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Needle Valves in CO2 Systems Poll

I've kind of enjoyed the CO2 poll my self-so I guess I ought to throw my
info into the hopper.

1)Describe your CO2 setup
I have two systems, one low pressure and one using Eheim diffusers (I guess
some would call that high pressure).
System A:  An old Sandpoint CO2 system with a Dupla CO2 reactor (the small
one for up to 100 gallons) with a 10# CO2 cylinder.  The Sandpoint appears
to be a no-regulator system.  As best I can tell, a fine metering valve is
simply hooked directly to the CO2 bottle.  The unit included a solenoid
which I leave on 24/7 (because I'm not interested in using a controller).
System B:  Victor 253D regulator (single stage; two gauges) with Nupro M
Series metering valve followed by two Eheim diffusers (better diffusion is a
large tank).  I use a 20# bottle for this setup.  Output pressure of the
regulator is set to 40 psi.

2)How long have you been using this setup?
Sys. A:  5 years
Sys. B:  3 years

3)Have you experienced any problems with this setup?
Sys. A:  None that I've noticed.  I have run the CO2 cylinder empty numerous
times without killing any fish or noticing any gasping or other ill effects.
I do not keep a pH monitor running on this tank-so I don't really know if
there is any dumping as the cylinder runs out of gas, but I don't think
there is much, if any.
Sys. B:  I have run this system empty twice since the CO2 dumping hooray
kicked up on the APD and monitored things closely.  I was surprised by the
results.  In both cases significant dumping occurred even with the use of
the metering valve.  As the pressure in the CO2 cylinder dropped below 800
psi, the output pressure of the regulator slowly increased to ~50 psi (this
is the amount Dave predicted based on his review of the specs of the 253D
regulator which is reported to be one of Victor's better single stage
regulators and not much of a dumper).  At 50 psi the Eheims clearly showed
more gas bubbling into the aquarium.  The pH (Pinpoint with new probe;
calibrated and run continuously) dropped from ~7.0 to ~6.0 over the course
of a couple of days as the output pressure rose.  At KH 5 this represents an
increase in water column CO2 from ~15 ppm to over 100 ppm.  In both cases I
stopped the experiment at this point and reduced the output pressure on the
regulator to 40 psi which restored the pH to ~7.0.  In neither case did any
fish die or exhibit gasping at the surface.

4)If you could change something(s) with your setup, what would it be?
Sys. A:  Nothing.  In fact I'm now quite intrigued by the idea of buying a
good metering valve and hooking it directly to the CO2 bottle (as Sandpoint
apparently decided to do a long time ago).  If I end up dying to know how
much CO2 is left in the bottle, I could always set the cylinder on scale of
some sort and simply weigh it.
Sys. B:  Metering valve setups do dump (notwithstanding a few reports to the
contrary), but perhaps not as badly as systems without need metering valves.
The valves provide some protection IMHO, and are worth the investment.
However, I would like to see the dumping problem solved.  (I think Dave is
close, but the APD flame war has dried up his CO2 business and once he does
have it solved, folks may not have confidence in the solution due to the
noise level of the discussion.)  We need either a regulator that doesn't
dump much or a control system after the regulator which solves the problem.
Dave (with Wright's advice) is working on a new angle.  There may be some
metering valves which create sufficient turbulence/perturbation through high
speed flow rates within the valve to effectively limit dumping.  That would
be a nice turn of events if it pans out.

5)What bubble rate and psi settings do you use?
Sys. A:  one large bubble every 6 seconds, like clockwork, month in month
Sys. B:  don't know (I use the pH to adjust the CO2 level); 40 psi on the
regulator output.

Regards, Steve Dixon in San Francisco