Re: [APD] Ooch

["S. Hieber" <shieber at yahoo_com>]

Is it that the pressure differential with the CO2 set-ups
is not great enough for a sonic velocity to be acheived at
the metering valve (critical pressure ratio?) or that the
metering valve opening in these setups is too large for
critical flow?

Scott H.
> On Monday, February 21, 2005, at 09:47 AM, Tom Wood
> wrote:
> 
> > "What the hell is Mr. Wood's problem?"
> >
> > I just don't like seeing misinformation constantly
> repeated.
> >
> >
> > Regarding the sonic nonsense. We don't use needle
> valves in a way that 
> > any of that applies. Sonic flow, Cv and all that
> applies when the 
> > valve is under relatively high pressure on the inlet
> side and is 
> > moving a lot of gas at a relatively high rate of flow
> measured in lots 
> > of cubic feet per minute (CFM) against a given head
> pressure. At best 
> > we are moving a fraction of a cubic foot per -hour-
> with relatively 
> > low pressures on both sides of the valve.
> >
> >
> >
> > We use the valves in an 'almost closed' position. We
> use needle valves 
> > because they have tiny parts with tiny threads that can
> create what is 
> > best described as a tiny leak. A small thread pitch
> coupled with a 
> > finely tapered needle and seat provide a higher degree
> of control over 
> > the leak through the valve. The bubble of CO2 gas
> ooches* around the 
> > needle and into the outlet hole on the other side of
> the chamber. It 
> > doesn't matter how fast it does that because it is
> laminar flow anyway 
> > at the tiny rate of flow we are using.
> >
> >
> >
> > It doesn't even have to be a 'needle' valve. We could
> use any type of 
> > valve, including any clunky hardware store valve, if
> they had fine 
> > enough threads and seats to create small movements and
> pathways. But 
> > they don't so we use needle valves to get that small
> movement with our 
> > big clumsy fingers.
> >
> >
> >
> > When used in the 'almost closed' position the way we
> use them, needle 
> > valves have the additional benefit of creating
> back-pressure on the 
> > regulator. Which keeps the regulator from over-reacting
> to the effects 
> > of a CO2 cylinder that is running empty. Without that
> back-pressure, 
> > the mechanism in a single stage regulator interprets
> the dropping 
> > pressure on its inlet side as a higher demand on its
> outlet side, and 
> > it opens up, thus creating the fabled end of tank dump.
> Add an 'almost 
> > closed' valve to create some back-pressure, and there
> is no dump. > Ever.
> >
> >
> >
> > TW
> >
> >
> >
> > * The Ooch is a unit of measure named after Beatrice
> Ophelia Ooch, who 
> > with her husband Horatio Octavius Ooch, died tragically
> while 
> > performing an experiment involving seven chickens and a
> garden hose. 
> > They were buried side by side with only their initials
> on the 
> > headstones:  B.O.O.   H.O.O.
> >
> >
> > _______________________________________________
> > Aquatic-Plants mailing list
> > Aquatic-Plants at actwin_com
> > http://www.actwin.com/mailman/listinfo/aquatic-plants
> >
> 
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