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Re: Pressure Regulator

Words, words, words..... use the wrong ones and you can only cause more

After reading what Casey Huang wrote to David, I'd just like to clarify my
use of terms, so that we are all on the same wavelength....

Cylinder pressure = Input Pressure to the Regulator (because the CO2 is
mostly liquid inside of the cylinder, this will stay at or very close to
approximately 900 psig while there is still liquid CO2 left in the cylinder,
i.e. it's not empty). This pressure is shown on the Cylinder Gauge on the

Output Pressure = the pressure of the CO2 gas leaving the Regulator, as
shown on the Output Gauge on the regulator.

The "decay characteristic" numbers that I used came from manufacturers'
published literature. I am making a BIG assumption in using them - that the
increase in output pressure of the gas leaving the regulator as the cylinder
pressure falls follows a "straight line function". In reality, I doubt that
this is the case, especially at lower cylinder pressure (i.e. as you get
closer and closer to a completely empty cylinder, the ability of _any_
regulator to maintain a constant output pressure would probably vary more
than the numbers indicate). To get an idea of what I mean here, go to the
Swagelok web site and look at the output curves for the Nupro M and Nupro S
series metering valves. These illustrate flow rather than pressure but
notice how at lower values the graphs are curved, not straight. I'm pretty
sure that a pressure regulator would exhibit the same sort of problem at
very low cylinder pressure. This is NOT taken into consideration in the
chart I posted.

Remember as well that this "thought exercise" is meant mainly to illustrate
ONE of the reasons WHY some regulators are $69.95 and other regulators are
$399.99. Additionally, as long as there is still liquid CO2 in the cylinder
(i.e. it's not empty), the cylinder pressure will stay at or close to 900
psig.  For OUR purposes, where we are injecting very small levels of CO2
into one or just a few aquariums, the lower scale conditions need not
_really_ concern us, unless you are hopelessly anal (I plead guilty to
that...<grin>). It can be likened to the difference between Fluval and Eheim
filters - both work, one is just built to closer tolerances.

A good needle valve downstream of the regulator should prevent any "dump" of
CO2 into your tank and even if it didn't, so long as your tank is not
hermetically sealed at the top, any excess CO2 should escape harmlessly into
the atmosphere. CO2 is not terribly toxic and it won't support combustion.
Just don't sleep on the floor next to an almost empty cylinder without
keeping your window open (<grin>).

And finally, as Smokey the Bear used to say "Only YOU can prevent forest
fires!", only YOU can decide how much weight you want to give this
information when you are about to lay out you hard earned cash for a

For all of the "non-techies" being bored silly by this, sorry........ <grin>
but the APD is the ONLY forum where such information might be thrashed out
publicly and under the scrutiny of a wide range of people from a number of
disciplines. If I've made a mistake (and we all know that I'm far from
perfect, especially with math), we can be reasonably assured that someone
who REALLY knows will step forward and correct any misconceptions. This
benefits us all.

James Purchase