pH readings wild when pump runs

[piscinepleine]

Hi there,
I have a home made controller with a pH probe that I mounted on the tubing after my pool filter.
So far I've had no luck with value accuracy as you can see in the screenshot.

• Pool water in a jar: Stable
• Pipe with pump off: Fluctuates
• Pipe with pump on: Fluctuates wildly

I read online that it's best to use an analysis chamber (which costs a lot) or maybe a bypass (like in the raspipool diagram). However when I look at diagrams or peoples' installations online, a lot of them just have it inline with no issue.
I also read the value could be disturbed by residual current in the water and you need to ground the water to earth.
Any idea?

 

[PoolGate]

Sounds like maybe there is air in the line coming from your filter. Do you ever see bubbles coming out of your return?