Strange Problem - Low PH Reading - USB Leak

[MDN]

I am fighting a very odd issue with my RA. Robert and I have been discussing in messages but I think we are both out of idea's so I thought I would see if someone has any input.
I have an RA that I setup about two weeks ago. All was working great and the PH probe was reading correctly in my tank.
I decided to setup the Client software to start uploading to the portal. Again, all was going well and I was watching my PC screen with all my data on it.
Then the next day I noticed that my PH was reading about 2 or lower. Robert and I did a bunch of troubleshooting as I thought I had a bad probe. If I took probe and put in it testing solution it read correctly (7 or 10). In my tank 2.
Robert had me put the probe in a cup of tank water, it read correctly. Robert told me a I had a voltage leak in my tank. I checked everything and nothing really made any difference. I tested with a multimeter and it shows .021 volts ( basically nothing).
Then I stated to think about when this bad reading stated to happen, it was when I hooked up the computer.
I disconnected the USB-TTL cable from the computer. Bam - the PH readings returned to normal. Plugged the cable back in and the PH dropped to 2.
We found it right, a bad computer and somehow had screwed up USB ports.
I loaded a totally different PC - same problem - USB port does not matter, tried them all.
I don't understand how the power in the USB could be affecting the PH probe ?
Has anyone else seen something like this ? Anyone have something to try ?

Sorry for the long winded post.
Mike

[rossbryant1956]

I have an issue like this in one of my tanks. Can someone tell me the process for testing for stray voltage? I know it needs a multimeter but how\what do you do with it? Thx in advance.

[MDN]

Put your MM on AC voltage
Place the black probe in the ground hole of the wall outlet
Place the red probe into the tank water.

[MDN]

More information:
I have now moved to a PC laptop. I loaded everything up and the client is up and running.
PH is still reading 1.92 or something close. I unplugged the power brick to the laptop. Bam, PH is back to normal.
On wall power, no go. On Battery power - go.
This is the strangest thing I have ever seen. I also tried a grounding probe, no help.
There must be an issue inside the RA head unit. I can think of anything else it could be.

[rimai]

Let's try this:
Leave the laptop plugged in the wall.
Your ph should drop, right?
Now, keep unplugging every single electrical device you have, even the ones not connected to RA.
You should end up with one that you will unplug and ph will start rising.
Which one is it?

[MDN]

I already tried that a couple days ago. I turned of everything and the PH did not come up.
But I will try again.

[rimai]

Try unplugging everything from the wall and relay box

[MDN]

Robert - Again you are correct.
It is my Chiller that is causing the problem. I never checked it before because it was plugged into a RA relay that is powered off. The chiller has no connection to the water. It has water lines but they drain when it is not running. It has no probe in the water (I am using the RA temp probe). How can an item that is not powered on cause stray voltage ? and it is tied to the computer some how as everything works if the USB cable is not plugged in (I.E. RA only with no monitoring)? Should I not use the RA relay box to run a Chiller ?
Still confused but happy I found the culprit.