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[APD] Ooch
"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.
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