I have a water tank that I want to measure the temperature of. Unfortunately, I only have room for one thermowell and I would like to share the temperature reading with three inexpensive controllers. Can I connect a transmitter and convert to 4-20 ma and run the three controllers in series? Are there other options?

 

See the blog on this topic at the link below:
http://tinyurl.com/mtaqm6f

> Can I connect a transmitter and convert to 4-20 ma and
> run the three controllers in series? Are there other options?

Yes, but . . . .

Depends on how much (analog input) resistance the transmitter can drive.

You could run the transmitter's output current signal through one or two controller analog inputs (in series) and parallel the voltage drop across the analog input of one of those controller inputs to the 2nd or 3rd controller input.

That might have a common mode problem. You don't know until you try it.

 

Yes, that should work, assuming the total loop resistance is within the transmitter's limit and the controllers are passive and not designed to wet the loop.

Other options: you can get probes that have two or more tc junctions or RTD elements in the same probe. You can also get transmitters with more than one output or convert to a single 4-20 signal and there are also isolators and signal conditioners that will give you multiple copies of the same signal.

 

If you connect three controller to the same RTD, which is technically possible as other posters have indicated, and you are using feedback control, your controllers will all share the common controlled variable and will be cross coupled (not independent). This can result in the controllers fighting each other.


William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.

 

Yet another way

Convert the temperature to mA (4-20) then send it to one receiver.
If that receiver is normal 250 Ohms you get 1 - 5 Volts across the input terminals, run that to the other receivers set up for Voltage input.

There are several other options.