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Inspired by the thread on CO2 controllers, and @Art experiment with modified pH probe / drop checker, I came up with what is to the best of my knowledge a new approach for our hobby.

I am not strong on the chemistry of CO2 in water, so it would be great if others chime in and comment if this approach makes sense.

Use the principle of Henry's law and fill the space under a closed aquarium lid with the correct concentration of CO2, using an affordable 65 USD sensor or similar with solenoid to release CO2 above the water. Control CO2 partial pressure so that the tank water will reach equilibrium with this gas pocket at about 30 ppm in the water. This would allow for very stable CO2 in the tank, and a big saver in CO2 as we don't rely on the outgassing via surface agitation to achieve stability. Besides that, it will be only weakly dependent on water chemistry (this may need to be tested and/or confirmed/quantified by chemists, as the chemistry and temperature do have some influence on CO2 solubility?).





Notes:

• My first thoughts/estimations are that one can reach 30 ppm in the tank water very fast, and have probably more than 90% savings on CO2 consumption. As we now measure CO2 in the gas pocket, independent of water chemistry, it would (hopefully) also be the first time that we can forget water chemistry as a major factor, complications with pH probes and using pH as a proxy, and indeed know the real water CO2 ppm more accurately and with less risks of misinterpretations of measurements.
• We don't need a precision expensive CO2 regulator anymore, no diffuser or reactor, no pH probe or drop checker. Just a sensor with electronics that switches a solenoid on and off. It would probably be good to have a simple air pump added, so that O2 and other gasses can exchange, but this would give very low CO2 losses.

From a physics and chemical perspective the approach is very similar (in reverse) to what professional dissolved CO2 probes do (see attached: measure CO2 concentration in a gas pocket that is in equilibrium with water), so that gives me some confidence that the above will work. However I am not confident that my understanding of chemistry of CO2 in water is good enough, and I may miss important aspects. Hope others chime in, and help to quantify if the chemistry is a minor or perhaps a major complication.

Attached file: Datasheet explaining the principle and use of a dedicated dissolved CO2 meter (for most in the hobby too expensive, but similar scientific principles applied as in above ideas).