Alternate Plan to Using Probes
Chrisbes,
I have been building hobby hydro systems for many years (am currently working on another "flood-and-drain" system, also using expanded clay pellets - Hydroton). I once considered using probes (and other more exotic approaches) for level sense, but abandoned them all as they all had inherent problems.
Now I use a very low-tech solution which works VERY well. My nutrient feed line is flexible tubing, running from the pump (submerged in the nutrient tank), which is inside much larger diameter (I like 1") PVC pipe. The PVC pipe is used as an overflow drain, set at the desired height (below the media top so algae doesn't grow) within the growing chamber. This allows you to pump nutrient with no fear of overflow (you actually want overflow into the PVC pipe, not overflow in you growing chamber), while the returning nutrient (via PVC pipe) falls back into the nutrient tank and thereby aerating it. Nutrient tank aeration is one of the key reasons to do it this way! Obviously, when the pump is turned off, your growing chamber will drain back through the pump, with no aeration. In addition to nutrient aeration, the vacuum created by each draining cycle of the growing chamber pulls fresh air into the root zone; one of the reasons why flood-and-drain is a great system and why plants grow SO fast! Enjoy.
Richard