Air pressure and water height vs valve operation time

[MrCedgy]

Ok, so I am trying to build myself a small water tank (about 1 L) with 2 tubes coming from the top. The first one goes all the way down to the bottom (thus in the water) and acts as the outtake. The second one is an air intake where the air pressure is regulated, say 5 psi. Now, I want to be able to time the valve regulating the air intake so as to control the amount of water expelled from the tank (for example, I could want to expel 300 mL).

I know from my classes (I am a future mechanical engineer who freshly started his degree) that the air pressure to maintain a water column of height 'z' to be P=z*rho*g, which would also be equal to (n*R*T/V) by the ideal gas law.

Now, my hunch is that in order to time the valve, I need the number of moles of air that goes from my compressed air to the water tank every second, which could be evaluated from the pressure difference.

The main reason I am doubting myself is because I am pretty sure there is an easier way to deal with this, especially since my air pressure is normalized.

If you need any clarification, I would be happy to oblige!

Have a nice day!

 

[anorlunda]

Allowing air in the tank makes it unnecessarily difficult.

Water is incompressible. If the tank contains only water, it is much easier. To expell 300 ml of water, inject 300 ml of water.

The best way to do that in a calibrated fashion is with a positive displacement piston pump. In your case, I recommend a syringe.