Dipstick Float Valves for Watering Plants - Part II

Dipstick float valves would be a convenient way to automatically add nutrient solution to long term crops. Dipstick float valves could maintain a constant level of nutrient solution in a hydroponic tank and could be installed after 40 to 50 percent of the initial nutrient solution remained in the tank.
Float blocks could be crafted from extruded polystyrene and these would provide the buoyancy force to stop the solution flow and thus, maintain a constant liquid level. Corks could also be employed as float blocks. In addition, a hybrid model was discussed where a centrifuge tube and an extruded polystyrene block were employed to provide the buoyant force.
Safety of the materials was also discussed.
I welcome suggestions for improvements for these dipstick float valves.