I hacked my soda stream to provide more co2 for the buck, connecting a hose instead of the cartridge to a large 20lb tank of co2. The only issue I'm running into is the co2 is intermittent, only ever another pump (or so) does it come out as forcefully as the old tanks that soda stream sells. Sometimes if move the tank or soda stream unit it will return to its normal force but then die back down. I'm wondering if hose length or path should matter or where I should be looking for issues because I'm stumped.
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How are you controlling the pressure delivery with your new tank? A regulator? What is the volume of your tank?
Here is my understanding of the physical chemistry of this; perhaps a physical chemist or chem E can correct any mistakes I have made, for I am just a lowly synthetic chemist.
The pressure inside the soda stream tank should be determined by the vapor pressure of carbon dioxide at room temperature, looks to be a little over 800 psi. That's pretty damn high pressure for something a regular consumer would be using, so those tanks that soda stream sells must be pretty thick aluminum.
The volume of the tank (minus the volume of non-gaseous CO2) will determine how much gas you can deliver. As you deplete the reservoir, the non-gaseous CO2 (I believe it is liquid under pressures/temperatures typically in a sodastream bottle; e.g. room temperature and a few hundred PSI) will begin to re-equilibrate with the system, eventually bringing it back to the original vapor pressure of CO2 at the given temperature. I think larger excesses of CO2 will allow for faster re-pressurization of the tank. Eventually you run out of CO2 and the pressure goes down.
Since your tank is running into the slow down stage, I would guess it means you are low on CO2. This happens towards SodaStream bottles as well, when they run low on CO2. Therefore, you should refill it.
Please keep in mind that you are working with high pressure gases... would be wise to learn about safety mechanisms, how much pressure the tank can handle, how much pressure is made by the CO2, etc...