thermocouple inputs in parallel


Thread Starter

Gantt Robert

Hello, we want to monitor (and later control) some 16 temperatures currently connected to independent single loop controllers. We'll be using PLC Direct 8 input thermocouple modules (for the DL405 series) for the
temperature monitoring. My question is, can we connect a thermocouple to both a single loop controllers and the PLC module? Is there a possibility of inaccuracies with this scheme, due to the low signal levels?

I know we have to be careful about not exceeding common mode voltage limits for the PLC modules, and of course we'll be using proper t/c extension cables,
etc., but our concern right now is only about possible drifts and inaccuracies. Thanks in advance for any comment.


September, Clyde

One of two options -
a) Dual pole thermocouples - allowing two circuits to read of one
b) Converter providing a current source e.g. 4-20mA for than 1 sensing
Both should provide reasonable accuracy's, but what is reasonable a trial
should be able to tell you.
...Hope it helps..

Clyde September

Thanks for the advise, but in my case option (b) is really not an option since I already have the thermocouple input PLC modules and the thermocouple input loop controllers. As for option (a), I don't know what you mean by dual pole thermocouples: are they just regular tc's with 2 connectors? Does that mean that I will get correct readings if I just add another connector to my "normal" thermocopule, connecting the PLC input to it?

Of course, I could just go ahead and test, but since I have to schedule a maintenance stop to make the changes, I would prefer to know beforehand of any possible problem to minimize downtime. Thanks again, Bob.
I have connected a single thermocouple to two seperate instruments with no problems in accuracy, but there is another problem of which you should be aware: Most TC single loop controllers have a burn-out detection feature that will not function if another instrument is connected to the line. If your TC burns out, or corrodes out, which it eventually will, then your process might run away. Most TC manufacturers make a dual element TC probe, that is a TC probe that has two seperate jucntions in the same sheath with independant connections, one for the loop controller and one for your PLC. This is a much better approach.
I have not tried this ever.
But i believe that most TC inputs put the TC in a bridge and therefore
connecting to two places should create some confusion to the systems and
result in different bridge values.


Trevor Ousey

I would be cautious. What about using temperature controllers which can re-transmit to an analogue signal like 4-20mA. I would think TC inputs are relatively expensive compared to standard analogue. Like wise use some other form of converter.

gerald beaudoin

Rather than run a single thermocouple to multiple input devices(which I have never tried), I would prefer to run the single thermocouple to a 4-20ma converter and then put all the input devices on 4-20ma. This way you are sure of no interaction and the thermocouple is totally isolated and seeing only one device as a load. If you haven't already purchase your hardware, this may be an option.

Gerald Beaudoin

John Paley--Graphic Pkg Corp


I've done it--in a pinch--but do not reccomend it. We have VOC oxidizers wherethe temps have to be controlled by one instrument, and recorded by another. The solution was to install dual element tc's (one probe with two thermocouples). Hope this helps.

j paley

Michael R. Batchelor

> > My question is, can we
> > connect a thermocouple to both a single loop
> > controllers and the PLC module? Is there a
> > possibility of inaccuracies with this scheme, due to
> > the low signal levels?

Yes, you can do this. No, it doesn't work except under some very special
circumstances. I've tried it, and I've found that generally speaking you need to plan for a lot of problems. Unless your maintenance labor is cheap and your downtime loss negligable, this doesn't describe any factory I know, then you would do far better to decide on one instrument to act as a relay to the other. Just use a controller with a 4-20 PV output, or use an output card on the PLC to send the PV to the controller.

Michael R. Batchelor - Industrial Informatics & Instrumentation, Inc.
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