Don’t build the ADC/DAC thing. If your board resets or the DAC glitches, the PLC sees a level jump and you’re now the person who tripped the site. Anything you put in that loop must keep the PLC’s reading correct even when your side is dead.
Two things I’ve used, both off the shelf:
- A loop-powered signal splitter/isolator, one input, two isolated 4-20 mA outputs. The PLC gets one output, your node the other. Loop-powered ones take the power from the loop, so check the voltage budget: supply minus transmitter minimum minus the isolator’s drop minus the PLC input’s burden must stay positive at 20 mA.
- Aisha’s idea done safely: a small series shunt with an isolated analog input module that has Modbus RTU output. The isolation removes the ground loop Ciarán worried about. Your gateway reads it over RS485 like any other Modbus device. Same voltage budget check, a 50 ohm shunt costs 1 V at 20 mA.
I prefer 1 because the PLC’s signal path is a certified part and not a shunt someone may remove later.
Then in Hermetry send the raw mA and convert there. In the root flow, a Scale block before Save telemetry maps 4-20 to your tank range, and you keep the raw value for fault finding (below 3.8 mA means a broken loop):
Scale: in_min 4, in_max 20, out_min 0, out_max 100 -> level_pctThe derived value example is on Derived values.