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# Cv and valves

```At 03:48 PM 9/13/00 -0400, Bob asked:
>Micro-mite Forged Metering Valves: 1600 Series     Cv = 0.0008
>
>Is this Cv rating sufficiently low that it could be used straight off
>the CO2 tank valve w/o a regulator?

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)

P1 is typically about 15, P2 about 2, delP about 13, SG about 1.5, T=70,
CFM (at 1L/day)=1/(30*24*60),
so Cv=1/(30*24*60*22.7*sqrt(13*15/530*1.5)) or about 1.8*10**-6, .0000018

THE CRITCAL POINT HERE IS VERY LOW FLOW.

Running straight off the tank, P1=delP=800,  so it works out like
Cv=3.6x10**-8 or .000000036

That's very very small.  You won't get there with a needle valve that you
can afford.  And it wouldn't be thermally stable anyway.

Just got a price quote on a Hoke valve for giggles.   \$107 for the
valve.   Then you need Gyrolok fittings to get out of their proprietary
connection system, those are a couple of bucks a piece.   Then you would
need an adapter from NPT to barb, those are a buck or so each.   Then you
would need a barb and barb to get to the right size, that's another
buck.  And a short piece of 1/4" hose to connect the big barb to the
reducer.  And in the end you would have a great valve jury-rigged to
work.   Not what I would call workmanlike.   You see, our flow rates are
way, way, way too low to be anything like a close match to what is offered
commercially.  But Hoke makes a great valve.

--
Dave Gomberg, San Francisco            mailto:gomberg at wcf_com
NEW Planted Aquaria Magazine:        http://www.wcf.com/pam
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