On a battery storage site we look after, there’s a 4-20 mA tank level transmitter for the coolant loop that goes into an analog input of the site PLC. The PLC is the vendor’s, program locked, no comms licence, and we’re not allowed to touch it. I want the same level in Hermetry, through the ESP32-based node we already have on site.
My idea was a little isolator of my own: read the loop with an ADC, pass it over an isolation barrier, and regenerate a 4-20 mA with a DAC for the PLC, so the PLC sees the « same » signal and I get a digital copy for free. Has anyone built that? Worried about accuracy and what happens to the PLC reading if my board resets.
Could you just add a second resistor in series in the loop and read the voltage across it with the ESP? That’s what I did with a pressure sensor at home.
Dumb question maybe, but why not read the value from the PLC over the network?
@Kevin H. not dumb, but no comms licence on that PLC and the vendor won’t sell one for a site this size. @aisha.maker that’s a ground loop waiting to happen here, the ESP is powered from a different supply.
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.
Went with a loop-powered splitter, the budget worked out with 4 V to spare. The node reads the second output, Scale block gives level_pct, and I kept the raw mA key with an alarm below 3.8. The PLC reading didn’t move when we cut the node’s power to test.
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