Acutally no matter how the conbimation between water, sugar and yeast. The life time for DIY CO2 should be around 1~2 weeks (the best may also just 3 weeks).
This is because:
Case 1: more yeast -- stronger CO2 at the beginning, however, they will "eat" all the sugar faster, that mean this will make short period but large CO2 per second.
Case 2: more sugar -- yeast will have more "food", however, yeast will generate "酒" after "eating", and such "酒" will die yeast. That mean this situation will make CO2 normal but still not too long time
Case 3: more water -- more water that mean less food, the sugar concentration of water will be less, then yeast will not be ease to die by "酒", however, yeast will die due to no "food".
As a result, 1/4 sugar 3/4 water and around 1 teaspoon yeast is ratio to get longer time. You will to adjust each time, if you feel the CO2 is too weak, you add litle more yeast, but as the same time, it will get shorter time. On the other head, you feel you can accept weaker CO2, then add few yeast.
That mean sugar and water is fixed everytime, use yeast to adjust how long and how fast of the CO2 generate.
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