Using an pH meter and EC meter at the same time?

Hey!

I just realized i cant have an EC probe connected to ground and/or VCC at the same time as the pH meter while they are both in the water. Because of trouble with getting good readings.

So when I'm gonna use the EC meter i want to isolate the pH-meter completly from the power supply, both + and - leads must be cut off. And vise versa.

Anyone know a way to build a circuit to solve this problem? Please help!

  1. let your circuit run off batteries.
  2. depending on the EC probe you use: isolate it using capacitors.

My problem was more than the EC went haywire the moment I had the pH probe in the water. Same underlying problem. Changed the EC probe design by adding the caps, haven't tested it in combination with the pH probe yet.

Thank you for replying!

I will try with batteries. But, I'm not sure about what you mean when you are saying isolate it using capacitors. Am i supposed to set the capacitor in parallell with the EC probe or one cap in serial with the + and one with the -?

That's what I'm doing now with my EC probe. Two caps, one on either side. The AC signal goes right through (2.2 uF ceramics work great for me). Whether it works for your probe, depends on what probe and what technique you use to measure the EC.

Have a look at MAX407 datasheet for lithium cell powered pH amp.
Works a treat, 10-years life from the cell and no ground loop problems.
Any single supply op amp with FET input and very low input bias will work
Look up electrometer amps for pH examples, but essentially, no problem, plus the bonus of not having to use fussy coax cable

I've seen the schematic, don't recall a mention of no ground loops, and they still suggest to connect to a coax cable.

The problem is that the coax output connects directly to the MCU, so to make this work you still have to connect the shield of the coax to the ground of the MCU, and there you get back your ground loop. No complete galvanic isolation between the two, which anyway is tricky for an analog voltage.

Agreed, but most commercial combined electrodes will come with coax - ideally, you put the op amp input directly onto the electrode inside the cap and old applications went to a lot of trouble to ensure guard tracks on the PCB and using a "leg flying" method to connect so the op amp input pin was clear of the PCB.
Ref the ground loop, I can only say that having put quite a few of these together in industrial situations, there was no evidence of problems, and where there had been a problem before, using the battery approach solved it for me.
I had to work in tricky situations where the main earthing relied on local earthing rods and the pH setup often had to be used next to say conductivity electrodes which were directly in contact with "ground" and all connected to the main data collection (SCADA) system.
All I can say is that it worked where other approaches had failed and there was pressure to provide a solution.