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RE: [APD] CO2 accident

John Thompson writes:

" Learn how not to set up a pressurized CO2 system from me.  Last night I
upgraded my CO2 from a DIY yeast sytem to a pressurized CO2 cylinder.  I
set the system up at approximately 15 bubbles/min using a low regulator
setting and a needle valve.  I woke up this morning to a fish massacre. 
Over night the bubble rate increased until there was a continuous flow of
CO2.  I'm not sure why this happened but I suspect it was because I did not
allow the cylinder to warm to room temperature before setting the flow
rate.  When I picked up the cylinder from a welding supply store it had
been stored outside at around 50 deg. F.  I plan to reset the flow rate
later after the CO2 level drops.  Currently at 140 ppm and Ph was to low to
measure.  Be careful and keep an eye on a new set up."

John, I've used several versions of manual CO2 systems for a long time and
here is what I've learned from experience:

In a pressurized CO2 system which does not include a controller, the needle
valve/metering valve is the weak link in the systems ability to provide a
consistent CO2 level in the aquarium. In particular, temperature changes to
the metering valve can greatly influence a set flow rate. The metering
valves are metal and are subject to expansion and contraction which changes
their ability to keep a constant flow rate. Even the very expensive
stainless steel Nupro/Swagelok valves are subject to this phenomenon. What
happens is that you set the flow rate where you would like it but over time
the gas cylinder warms up to room temperature and so the regulator and the
metering valve warm up as well because the gas that is flowing through them
is warmer. This causes a very small but very important expansion in the
metal parts. The regulator has no problem coping with this but the metering
valve is now letting too much gas pass through it.

I tested this theory when I was having problems getting an ARO #1 valve to
stay in adjustment. This is an inexpensive, mostly plastic valve. I took a
variable temperature heat gun and gently blew warm air over the valve and
immediately saw an increase in bubble rate. I put a thermometer next to the
valve to see just how much I was raising the temperature as I wanted to
have realistic expectations and valid results. While I didn't record any of
the data, I can say that a small change in temperature of only a couple of
degrees was enough to change the flow rate. I thought more expensive valves
which offered much finer control would solve the problem and, while they
are an improvement, they do not solve the problem. The problem of gas flow
regulation remains a function of the temperature of the valve body as far
as I can tell for the models I have used and tested; Nupro, ARO, and

Currently, the phenomenon happens to work in my favor: The room temperature
that the aquariums are housed in rises significantly on a bright sunny day
and I can correlate this with an increase in the CO2 bubble rate.
Fortunately this happens during the photo period for the plants so they can
use the extra carbon. When the sun goes down the room temperature drops and
so does the CO2 bubble rate.

Setting up a manually adjusted (no controller) pressurized CO2 system is
relatively easy but they do require close supervision during the
"adjustment phase" which can last a couple of days. Most of the adjustment
phase is waiting to see how the system settles in to a stable operating
temperature. If you want to get technical, there are several factors that
contribute to the net operating temperature. Some of these are: the
temperature of the contents of the cylinder, the ambient air temperature
and any changes in it over time, the temperature drop caused by the gas
decompressing (Boyle's law), the temperature increase caused by friction as
the gas is restricted in the metering valve, the temperature increase
caused by conduction of the heat created by the operation of a solenoid if
one is used, the temperature change caused by blah, blah, blah, etc. The
point is that every CO2 system is slightly different but the best way to
get one operating correctly is to make small adjustments and observe it
over time. The biggest contributor to an unstable CO2 delivery system using
the manually adjusted continuous delivery method (CO2=ON 24/7) that I have
seen has been air temperature changes which I think greatly effect the flow
rate through the metering valve. If you are planning on setting up a CO2
system where the room temperature changes by more than a few degrees then I
would recommend first trying the solenoid adaptation to control when the
CO2 is being delivered. If this does not stabilize the ecosystem within
reasonable bounds then a pH controlled system may be necessary.


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