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Re: End-of-tank-dump more comments -- or Still more gas about pressure

If I understand Wright Huntley correctly, he has suggested
that a metering valve can avoid end of tank dumps, ie.e,
the condition where a CO2 tanks quickly discharges its
contents when it is near empty.  And this depends not so
much on the regulator as the metering valve.  I think
Wright is absolutely correct that regulator quality has
little to do with dumps.  All spring/gas pressure mechanism
regulators (which are the kind we're talking about in the
hobby) require a minimum pressure to work as a regulator. 
As you near the minimum pressure, the regulator's accuracy
generally decreases, some more expensive ones are more
accurate over a wider range of pressures than some cheap
ones.  Ones that are really accurate up at 800-1000 psi and
also way down around 100 or 200 psi are absurdly expensive
for aqurium purposes, imo. But they *all* give out at some
point and that point is when there is *still* pressurized
gas in the CO2 tank.

I understand how a metering valve can prevent a lethal dump
of CO2 into your aquarium.  (And I recommend folks use
metering valves on compressed CO2 systems.)  I believe this
is true under conditions that often do occur *and* that
often do not occur for many CO2 users -- and it's hard for
most of us to know if those conditions exist until the the
dump occurs.  

I believe the idea is that if the regulator is set to a
certain point and the metering valve is set to a certain
point, then you will have a desired CO2 flow rate into your
aquarium (when the CO2 tank is not near empty) *and* yet
the valve will sufficiently restrict the flow rate that a
rapid dump into your aquarium cannot occur (when the CO2
tank is near empty).

I think for many of us, adjusting the regulator and
metering valve (and I've used Swagelock and clippards)
requires some back and forth adjustments between the two
devices to a achieve a desired flow rate.  At any setting
of the metering valve that allows an acceptable flow rate,
an increase in regulator (low side) pressure causes an
increse in flow and a decrease in regulator pressure has
the reverse effect.  Wright pointed out that a metering
valve attentuates the variances.  Indeed, but he variances
are still significant ime.  And the balance changes
somewhat with changes in ambient temperature, confound it! 
Now after I've fiddled and tweaked and got the rate just
where I want it, is the metering valve closed down enough
that a dump cannot occur?  Maybe.  I'd guess "Probably not"
but it's just a guess.  

I do know that a couple of different things can occur when
the CO2 tank gets near empty and neither is all that
desireable.  One thing is that the metering valve might not
be sufficiently closed to prevent excessvie CO2 going into
the aquarium.  The second is that, if the metering valve
will not allow an extraordinarily excessive flow of CO2,
then when the CO2 tank nears empty and the regulator's
diaphragm mechanism "gives out", then the low side pressure
will build and the safety blow-off valve on the regulator
will allow a dump of the CO2 tank in the room.  That last
situation isn't as bad for my aquarium as excess CO2; it
just means the aquarium stops getting CO2 earlier than I
might have expected or provided for -- but running the tank
without CO2 is not what I really want.

Of course, there is a third thing that can happen, namely,
that the valve restricts excess flow, the high side
pressure builds but the safety blow off doesn't blow.  My
aquarium is fine then, no CO2 overdoes and I didn't run
without CO2 for any unanctcipated length of time, but I'm
not all that happy about the performance of the safety
valve :-\

It still seems to me the best and surest course, and a very
easy one to follow, is to get the CO2 tank refilled when
the high-side pressure drops substantially below 700 psi. 
At normal room temps, the liquid CO2 is all gone from the
tank and the remaining gas is worth only a buck or so (in
most places).

You can buy a really expensive metering valve, but I don't
think it will change the picture much.  And the extra cost
of an expensive valve would be hard to offset with "saved"
CO2 costs.  On my 30 gal aquarium, running down from 700 to
400 psi would save me about 50 cents per year so it would
only take about 36 years for that to make up the the
marginal cost of an expensive valve over a clippard (or
something comparable).

I think $15-$18 Clippards work as well as the Swagelock and
you needn't spend more.  I also think $30-$60 bucks is
plenty to spend on an adequate regulator.  I've spent more
and less.

So I offer these rules of thumb:
Get a moderately priced metering valve; get an adequate
regulator; don't try to run your CO2 tank down to the
bottom -- refill when the liquid is gone.  Your saftey
margin is wider with a metering valve than without, but
it's not a sure thing.

Scott H.

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