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CO2 -- High Pressure/Low Pressure

Dave G. and I talk a lot about CO2, but I hadn't read his FAQ before.
Another Dave (DaTrueDave) quoted Dave G's FAQ as follows:  "A low pressure
system runs about 1 psi, a high pressure system about 15 psi. A low pressure
system is hard to regulate. The regulator will regulate well from about
10-25 psi. A low pressure system needs a way to drop the 10 psi to 1 psi.
Usually a needle valve is used, but this is not very reliable, since a
needle valve is not designed to regulate pressure, but flow. If you set the
regulator very low, the pressure will wander up and down. If you set the
needle valve almost closed (as you must to make a big pressure drop) the
flow rate will vary a lot."

I don't agree with Dave's conclusion on reliability of needle valves at all.
I've shown Dave my low pressure CO2 setup in which my Nupro fine metering
valve (M series; I would buy the S series now) puts one bubble of CO2 every
four seconds into my low pressure Dupla CO2 reactor, day after day, month
after month, just like clockwork.  Once one finds the sweet spot (a couple
of twists one way or the other) it stays right there on the money for months
at a time.  And there is no reason at all to set the regulator to an
arbitrarily low output pressure.  That's just asking for trouble.  I set
mine at 40 psi where it seems to work well.  The Nupro valve manages this
pressure (or 10 times that pressure according to the specs) quite well.  And
just to poke Dave in the ribs a little :-) my low pressure system seems to
be much more reliable that his high pressure system (regulator plus Eheim
diffuser) which I have observed wildly bubbling CO2 into Dave's tank on
several occasions.

I've reminded Dave G. of how well my low pressure system works repeatedly.
Dave often talks about $100 Hoke valves; but I can report based on 4 years
of experience that my $34 Nurpo valve also works very well and Tom Barr has
a couple of even cheaper valves that he claims work well.  (Monolith Marine
Monsters and even Pet Warehouse claim to be selling good quality fine
metering valves for reasonable prices.)  What I don't think Dave G. has ever
done is buy this type of valve and actually test to determine whether his
statement is true.  In my experience it is not.

While there are some important issues in the CO2 area, it is simply NOT as
difficult as some would make out.  I suppose I should say that based on the
recent CO2 thread and careful observation of my own setup as the bottle ran
out of gas, I have come to believe that even good quality single stage CO2
equipment with a good metering valve will dump some CO2 as the tank runs out
of gas.  I calibrated by Pinpoint pH monitor (with new probe) and took
measurements twice a day.  As the tank pressure dropped from 700 psi to 200
psi the pH dropped a total of 1.0 degrees in two steps.  The output pressure
of the regulator rose to 50 psi (25% increase) and the Nupro let enough
extra gas through to drop the pH by a whole degree over a couple of days.
This was much more than I expected and wanted.  I intend to change out the
CO2 bottle before it runs out of gas in the future.

Regards, Steve Dixon