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I am doing a mini waste water plant with cascaded PVC tanks (and PVC fittings) for my university project. Each tank has a mixer (the motor is grounded). I made a pH amplification PCB which works with its own battery then send wirelessly the information to my computer.

I was advised that it could be a ground loop, I trace the supposed path on the graphic. Near 1 unit of pH error in the 2nd tank if I compare with a sample taken in a beaker. The error seems proportional to the water flow.

What could be the best approach to solve the issue? Independent PCB for each pH-meter? ground electrodes like stainless steel faraday cages around electrodes connected both together? Or even connected together and with the negative pole of the battery? enter image description here

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  • \$\begingroup\$ Most likely the error is a common mode rejection or range error with your bridge amp or sensor error. Show all details with links to spec sheets \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 16 at 5:08
  • \$\begingroup\$ is it 59.16mV / pH unit ? with temp. compensation? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 16 at 5:17
  • \$\begingroup\$ @SunnyskyguyEE75 I am using LMC6482 as opamp with unity gain followed by an ADC working -1 to +1V with 16 bit resolution. \$\endgroup\$ – Vinlar Jun 17 at 14:18
  • \$\begingroup\$ By putting linked faraday cages around the electrodes, it seems that problem is fixed, the offset error is gone - I have still to try in process conditions. Next time I will use only one opamp/battery per electrode. I didn't implemented T°C compensation because working near 7. I will check mV per pH unit. \$\endgroup\$ – Vinlar Jun 17 at 14:28
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If somebody is having the same problem, I found a solution: Both electrodes cannot share the same ground, a ground loop forms between each reference electrode of both pH meter. Each pH meter must have its own ground (own current generator or use a DC-DC isolated converter).

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