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.
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