# Re:Cv of valves.

``` From: Dave Gomberg <gomberg at wcf_com>

<< Here is the sad part.   If your input pressure is 5psi (go below this and
you risk serious instability problems) and your output is 1.5 psi (36" of
water) and your temperature is 70F, then the Cv you need is .000007 for one
liter per day.   If you want one 2mm bubble of CO2 per second, that is
about 4mm3/sec or 15cm3/hour or .4L/day or a Cv of about .000003

That is a long way from .004, or the Hoke value of .0008 which still
doesn't hack it.

This is why I advocate high pressure systems, they work. >>

the only thing to remember is that the Cv ratings are wide open.
And the math requires you to subtract the back pressure of Xinches
of water, and than remember the possible resistance of an air
stone to make the bubbles fine and speed up the absorption.

So as you said a Valve with a Cv of .004 full open (usually 10 turns)
when shut down to one turn drops the Cv to .0004.  Turn it down
further to just 1/4 turn and you have .0001.  This would be for an open
ended air line tube.  Now add in the pressure resistance that you get
from a fine air stone at the end of your tube and I found this is about
right for 4psi of pressure.

Where I will agree with you is that higher pressure will result in less
fluctuation.
Looking at that idea the hooker valve of .0008 Cv turned down to about 2 turns
would give less than .0002 Cv.  Therefore the pressure could come up
considerably and the system would be much more stable.

The real key here is finding the right air stone to break up the bubbles and
add enough back pressure.  Remember the equation involves pressure
differential besides the Cv.

Dennis
```