etc.). This just doesn't work very well. Yes; the motor RTD leads are in the same junction box on the side of the motor as the motor leads, but, the junction box usually has two or more penetrations for conduits--one for the high voltage wiring, and one for the low-voltage RTD wiring, which should also be run in a separate cable tray with other low voltage/low current wiring.

Hope this helps!

 

Thanks for u time and explanation. It really helped.

 

> Thanks for u time and explanation. It really helped.

I thought CSA's explanation was excellent.

Just adding to the discussion

3 wire is adequate for most installations, you might only use 4 where you need to measure temperature to a decimal place. If you have a 2 wire RTD just twist two of your lead wires together in the RTD termination head and ignore the short length from head to the actual element.

If you have multiple RTDs or a combination of RTDs and 4-20 mA devices terminated to a multipair junction box it's ok to use 1-1/2 pairs for a 3 wire RTD.

Often RTD measuring devices are multiplexed so for the home run multipair you can use just one compensating wire, for example you have 6 RTDs into the J box, you need 6-1/2 pairs.