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CliveC

PT100 on V350

6 posts in this topic

Hey, I am new to this programming/hardware environment and I am struggling to get my head around how to get a PT100 temperature gauge to work? The PT100 I am using consists of 2 leads, and I am using the V350-35-TRA22. I have changed the jumper settings according to the manual, and setup the inputs in my hardware configuration, however when trying to access the value via a HMI screen, it outputs 32767, saying the sensor is not found?

Is there something that I am missing out??

Any helps or pointers would be appreciated, Thanks!

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Most PT100 sensors have three leads, and that's what the Unitronics input is designed for. Are you sure you don't have a thermocouple? Are the leads different colors? If you measure between the leads with an ohmmeter, do you get some value above 100 ohms or do you get a dead short?

Joe T.

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Hi Clive,

To get the most accurate measurement we highly recommend to use 3-4 wire RTD sensor.

Using 2 wire sensor may lead to very unstable (sensitive to noises) and un-precise (affected by wire impedance) reading.

If anyway you have two wire RTD sensor, you need to connect one of the wires to T+ and T- (both have PT label as well) and the second wire connect to CM.

You need to define selected input as PT100 in hardware configuration in accordance to used PT100 type (alpha=0.0385 or, alpha=0.0392) assign MI and select C(celsius) or F.

Then you will see in assigned MI raw number, which is 10xt.

Example: you see raw number 247. Temperature is 24.7 C.

When MI = 32767 Sensor is not connected to input, or value exceeds permissible range,

or sensor is different than PT100 (for example, PT1000).

When MI = -32767 Sensor is short-circuited

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I am curious,

I have used 2 wire 1000 Ohm RTDs for about 12 years and have never had a problem with noise (that I am aware of). I have not used an RTD with a Unitronics, but is there any test data that anyone knows of that would show me the relative affect of having a 3 wire RTD over a 2 wire RTD in similar noisy environments. Or is it simply a matter of accuracy in that a 3 wire is more accurate than a 2 wire in all conditions. I know that the 2 wire units we use have always had better accuracy and much lower noise that any thermocouple I have ever used which is why we went that way in the first place.

It is completely accidental that I have allways used 2 wire units, it is just what was selected from a companey that has a very inexpensive converter from 1000 Ohm RTD to 4-20ma that we needed at the time and we have never altered what we are doing with that product.

Thanks

Keith

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Keith-

A 1000 ohm RTD is typically two wire because the ohm or so of lead resistance has a small (< 0.1%) effect on the temperature reading. A three wire 100 ohm RTD is much more common and allows accurate resolution of 0.1 degree but obviously lead resistance becomes an issue. The third wire is used in an adaptation of a 4-wire ohmmeter to actually measure the lead resistance and subtract it from the RTD resistance.

Joe T.

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Along with Joe's post, the main use of the 3rd (or 4th) wires is to compensate for wire resistance. I suppose it could also eliminate noise if the noise was affecting the main sensor wires as well as the compensation wire, so the noise was then cancelled out.

If you only have a 2-wire probe and the interface expects 3 or 4 wires, you can trick the interface by bridging the compensation terminal to one of the RTD terminals locally at the interface. This of course defeats the purpose of compensation, but at least allows the interface to operate. If the resulting accuracy is sufficient for the application, then you have achieved your purpose. Take a close look at the Unitronics wiring diagrams when doing this, as wiring errors are a common reason for non-functioning RTDs.

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