# Cv and flow

```At 03:48 PM 9/23/1999 -0400, Peter G. Aitken asked:
>I have been looking into the Swagelock needle valves. The "S series metering
>valves" is indeed the lowest flow rate valve style they make. The maximum Cv
>is 0.004 (valve fully open) but according to the flow data graph I have you
>can get a Cv as low as 0.00025 with the valve open 2 of its 10 turns. I
>think this will be fine for a low pressure system - if not someone please
>enlighten me before I order one!

For gases:

CFM=22.7*Cv*sqrt((delP*P1)/(460+T)(SG))

where delP=P1-P2
P1= input pressure (psi)
P2=output pressure
SG=specific gravity of the gas relative to air=1.0
T=temperature (F)

If you are trying to use a valve to drop regulator output from a stable
10psi to the 1psi you need at the bottom of a two foot deep tank to make a
bubble, and the gas is CO2 and the temperature is 75F, this means:

CFM=7.2*Cv; Cv=CFM/7.2

If we want 1 L (about a quart) per day, that is about 1/40700 CFM.

Cv=1/7.2*40793=.0000034

That is for full flow.  To be able to regulate to +-10%, you need to be
able to modify the Cv by:

.00000034

Do you really think you can do that?   Is this the right design?

--
Dave Gomberg, San Francisco            mailto:gomberg at wcf_com
For low cost CO2 systems that work:  http://www.wcf.com/co2iron
Tropica MasterGrow in the USA:      http://www.wcf.com/tropica
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```