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Re: Cv for your micro-metering valve

Dave Gomberg <gomberg at wcf_com> writes:
> At 03:48 PM 5/9/99 -0400, Steve Pushak wrote:
> >The less expensive valves such as the ARO NO1 (part # 104104-NO1) have a
> >full open valve coefficient Cv of about 0.04 whereas the Nupro S series
> >has a Cv of about 0.004
> 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

I think you are referring to transition from laminar to turbulent flow
regimes here. Normally when the input pressure is twice the output
pressure for compressible gas, the flow regime is fully choked (i.e.
sonic) and turbulent. You have to add the ambient pressure (14.7 psi) to
both inlet and outlet pressures when calculating choked flow. I have
never experimented with using a pressure below 5 psi to see if the flow
regimes are unstable but I'm not sure that the theoretical models would
predict that. Are you sure of this Dave?

> 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.

The Nupro has a Cv of .004 FULLY OPEN. We always operate the valves in
the nearly closed position.

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

The valves also apparently work! The fact that we can achieve stable
bubble rates in the ranges you quote implies that we are able to adjust
the valves to those co-efficients. 

I'm not arguing the pros or cons of the "high" pressure diffusers. In
fact, since I haven't yet purchased any valves, if I could find
diffusers at a competitive price, I'd be very interested. I believe we
have Eheims available here in Vancouver but I am under the impression
that they cost about $100.

Wouldn't you still want to have a powerhead or similar device to induce
currents into the aquarium when using the diffuser? This greatly
improves the ability of the plants to absorb CO2 from the water probably
by a factor of 100 or more.

Steve Pushak                              Vancouver, BC, CANADA 

Visit "Steve's Aquatic Page"      http://home.infinet.net/teban/
 for LOTS of pics, tips and links for aquatic gardening!!!