disposable CO2 gas cylinder
Clarke 600 g disposable CO2 cylinder

I ran out of CO2 gas today for the Fireplace Aquarium and swapped out the old CO2 supply cylinder for a new one.  I get 600 g CO2 at a time in disposable cylinders used for MIG welding, and sure enough, the empty cylinder weighed 1200 g and the new cylinder weighed 1800 g.

End of tank dump

I few weeks ago, I noticed the CO2 bubble rate had increased substantially due to increased flow rate through the regulator on the CO2 tank.  Regulators are the first step in controlling aquarium CO2 and regulators work best when the pressure inside the CO2 tank is constant.  Most of the time is this easy because inside the cylinder the CO2 exists in both a liquid and gas phase in equilibrium, and so long as there is any liquid CO2 at all, the equilibrium maintains a constant pressure in the tank of around 860 psi.  When the CO2 is running out, however, there comes a point when there is no liquid remaining in the tank at which point the pressure of the gas phase CO2 gradually drops as it is exhausted.  Single stage* regulators “notice” the pressure in the supply cylinder is dropping and attempt to compensate by opening more fully, however they inevitably over compensate resulting in increased gas flow even though the source cylinder is at lower pressure.  In some cases this failure of regulation can happen quickly and dramatically and can “dump” all the remaining CO2 gas into the aquarium, poisoning the animal residents (the plants won’t be bothered however).

With my set-up the regulation failure is not so severe, and can be easily mitigated by awareness a problem might be coming and by adjusting the flow through the regulator down a little on a daily basis until the cylinder runs out gas completely.  I get more than three months of constant pressure CO2 and then a couple weeks of manageable instabiliy at the end.  I’ve set a 3 month reminder for myself to remind me when to start carefully monitoring the end of this new cylinder.

*Dual stage regulator alternative

A dual stage regulator is designed to maintain constant flow even if the source pressure changes and can avoid the end of tank dump that can happen with single stage regulators.  Dual stage regulators are much more expensive than single stage, can be easily and erroneously confused with “dual guage” regulators, and don’t seem to be designed for the disposable CO2 cylinders I like to use.

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