entirely safe and fun

After five days in the fermenter, 38 gravity points of sugar have been transformed into something happier. As of this evening, the wheat beer is down to a specific gravity of 1.016, near the final gravity it’s likely to settle at. Beer has almost happened here!

One of the things I’d like to do is correlate fermentation activity with bubble rate a little more confidently. Now, absolute bubble rate is not comparable across fermenter setups, or even between batches, and perhaps not even between measurements which require the top to come off. There’s no hope of reliably saying something like “oh, 30 bubbles per second means that we’re fermenting out 8 points per day.” But it’s possible that we’ll be able to say that, in general, fermentation rate within a batch correlate pretty strongly with day-over-day change in specific gravity, that is, that more bubbles means more fermentation. The rate of bubbling depends on the volume of the headspace between the top of the beer and the bucket lid, on the level to which the airlock is filled with water, on precisely how tightly the bucket’s lid fits… but also, we hope to show, on the speed with which CO₂ is being generated.

Here’s four days’ worth of measurements from this batch:

For the three bubble measurements available — a pretty thin data set, to be sure — the correlation coefficient between the bubble rate and the real fermentation rate (ΔSG/day) is 0.997, which is very significant, even more so than I expected. It will be interesting to see how it holds up with more measurements.

Part 1 of this series is here; part 2 is here.