This is the article that explains why “just turn the gas up” is wrong.
How the FizzWizz is plumbed
The video opens with the setup, and each part plays a role in bubble size:
- CO2 in: supply from your pressure regulator, set to about 25 to 30 PSI.
- Stone line: a quarter-inch pneumatic hose from the FizzWizz to the top of your carbonation stone.
- Separator: connects the FizzWizz to the CIP arm, with a small sight glass at the bottom. More on why that matters below.
- WizzTemp temperature sensor: in its own dedicated thermowell, via a spare well, a thermowell adapter on the sample port, or the WizzStone-Combo. See What Is a Thermowell.
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. Surface tension squeezes every bubble, and the smaller the bubble, the harder it squeezes. The extra pressure inside rises as the bubble shrinks (ΔP = 2γ/r, the Young-Laplace equation; see LibreTexts), and that extra pressure pushes CO2 out of the bubble and into solution. The effect only becomes large at microscopic sizes. Using a measured surface tension for lager of 55 mN/m (Atasi et al.):
| Bubble diameter | Extra pressure inside (approx.) |
|---|---|
| 0.1 mm (just visible) | 0.3 PSI |
| 10 µm | 3 PSI |
| 1 µm | 32 PSI |
| 0.3 µm | 106 PSI |
Those micron sizes overlap with the pores in a fine stone; sintered stainless stones are sold with pores as small as 0.2 µm (GW Kent). A stone that releases bubbles close to its pore size gives you bubbles that want to dissolve.
Depth. The weight of the beer helps too, in a different way. Liquid depth adds roughly 1.4 PSI per metre, so the stone at the bottom of the tank sits at a higher pressure than the headspace.
What happens when you flow too fast
In the video’s animation, the FizzWizz runs in carbonation mode at its default of 2.4 volumes. 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. Or add a second stone: the Splitter-Wizz runs two stones in one tank at the same time.
Every stone is different. Contact us and we’ll review your setup and help you dial in the right flow rate for your stone. For a quick estimate, try the Stone Flow Rate Calculator.