That's really the same scenario, but the friction of the plug would actually make it worse; as resistance to movement increases the air compression.
It's also similar to another existing DIY system. Use an off-the-shelf plastic syringe. Eliminate the air pump, and instead use a motor to directly drive in the plunger via some threaded rod and a nut. This works well and is very consistent, the only limitation is the syringe size.
Though with access to a machinist, you could create a very large syringe.
i don't think you would need to pressurize your liquids (though you could)
if the idea is that it doses based on fert lvls, then it would do more quantity of doses to compensate for less volume per dose correct?
also how do we keep things from growing in the liquid ferts, i use dry, but i thought liquids grew fungus over time if left un checked
My orignal plan was to do a one inch cylinder but after realizing I would need a hydro cylinder I had the CAD guy increase the size to 2 inches. hoping this will give me enough volume that A) I can spend less time filling the tubes B) smaller shaft requiring a smaller motor to run. So looks like I will see what I can find for DYI hydro cylinders.