Does anyone know of a device or method to measure a 4-20mA signal then superimpose the value of this signal via HART on a second 4-20mA loop?
This would basically be the opposite of what a Rosemount Tri-Loop does.
I have a field device that has two 4-20mA outputs for process variables, but only 1 twisted pair cable back to the IO. I'd like to superimpose one of the process variables on the 4-20mA of the other variable, then use a Tri-Loop 333 to split them again in the IO cabinet.
Moore THZ3 can take 2- 4-20ma and convert to Hart, then use a triloop.
Viatran 970 pressure transmitter uses Hart. It looks like you could use a tri-loop to access 3 of these transmitters similar to what is shown on page 13 of the manual referenced above. Or if you used an Emerson DCS, like Ovation, that has Hart input modules, you might be able to access multiple Rosemount smart transmitters on one analog input.
Another option is to use a Hart concentrator and convert the twisted pait to modbus 485. Moore industries has a Hart Concentrator:
I've never done multiple Hart like you want but it sounds possible, good luck!
No but I have several analog to rs485 devices that would potentially work for you. They come in ethernet too.
Thanks everyone for your answers. After doing more research, it looks like I should move to Foundation Fieldbus for this problem. I found that Smar makes IF302 and FI302 converters that will do three channels of 4-20mA to Fieldbus and then back to 3 channels of 4-20mA.
I think FF would be overkill. Foundation Fieldbus doesn't need to use 4-20 mA. It takes the signal (for example pressure) and converts it to engineering units over the whole range of the cell e.g. 0 - 250"WC and transmits it as a digital signal. It's not scaled in the transmitter.
I may not be 100% correct in the units it may be some non engineering number like 0 - 10,000 wigets, but not analog.