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re: CO2 FAQ

On Tue, 19 Sep 2000, DaTrueDave wrote:

> Ok, I've gone back and reread Dave Gomberg's coČ FAQ.  It does contain
> a lot of good information!  However, he mentions several times that a
> "low pressure" system isn't as stable as a "high pressure" system.
> I've been leaning towards wanting a low pressure system, but now I'm
> hesitant... 

Of course.  This is the problem with anyone with a vested interest
providing "free" information on the side (sorry Dave).  Of course Dave
will say the ceramic disk approach is better.  That's what he believes in
and sells.

I'll say right upfront that my experience with the ceramic diffuser is
different.  Being a big proponent of unbiased comparisons, I bought an
eheim diffuser a year and a half back and tried it out. After a day or
two, it "popped" and killed a tank of fish. I don't use them now.  After
hearing that it can happen at the end of the tank as well, I don't think
it's worth it for my tanks (most of which are breeding tanks with fish
worth more than the CO2 system, let alone the delta between an eheim and a
good needle valve).  On the other hand, I've never lost a fish in my tanks
fed with even the crappy ARO-1 valves.  So that's MY bias.

> Here's the section of the FAQ that confuses me:
> <<<What's the difference between high pressure and low pressure
> systems?
> 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. Ususally a needle valve is used, but this
> is not very reliable, since a needle valve is not designed to regulate
> pressure, but flow. 

As Wright is so fond of writing, "HOGWASH!"  All systems ultimately are
delivering CO2 by means of flow.  The flow is very low, but it is still
flow.  The needle valve system (I hesistate to call it low pressure.  See
below) is designed to put out 1~5 bubbles per second.  The ceramic disk
method is designed to put out a fixed number of microbubbles per second
(I'd presume about the same volume as the needle?).

> 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.

Most everyone using a needle valve sets the pressure of the regulator
between 5 and 20 PSI, which is quite reasonable, and sets the needle valve
to a very low flow.  If it's a cheap needle valve (i.e. the infamous
ARO-1), fluctuations occur, but it's not too bad if one keeps it in the
same place and roughly the same temperature.  If it's a $40 needle valve
like the Nupro, the fluctuation is very low and stability very good.

Now this whole thing of calling the diffusers "high pressure" and the
needle valve system "low pressure" is a bit misleading.  Both systems have
a "first stage" pressure regulator (the standard CO2 regulator) that is
set roughly the same (though the ceramic disk requires a more precise
setting... too low, no bubbles will get through the disk; too high, the
thing will pop).  Both systems have a "second stage" (the needle vs. the
ceramic diffuser) that regulates the flow down to about the same rate.  
The only difference is that the needle valve is usually coupled near the
regulator, while the diffuser is coupled far away from it.  OK, I don't
want to start another American Flagfish Naming Debate, so I won't bring it
up again.

Anyway, my take on the pros and cons between them.  

Needle valve/"Low Pressure"
PRO	allows actual control of flow by adjusting the needle
	does not suffer from "end of tank" dump
	all high pressure parts are coupled near the tank
	downstream reactor can be more efficient than ceramic diffuser

CON	more expensive if low fluctuation is desired
	requires reactor, powerhead, or diffusion bell downstream		
	requires fiddling with needle to get right flow right

Ceramic Diffuser/"High Pressure"
PRO	diffusion built-in.  compact, non-intrusive in tank, and cheap!
	cheaper than good needle valve
	no fiddling, just set regulator pressure
	very stable output through most of CO2 tank life

CON	no ability to set CO2 diffusion rate.  range is limited
	suffers from end-of-tank problem
	potential hazard of dumping all CO2 into tank if diffuser "pops"

  - Erik

Erik Olson
erik at thekrib dot com