Keeping on the hop, it’s time for part seven of this fermentation instrumentation series. In part four, I placed a few different sensors in some actively fermenting beer to gather data. In the previous post, I looked at data from a commercial sensor. Now it’s time to examine the data from my experimental pressure sensor.
I have two atmospheric pressure sensors collecting data while this beer ferments. One is outside the carboy, while the other is in a non-rigid (i.e. squeezable) container near the bottom of the inside of the carboy. The idea is that as the beer ferments, it will become less dense (because alcohol is less dense than sugar), and thus the same volume of liquid will put less pressure on the sensor.
Let’s start with predictions. I took the long way around and made a table that says that if my sensor is four inches below the surface of the water (it is), it should see about nine millibars of pressure more than just sitting in the air, regardless of specific gravity. This was the long way around, because it turns out “water-inch” is a known pressure unit (equal to 2.49mbar).
Here is what my sensors measured:
Unfortunately, there are two problems, relative to my predictions. The first is that the curves are not 9mbar apart. The second is that the red one is the internal sensor, consistently reading lower than the external sensor. Before I put the sensor in the carboy, I saw about the same difference. It’s possible that the missing 9 mbar is due to the fact that the sensor housing is not laying on the bottom of the carboy, as in the picture at the top of this post, but is instead resting with one end higher than the other. That means the pressure on it is not uniform, and I could be losing all of the additional pressure to the top end (which also happens to be the most flexible end). This might be enough to declare the experiment invalid, but let’s continue looking at what I have anyway.
What is more interesting from the earlier table is the difference we’re supposed to see as the beer ferments. I measured an original gravity (OG) of 1.040 when I pitched the yeast, and a final gravity (FG) of 1.00075 when I bottled (there are some sugars the yeast won’t eat, so we don’t reach 1.000). The predicted difference in pressure is just over 0.3 mbar. The pressure varies by over 15 mbar just due to the weather, though, so how can we tell? By subtracting the external, weather-only pressure from the internal, weather+beer pressure:
Hooray! We do see relative pressure change in the carboy. It’s noisy, but I think we have to compare the top-ish of the hump with the resting level of the plateau on the right. Why not compare the start point at the left with the resting point at the right? The climb on the left is likely one of two things: something similar to what the BeerBug sees, as discussed in part six (i.e. initial oxygen consumption, carbon dioxide production, or yeast proliferation), or the sensor moving. In either case, it does take the yeast 12-24 hours to really get working, and that’s about where the climb levels off, so that’s where the conversion of the sugar really starts.
Drawing some lines across the difference graph, I find the “max” pressure to be about -1.65, and the “end” pressure about -2.05, a difference of 0.4 mbar. That’s 30% off of the hypothesis. This doesn’t seem like a bad error (for a first attempt), but I’m skeptical that we saw this much pressure change without seeing the entire pressure difference (the missing 9 mbar).
So, let’s see if we can answer some unknowns. Before removing the sensor from the carboy, I attempted to figure out if the pressure leveled off higher due to pressure from the airlock. There is about a half inch of water that has to be moved out of the way, which would be 1.2 mbar. That’s nearly double the difference between the start pressure and the resting pressure, but to check, I let the gas out a couple of times between 5pm and 7pm in this graph:
There is no dip in pressure, just added noise from me jostling the cable. The resting pressure change is not from pressurizing the airlock. Follow-up experiments will be necessary to determine if it has anything to do with the specific mixture of absorbed gases, or the presence of yeast cells, I think for now the most likely answer is that the sensor moved.
I also tried to find the missing 9 mbar. After removing the sensor from the carboy, I put it in another container under about 4 inches of water, fully horizontal this time.
That graph starts with the sensor in the open air. It looks like I found about 6 mbar once I got the sensor truly in the bottom of the container. I think the last 3 mbar can probably be attributed to the rigidity of the container – it probably couldn’t deform further.
Perhaps most importantly, how does this compare to the BeerBug’s specific gravity curve? If I do just a little scaling and shifting, I can lay the curves on top of each other:
The two carboys do contain different strains of yeast (the original reason for using three separate carboys), and the BeerBug’s carboy started noticeably faster. So, the different in the start of the curves is to be expected. The head in both, and the bubbling out of the airlocks of both did seem to reduce at about the same time, so the simultaneous arrival at finishing plateau is expected as well.
Overall results are, unfortunately, inconclusive. It looks like the end of fermentation was signaled correctly. I would not have been able to predict the finishing gravity, though. There is enough here to warrant future experiments, I think. This was something of an opportunistic test. I was brewing these three batches anyway, so why not try the sensors? Something with more control (i.e. taking fermentation out of the equation) should be illuminating.
If you want to explore this data yourself, I’ve posted the data in CSV format in a gist.
Stay tuned for the final episode analyzing the accelerometer data soon!
Update: accelerometer data is live in part eight.