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Re: Avoiding Dumping CO2

At 03:48 PM 5/22/02 -0400, Alan K wrote:
>When using a pressurized CO2 system with a needle
>valve, what is the optimal pressure at which to set the second stage?
>I have been keeping the second stage at around 10# psi, but I heard
>recently that it is better to keep the second stage setting higher and
>the needle valve more tightly closed.

I assume that you are asking what is the best pressure to use coming out of 
the regulator into the needle valve.   This is the $64 answer (more than 
you wanted to know).  It turns out that all the stuff I posted about Cv 
(see archives) is correct but irrelevant.  The real way that needle valves 
work is by running "sonic", which refers to the fact that the gas in the 
most tightly constricted part of the valve reaches the speed of 
sound.   That is very hard to do with the very low flow rates we use, which 
explains why only very fine valves do a good job of it.   The best is 
likely the Hoke at $130-200 (micrometering valve).   The next best I have 
found is the Fabco NV-55 which you can get from your local pneumatics 
dealer or from me.  It is closer to $25-30.  It is a much better deal than 
the Hoke but not as good a valve.   To ensure sonic flow in either valve, 
you need the pressure drop to be about 50% of absolute pressure.  If your 
output from the needle valve is a couple of psi, this means an input gauge 
pressure of about 20psi to be safe.   Less and your valve will not be 
running sonic (possibly) and it may not be providing the protection you are 
hoping for.   Please ask about anything I did not explain well.

Dave Gomberg, San Francisco   NE5EE     gomberg at wcf dot com
http://www.wcf.com/co2iron for low cost CO2 systems that work!

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