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I have a CO2 production for my aquariums (currently 2, but the solution should be extenable to - let's say - 5) based on fermentation of bio mass (CO2 is used as fertilizer there) which consist of a bottle, a connector, valves and diffusors and flippers which help dissolving the CO2 underwater. They're connected with 4/6mm tubes.

Since the efficiency of the fermentation is irregular the presure in the system varies. Extranous CO2 is released into the water, but can't be dissolved and thus is waisted.

I want to avoid that by adding a storage which collects CO2 if the pressure in the system is above 2 bar and releases it when it's below. What components can I use for such a system?

Plan:

CO2 source -->--|-- valve 1 -- Aquarium 1 (diffusor, underwater)
                |
                |-- valve 2 -- Aquarium 2 (diffusor, underwater)
                |
                |-- ...
                |
                |-- storage

Functional sketch of my idea for a storage:

                                                      |---------|
-->- backstop valve opening > 2 bar before storage ---- Storage ---- pressure regulator opening < 2 bar after the storage --
                                                      |---------|

The storage isn't necessary in the scenario since the CO2 is cheap and over-pressure could easily be released in the room (1 1.5 l PET bottle with water and sugar can't cause a hazard). It's more of an engineering exercise.

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    $\begingroup$ Why are you fermenting anything in aquariums? That sounds like it would be really bad for the fish. $\endgroup$ – Olin Lathrop Jan 29 '17 at 21:13
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    $\begingroup$ It's for my aquarium in in PET water bottle (water, cristal sugar and yeast). And yes, that'd be bad for the fish... $\endgroup$ – Karl Richter Jan 30 '17 at 7:57
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Your option 1, done "according to specs":

Two check valves and a pressure regulator with separate output and regulation input (these are usually joint; output pressure being the regulating/feedback pressure; you need a variant with these separate).

The regulator has regulation input open to air, throughput input in your circuit, output to the tank, through a check valve. Set to two bars, allows pressure in only when pressure in circuitry exceeds 2 bar, filling the tank.

A separate, parallel connection through the second check valve depletes the tank whenever the pressure in the circuitry drops below the pressure in the tank.

In my opinion this - while meeting the specs - is absolutely unnecessarily complex. What normally is used in your situation is a buffer tank:

Option 2: Adjusted specs. Extra storage for CO2 filling when pressure rises, depleting when pressure drops, no hysteresis on filling.

Simply connect the tank directly to the circuitry. No check valves, no extra parts. Fill it with CO2 at ambient pressure initially. As pressure grows, it accepts more gas, in proportion to the pressure. As gas is spent, the tank releases its storage. Simple, robust, no extra complexity or cost, and does its job roughly as well as the original design.

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Your setup as with the pressure regulator could work. Before buying one, you need to know the approximate pressure you regulate to: Static pressure + dynamic loss pipe + dynamic loss diffusor

  • Static pressure: 1 cm water column above the diffusor equals 1 mbar
  • Dynamic loss pipe - dependant on flow rate, use Darcy Weissbach.
  • Dynamic loss diffusor - dependant on flow rate, impossible to know without knowing the diffusor.

The gas from the fermentation will be at saturation moisture, condensation inside the regulator can happen. I would position that regulator so that drops can leave and see what happens, though this may damage the regulator. Alternativly preheat the gas somehow upstream of the regulator.

Avoid low points in the tubing, put tubing with a consistent grade downward towards containers so that condensate can flow along.

I don't really go into safety considerations here, you have small amounts of gas (I assume?) and PET containers. Make sure that the whole setup is in a well ventilated place.

Make sure the include vents with manual valves so you can de-pressurize your system, before opening.

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If I am understanding your design correctly, I think you are going to need a little more than a regulator to operate efficiently. When your storage pressure is only a little higher than your use pressure in a system like this, your storage volume goes up, as does your waste.

I would recommend a gas/water diaphragm pump with a pressure switch or level switch. This would let you store at a higher pressure and capture all of your waste gas instead of just what is above 2 bar. Returning the gas could be done with a regulator or control valve.

In the sugar production industry, a device used to mix CO2 with water is called a carbonator. That might help you locate some other references. Membrane carbonators might also be an option. There are lots of brands out there, but they basically allow you to add CO2 in a more controlled manner without the large footprint or gas recirculation of a bubbling type device.

If you give some more details about what your are trying to achieve with your setup I may be able to provide more specific information.

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    $\begingroup$ The CO2 is disolved into the aquarium water through the diffusor where it's used as fertilizer for the plants. The storage isn't necessary in the scenario (I expanded on my motivation in the question). $\endgroup$ – Karl Richter Feb 7 '17 at 1:12
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I guess you want to keep fermentation process anaerobic. Just snap rubber/latex balloons on top of bottle, bleed of excess gas automatically by poking a needle hole on the balloon. The hole will release excess CO2 but could withstand water ingress.

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