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Re: [APD] Ooch

Now I understand "Mr. Wood's problem"!
Thank you for a very good explanation of the use of a needle valve to control an almost zero gas flow. I spent the first part of my engineering career working with lots of valves and various pressure gases, mostly air, plus using sonic flow nozzles on a daily basis. I agree with your explanation. But, the "ooch", like the hertz, came on the scene after I left it. Alas, poor Mrs. Ooch. At least she didn't meet her doom by using CO2 cylinders upside down.

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.


* 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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