What’s Happening in Your Tank: The Physics of Bubble Size
This is the article that explains why “just turn the gas up” is wrong.
The three controls on bubble size
- The size (length) of your carbonation stone
- The pore size inside the stone
- How fast you flow CO2 through the stone. On FizzWizz, that’s a flow controller measured in cubic feet per hour (CFH).
Why small bubbles win
Buoyancy. Big bubbles are mostly gas, so they race to the headspace and never dissolve. Small bubbles barely rise; they linger in the beer, and lingering time is dissolving time.
Surface area. Fit many small bubbles in the space of one big one and you get far more gas-to-beer contact area for the same volume of CO2. It’s the same reason crushed ice dissolves in a gin and tonic faster than one big cube.
Laplace pressure. A small bubble surrounded by beer is squeezed by the weight of liquid above it. The smaller the bubble, the higher the internal pressure: a bubble the size of a cat’s whisker can hold 30 to 100 PSI inside. That pressure differential is what drives CO2 into solution. (Look up “Laplace pressure” if you want the full derivation.)
What happens when you flow too fast
At 1 CFH you get a fine cloud of tiny bubbles that dissolve locally, and almost nothing reaches the headspace. At 4 CFH the bubbles grow and more of them escape upward. At 8 CFH you get big lazy bubbles, foam on top of the tank, and poor transfer. Stouts and other highly foamy styles are especially punished: they need slower flow and smaller bubbles than, say, a pilsner.
The sight glass: your only warning
In a real tank you cannot see foaming at the top. That’s why the FizzWizz separator has a sight glass. If you see beer dripping into the sight glass, you are flowing far too fast. The fix is not to nudge it down: cut the flow rate to at least half and let the bubbles shrink back.
And if you need to carbonate faster?
Don’t turn the flow up. Get more small bubbles instead: a longer carbonation stone gives you more emission area, so you can run a higher CFH without creating big bubbles.
Every stone is different. Contact us and we’ll review your setup and help you dial in the right flow rate for your stone.