Hello.  I am trying to figure out how to determine what the pressure is inside of a tank which is being filled with water and air at the same time, with known flow rates.  I know that there is a pretty easy way of doing this, but I can't think of it.  The initial volume of water within the tank is not known, only the diameter.  There will have to be an open vent in order to allow air to escape.  (the tank is designed for only about 2 psi.)  I also need help in figuring out the size of this vent.  I was thinking that it could be sized to let a certain amount of air escape, but I don't know the velocity of which.  The vent will be simply  a hole in the top of the tank, covered so as not to allow rain to get in.  

Thanks for any help you can give me.

Kay

Kay,

If the tank has a direct opening to the atmosphere, the pressure will be atmospheric, provided the opening is big enough.
Concerning the diameter of the venthole, this depends on the fillingrate of the tank.  Normally the velocity of liquids being pumped is limited to 1 à 2m/s, but gasvelocity is allowed to be much higher ca 10m/s at least.
This means that the required diameter of the vent hole could be approximately .3 times the diameter of the liquid inlet nozzle.
One other thing, overpressure of the tank can be a problem, but so is underpressure, if the outlet is blocked whilst you are unloading.  In that case the tank may collapse.
Wim