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Re: Needle valves in CO2 systems

I'm just getting caught up, so this is a bit delayed: 
Dave Gomberg wrote:
> Now assuming we set the regulator for 30psi and want 15psi out of the 
> valve, with a flow of one liter per day, we need a valve operating at a Cv 
> of .000001
Woah!  Not so fast there.  The Cv values quoted for valves are pretty
good for sizing valves for liquid flows, but somewhere between suspect
to downright useless for gas flows.  Particularly if you're talking
about a large absolute pressure drop across the valve (> 1.9).  I was
reminded of this when zxcvbob wrote:
> I suspect that at its most restrictive point, the gas
> velocity is approaching the speed of sound (thus establishing a
> maximum flow rate at any pressure).

Yes indeed!  When you hit the realm of sonic flows, you can't just use
a Cv and pretend that you're dealing with an incompressible liquid.  In
particular, a calculation using Cv will in general overestimate the gas
See, for example:
I have used a ARO NO1 valve to control CO2.  While it did not provide
the fine control that I would have liked, it was certainly capable of
keeping a pretty stable CO2 flow at about 1 bubble/sec with an input
pressure of ~20 psig.  I never measured output pressure, but it would
have to be considerably less than 1 psig.  This gives an absolute 
pressure ratio of over 2, so we're dealing with a sonic flow.  In this
realm, the flow rate is fairly stable with regard to changes in
the input (or output) pressure.  The Cv on this valve is nowhere near
the low numbers that Dave is looking for.

I now use this same ARO valve to restrict the maximum flow, but use
an Eheim diffusor as well, so I control my flow rate by adjusting the
regulator pressure.  And my fish recently survived a leaky connection
that allowed my CO2 tank to empty.  The only problem that I have with
this setup is that I have to make a small adjustment of the regulator
and then wait 20 minutes to see what the final stable bubble rate is.

David Ozenne
Fremont, CA

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