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PT100 4 Wire.

06/06/2012 1:55 AM

dear engineers,

i have pt100 4 wire used to sense motor winding temperature, i have to connect it with temperature monitor system (TEC NT-538) where i have only 3 inputs.

pt100 4 wire = two red + two white.

tempt. control device = two red + one white.

please advice, if i connect two red and short two white at temt. control device, is there any complication.

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#1

Re: pt100 4 wire.

06/06/2012 2:29 AM
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#2

Re: pt100 4 wire.

06/06/2012 2:33 AM

I'm not sure, but I think on the 4-wire RTD you should use just one of the white wires and ignore the other. (The idea is to keep the resistance the same in all of the used wires.) But to be sure, you should check the RTD wiring diagram.

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#3

Re: pt100 4 wire.

06/06/2012 2:37 AM

Connect red and white to resistance inputs, other white to 3rd terminal. Cut back (if long) and insulate remaining red.

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#4

Re: PT100 4 Wire.

06/06/2012 4:02 PM

Both Tornado and JohnDG are correct: don't connect the second white wire of the RTD to the "white" terminal on the monitor system. The reason for this is that 3-wire systems are able to compensate for wire resistance only if the two current-carrying wires have exactly the same amount of end-to-end resistance, and therefore drop exactly the same amount of end-to-end voltage.

If you were to land the second white wire to the same monitor terminal as the first white wire, it would parallel the two white wires, resulting in that wire pair having approximately half the resistance as a single red wire. Therefore, a measurement error will result. The longer the cable, the greater the error, all other factors being equal.

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#5

Re: PT100 4 Wire.

06/07/2012 10:21 AM

Imran,

A 4 wire RTD is used for more accurate measuring where resolution and reliability is critical:

1. Both red wires are attached to the (+) (same point) junction on the RTD.

2. Both white wires are attached to the (-) (same point) junction on the RTD.

3. A pair (One (1) red and One (1) white) provide the actual feedback signal to your monitoring device (PLC I/O, controller, display etc.).

4. There is no way to get exact wire/cable length distances when the instrumentation is installed, and in critical applications a deviation of small magnitude can cause significant process control error(s).

5. The second/other red & wite wires are used to measure the impedance/resistance of the circuit so that the monitoring device can "null" the impedance of the cable/wiring between the RTD and the monitoring device.

6. This provides maximum temperature signal feedback accuracy and a means of isolated feedback alarming if either the red or white wire express "out-of-range' readings or failure.

The 3 wire RTD functions the same as a 4 wire RTD however, it uses one monitor wire in conjunction with one of the sensing leads to facilitate use of the feedback signal as a means of measuring the cable/wire length to "null" out the added impedance and correct the offset.

This may seem confusing:

Any 4 wire RTD will work in a 3 wire design application.

A 3 wire RTD will not work properly in a 4 wire design application as the "nulling" circuit will cause/show a "failed" RTD an/or the readings will be inaccurate (offset).

Most manufacturers provide connection diagrams with their product that illustrate how to adapt and install/use a 4 wire in a 3 wire application.

Go to omega.com on the internet and download their free technical files that come with their catalogues. You can get a hardcopy "hard-bound" set of reference books, cd, or e-form documents. (your choice) There is a wealth of information in their documentation that will help.

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#6
In reply to #5

Re: PT100 4 Wire.

06/11/2012 8:40 AM

Hi All,

Please refer to the vendor and request for more detail on the RTD/TC

Regards

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#7
In reply to #6

Re: PT100 4 Wire.

11/12/2020 8:02 AM

GA.

It's probably even in the instruction leaflet that came with it.

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