So here is how it works:
1) An air pump (~$10), controlled by an electronic timer ($8), powers the dosing. The airflow is split into two via a T.
2) A little airvalve adjusts the airflow through the dosing line. This is not extremely critical, but I find that if the flow is too high, droplets splash through the tubing and can collect after the pump has turned off, preventing an easy refill.
3) This check valve allows nutrient solution to flow into the tubing, and prevents it from exiting when the air pump is running.
4) This check valve prevents solution going up the airline, and therefore skewing the dosed amount depending on the level of the nutrient solution.
5) This tubing contains the volume that will be dosed, which depends on the length and diameter. In my prototype, I am using a 3in piece which results in just a little under a milliliter dosing volume. Extremely easy to adjust, of course.
6) I use mini tubing for this part, to minimize the amount of liquid that fills it to the level of the solution. Also, mini tubing should help to push the liquid out completely.
7) As described in the first post, this serves as a solenoid that is closes while the pump is working, therefore creating back pressure so no more liquid can enter through check valve 3. This syringe shows clearly how many ml are being dosed. First, the liquid is pushed into the syringe. As the "solenoid" closes, pressure builds up and finally pushes the liquid through a tiny needle hole out of the syringe into the dosing line 9 and into the tank.
8) This air valve determines how much air goes into the "solenoid", which is a very small amount, unless you want the little pop.
That's pretty much it, dosing a consistent amount each time the timer kicks of the cycle. I am sure this can be beautified and maybe simplified, but the basic concept seems to work.