Can two systems share sensors?

[ozone drone]

My vaults have Partlow chart recorders/controllers using 4-20 milliamp temp and humidity sensors. I have a gazillion dollar metasys (nemesis) system in the building that for some unknown reason doesn't monitor (for alarm purposes) what's going on in the vaults. Can the existing sensors for the Partlows be doubled as sensors for the metasys? or would that be mixing electronics from two systems?

[sysint]

Yes - I think so. What is the impedence of each controller?

[NormChris]

Yes, if the software allows you to. This is called a "Global Point" which is a software setting allowing a single sensor to share its reading with other controllers on the system. Metasys has this feature.

[control_noob]

I think what he is asking is if he can piggyback two different systems onto one sensor, wouldn't this split the resistance of the sensor? Metasys does have global sharing but the point and controllers sharing it need to be on the bus IIRC.

[engineerguy]

4-20 ma is specifically designed to allow one sensor to connect to multiple locations. You are limited only by total circuit resistance and power supply voltage. Total circuit resistance must be less than power supply voltage divided by 0.020 amps.

Just cut one of the wires and run the cut ends to the 4-20 input module in the other system. If its a voltage input module, it will need an external 250 ohm resistor.

[control_noob]

My bad, 4-20 ma, not resistive.

[ozone drone]

Originally posted by engineerguy
4-20 ma is specifically designed to allow one sensor to connect to multiple locations. You are limited only by total circuit resistance and power supply voltage. Total circuit resistance must be less than power supply voltage divided by 0.020 amps.

Just cut one of the wires and run the cut ends to the 4-20 input module in the other system. If its a voltage input module, it will need an external 250 ohm resistor.

Thanks, I'll give that a try.

[cmakowk]

I would just cut the wires to the other system and route them to the nearest JCI controller. Any JCI controller will
accept 4-20 or 0-10vdc except only AHU controller which only accept 4-20. After connecting the wires to the JCI controller you would have to define input in controller and built the point out on Metasys.

[innocon]

If you mix your signals, you'll likely get a ground loop and neither will produce a good reading. You can use isolation transducers, which can be placed in series with a 4-20mA signal. It has a very low impedence, so it does not place any drag on the original signal. It has it's own power supply and provides a scaled 4-20mA isolated output that you can send to the JCI. We got some from Kele and Associates for a chiller plant with 38 such recorders and, of all things, a Metasys control system. It worked very well. The Kele model no. is a DT13-24, and it was about $100 ea. You can also just put a $12 thermistor down there and read that, but that might just be too easy for us controls guys.