Jump to content

Recommended Posts

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!

Link to comment
Share on other sites

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

Link to comment
Share on other sites

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.



Link to comment
Share on other sites

  • MVP 2021


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.

  • Like 1
Link to comment
Share on other sites

  • MVP 2014

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.

Link to comment
Share on other sites

  • 5 years later...
13 hours ago, denci said:

I am using Unitronic Samba SM35-J-RA22 and pt100 temperature sensor. I confugered sensor as pt100 analog input on MI0 and when i want to read temperature from sensor i alwayg get value 32767, what i am doing wrong with temp reading??

When you measure Pt100 resistance it should be around 120 ohms or so. 32767 means you are out of range and you have open circuit between points where PLC measures resistance. Either your probe is faulty or it is a wiring error, which is more probable.

Link to comment
Share on other sites

I have double checked connection and measure resistance of PT100 and all seems perfect but plc still show value 32767. Maybe something wrong with setting of analog input of plc, maybe there is some trick to configure analog input. I read many description how to configure pt100 and I have no idea what is wrong. Maybe problem is because I am using 2 wire sensor and bridged one wire to second connection sensor with other wire (one wire of pt100 is connected on CM pin).

Link to comment
Share on other sites

Which set of inputs did you connect to ? 

Sensor 0 would use inputs 9 and 10  in reference to CM on input 11

Sensor 1 would use inputs 7 and 8 in reference to CM on input 11

I've set this wrong before thinking 7 and 8 are 0 and 9 and 10 were 1.

Also double check your jumper settings


Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...