4-20mA Ground Loop Condition Current Loop Receiver

I have a 12 channel 4-20mA receiver with I2C interface and three more on order (I have currently halted the order to clarify the situation) used for monitoring various aspects of water quality

I have been told by the manufacturer of the probes we are using that some probes cannot co exist in the same system without the use of a galvanic isolator.

“The inputs of your PLC are not isolated, differential sensors (P65 and R65) are using measuring water as “floating” ground and therefore interfere with each other.”

I just wondered if there was any product you have that is isolated by design or if its just the case that we need to purchase an external isolator such as something like this: https://www.asi-ez.com/member/~ASI451129.asp



could you share the part number of the sensor you will be interfacing with the 4-20mA board?


Hi there,

Thanks for responding so quickly, here’s a link to the datasheet.


a few more things

  1. What is the max number of sensors you will be connecting with one board?
  2. are you planning to use a single power supply with all the sensors?
  3. Have you tried using two or more sensors at one time using a single power supply.



1 We plan to use up to 12 ports on each board.
Per board we would be using two to three of this type of sensor which have a “Floating Ground” the rest of the sensors (Dissolved Oxygen, Torridial, Total Gas Pressure etc are “isolated by design”)

2 We are planning to use a 24v 1.2 amp PSU for a single system (up to 12 sensors) and a seperate 12v psu to power the 12 port receiver board.

3 We are currently using 6 Sensors with a single supply but we think we are getting a small amount of interferrence between the pH and ORP Probes. If we put a second ORP probe into the system all sensor measurments go dead



Sorry when I said all sensor measuremernts go dead its not quite right they go wrong.
Please see the attached graphs you can see at around 11:30 is when I put the second ORP Probe into the system.

1 Like

looks like at 11.30 the readings go down and around 12.00 they go back to original readings.

correct me if i am wrong
did you connect the second ORP at 11.30 and removed it at 12.00?
did you change anything else in the setup which brought back the readings to original readings ?

Yes, I took the other additional sensor that I put in back out of the water.

Lets do this
use two power supplies. one for first 6 sensor and one for reset 6 sensors.
and see if you still encounter the issue.

OK I can try this next week when I am back on site.
In the meantime I need to confirm that there isnt any other type of 4-20 receiver you make that has "built in " isolation. If not then I will just go ahead and unfreeze the order for the additional 3 12 port units and we can figure out a solution to fix the issue even if the answer is buy additional isolators for the floating ground probes.

Thanks for your awsome support by the way. I love this forum first policy, its a great idea, made even better by your quick response times.

at this point we dont have a 12 channel receiver with built in isolation ( we have one channel with built in isolation).
if the power supply fixes the issue then you will know its a sharing ground issue.
You can fix this with this part

your UI looks great. would love to know how you built this.


OK, thanks for that i’ll look into those isolators. Please can you post a link to the 1 channel isolated board so I can take a look, also, can it co-exisit on the same bus with the 12 channel board ? If its isolated does that mean it could share the same PSU with the rest of the sensors.

BTW, The UI is standard Grafana.

The solution stack is: 4-20 > NCD board > ESP8266 > mqtt > Node Red > Influxdb > Grafana



the isolated 4-20mA receiver is not on the website.
it looks like this( with on board power and signal isolation)

Yes, you can use it on the same bus. this board comes with on board power supply as well, so it wont need the external 24V supply.
this will increase your cost, so i will recommend using power isolators with 12 channel board you have.