Transmitter issue

Dear All
An instrumentation issue -
A machine is utilising three 4 to 20ma pressure transmitters for control purposes. They are all on different circuits which have their own supplies.
For certain reasons it has become desirable to have just one transmitter perform the function of all three (same pressure being measured) - how can this be achieved? A splitter has been tried and whilst this did indeed generate three signals from one instrument (the original and two additional outputs) it could not be connected into the existing installation - for each circuit a loop was required and it's own voltage was present, whereas the splitter generated its own power etc.
Is there a method or device that can do this?
Any advice is greatly appreciated.
Thanks in advance
Steve
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Steve & Lizzie wrote:

You could try these companies:-
<http://apcs.com/ssp235/ssp235.pdf <http://www.amelec-uk.com/
You could also talk to the company that provided your original splitter module to see if they do unpowered outputs on a similar module.
Finally, the decision to go with just one sensor will leave a single point of potential failure for your control scheme so I would try to get a second sensor in the mix somehow.
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If push comes to shove, simply convert the 4-20 ma signal to a digital signal, then provide that to three d/a converters to provide the final signals.
Michael
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Steve
Is the problem that all 3 devices that will receive the signal are trying to power the loop ?
There are isolators that have outputs that themselves require power (ie they look like transmitters)
Which country are you in ?
Steve
Steve YATES MTL Instruments Luton, UK www.mtlgroup.com
Steve & Lizzie wrote:

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If I understand correctly you want to lose two transmitters and just use the sgnal from one at three different places? Maybe one transmitter is used for pressure control and the other two are just for indicating displays somewhere. The first thing I would do is check all three units that receive the signal and see if any of them can be configured to accept "isolated inputs". By that I mean the 4-20 is powered from an external source and the units do not connect the signal to ground inside the instrument. If the input resistance is low enough on the receiving units it's possible that the power supply that's powering the loop has enough balls to drive two additional units. If that's the case wire the 4-20 in series with all three units receiving the signal. But that will only work if the display units can be configured for isolated inputs.
I once had an AB PLC that was controlling tank level. A Bristol-Babcock PLC needed to see the same signal. Both PLCs had powered analog input modules. I used a Weidmuller signal conditioner that would accept power on both loops. The only caveat was that the conditioner needed an additional 24 volt power supply. The conditioner I use is on page 19 of the pdf below:
http://www.weidmuller.com/downloads/pdfs/datasheets/LIT0310_AnalogSignal.pdf
The input resistance on these conditioners is only 50 ohms, which is pretty low. It's possible that you could insert two of these in series in the main control loop. That would give you the two additional 4-20 outputs you're looking for. You will need an additional power supply *just* for the conditioners however.
What are the three signals used for? What are the input resistance/voltage drops of all three receiving devices? What are the ratings of all three DC power supplies?
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