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Re: CO2 discussion

Please folks........is it possible for this list to discuss a topic like the
use of Carbon Dioxide and its associated hardware _without_ resorting to
turning the thread into a pissing match? Letting too much of your ego
intrude into the discussion clouds the real issue and probably inhibits a
full and accurate exploration of the topic. Many people who _might_ have
something valuable to contribute probably fail to do so out of fear of being
flammed or being called names by a few people who think they know more than
they probably do.

It _is_ possible to question another list member's comments without
resorting to personal insults. I realize that many of you might think this
request odd, coming from me - I can be pretty gruff and quick with the quips
at times, and have been known to get into a few flame wars myself (for which
I have been rightly chastised and sorry for). But I think that we all need
to remind ourselves now and again that we are here for mutual support and
learning. This can be accomplished much more effectively if we respect one
another a tad more.

Regarding compressed CO2, (as _I_ understand it), it does exist within a
cylinder as both a liquid AND as a gas. It is the vapour pressure of the gas
inside of the cylinder which maintains the relatively constant internal
pressure of the system. The regulator controls the release of the CO2 gas,
which we then feed thru needle valves, in-tank appliances, etc.

I can accept (and understand) that while there is still some liquid CO2
inside of the emptying cylinder, that the pressure displayed on the cylinder
guage will remain pretty stable and that the pressure of the CO2 leaving the
regulator's output port will also remain pretty stable. The problem comes
when all or nearly all of the liquid CO2 within the cylinder has evaporated.
The internal pressure of the cylinder then quickly falls and all hell can
break loose as far as the regulator's ability to control the flow of gas out
of the cylinder. This isn't a danger in the sense that anything is going to
break or explode, and various strategies have been suggested to either avoid
the situation or at least minimize its effects.

I also noticed, while looking at the stats for various regulators on several
manufacturer's web sites, that different regulators display different "decay
characteristics". This is directly related to a regulator's ability to hold
a stable output pressure as the internal cylinder pressure falls. As has
been noted, this is much more of a problem with materials that are entirely
in the gas phase within the cylinder, as opposed to CO2, which exists as a
liquid AND a gas within the cylinder. But the phenomenon still exists in
both cases.

Most two stage regulators are able to maintain output pressure to much
closer tolerances than single stage regulators but even here, some variation
exists (one model lists in its specs a figure of 0.1/100 psig while another
claims 0.026/100 psig). With single stage regulators, I have seen quotes of
4.8/100 psig, 2.4/100psig, 1.75/100 psig, 0.35/100 psig. From my on-line
research, and from talking to several manufacturers, it appears that usually
a regulator's abilities in this regard are related to its price, but this is
not always the case. How important this is to you can depend upon how
willing you are "fiddle" or monitor and adjust your regulator (or how
tolerant your set-up is to output pressure fluctuations).

To make this "decay characteristic" easier to see, consider the following
chart. Given a full cylinder with an internal pressure reading of 900 psig
and a regulator adjusted to produce an output pressure of 30 psig, if we
apply the manufacturer's stated "decay characteristics over the falling
range of internal pressures experienced as a clinder empties, we get:

    <- Single Stage Models -> | <----- 2 Stage Models ---->
    4.8/100 1.75/100 0.35/100   0.1/100 0.042/100 0.026/100

900   30.0     30.0    30.0      30.0      30.0      30.0
800   25.2     28.25   29.65     29.9      29.958    29.974
700   20.4     26.5    29.3      29.8      29.916    29.948
600   15.6     24.75   28.95     29.7      29.874    29.922
500   10.8     23      28.6      29.6      29.832    29.896
400    6       21.25   28.25     29.5      29.79     29.87
300    1.2     19.5    27.9      29.4      29.748    29.844
200    -       17.75   27.55     29.3      29.706    29.818
100    -       16      27.2      29.2      29.664    29.792

in    30       14       2.8       0.8       0.336     0.208

These figures _assume_ that the decay characteristics follow a straight
line, in reality the variation at lower cylinder pressures are probably
greater, and these numbers are only meant as a guide to illustrate a point.
But you can see that some single stage regulators can maintain a starting
pressure of 30 psig output much better than others, and that two stage
regulators are even more capable in this regard. How much variation you can
tolerate in your own system is something only you can decide. For a hobbyist
type set-up, it is probably true that 2 stage regulators are over-kill, but
it is wrong to call them "useless".

In all of this discussion on CO2 equipment, it helps to remain focused on
what we want to do - provide a more enriched environment for our aquatic
plants. I'll admit that for most people, there might be no need to go to the
added expense of a high purity analytical grade of regulator or use really
expensive regulating valves such as Nupro S or Hoke Micro-metering valves.
But remember that _all_ equipment is built to operate over certain ranges
and to react within certain tolerances. It's your tank and your money, so
only you can decide what to select for your own use. But it is much better
to know what to expect from your system before you spend your money than to
find out after the fact that it isn't capable of doing what you want or
need. For this, you need (or at least the person advising you needs) to know
some hard, physical data.

James Purchase
(..... still hoping that someone chimes in with some more information on
Check Valves......)