P4 conductivity probe read less every day..

[ReefClownMIA]

I have mine on the sump, previously I have it near the protein skimmer out take flow (vertex 150, just water flow), now I changed it yesterday to the first part, where the aquarium download flow is located (just opposite of the sock, and it´s imposssible to have microbubbles in those 2 positions but it works with the same values cycle

any more idea or sugestions?

Both locations are highly prone to microbubbles. I'd have a look at locating them in either lower flow areas, or active flow setup, such as a manifold, or better yet, the Measurement Cell GHL offers.