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Old 12-16-2011, 05:31 PM   #11
ajdelange
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Would this work?
I from time to time think about putting load cells under the fermentor mostly as an aid in filling, adding dilution water etc. I also thought about monitoring fermentation progress that way but decided it wouldn't be likely to work too well because:
1. Ostensibly, CO2 loss is all that is going on. The 0.11 g yeast biomass per gram of alcohol stays in the fermentor. But you also lose water vapor.
2. The load cells would have to be quite precise i.e. have to be able to measure gram or subgram level mass changes while at the same time having to be able to handle 200+ kg of beer plus the empty weight of the fermentor plus the weight of coolant.
3. Variabllity in the amount of coolant at the time of any particular weighing would probably mask the small changes in mass from CO2 loss.
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Old 12-16-2011, 10:22 PM   #12
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Originally Posted by ajdelange View Post
I from time to time think about putting load cells under the fermentor mostly as an aid in filling, adding dilution water etc. I also thought about monitoring fermentation progress that way but decided it wouldn't be likely to work too well because:
1. Ostensibly, CO2 loss is all that is going on. The 0.11 g yeast biomass per gram of alcohol stays in the fermentor. But you also lose water vapor.
2. The load cells would have to be quite precise i.e. have to be able to measure gram or subgram level mass changes while at the same time having to be able to handle 200+ kg of beer plus the empty weight of the fermentor plus the weight of coolant.
3. Variabllity in the amount of coolant at the time of any particular weighing would probably mask the small changes in mass from CO2 loss.
It could work on a smaller scale production though. If you were able to keep a smaller vessel of the same recipe at the same temperature, you could do it that way. I for example only have a 2 quart carboy (read "empty glass juice jug") that I'm experimenting with. During the most vigorous period of fermentation, I would expect to loose about half a gram of CO2 a day, which is an entirely readable amount by an inexpensive scale. As it slows, error is going to become a bigger part of your reading, but practical knowledge can still be gleaned ie. the trend of the data. All of this is just for fun though. By working backwards, and assuming you know a couple other variables, a hydrometer tells you most of what you need to know.

Also, about the water vapor loss. Are you talking about water evaporating and being forced out by the CO2?
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Old 12-16-2011, 10:44 PM   #13
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Also, about the water vapor loss. Are you talking about water evaporating and being forced out by the CO2?
Yes. After sufficient CO2 has evolved to expel the air from the fermentor headspace it contains CO2 and water vapor and the mix passes out through the airlock. But thinking about it for a minute this probably isn't a major factor. The vapor pressure of water at 20 °C (which is high for fermentation) is less than 17 mmHg and water vapor is less dense than CO2.
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Old 12-17-2011, 01:50 PM   #14
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I'd also recommend using a balloon with pin hole so there will not be significant mass loss due to evaporation of the airlock solution with your small scale trial.

There will be many statistical errors to track down, including CO2 absorption in solution, but I think you will get an interesting data set and graph, post results here!

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Old 12-17-2011, 04:36 PM   #15
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Originally Posted by Hex View Post
I'd also recommend using a balloon with pin hole so there will not be significant mass loss due to evaporation of the airlock solution with your small scale trial.

There will be many statistical errors to track down, including CO2 absorption in solution, but I think you will get an interesting data set and graph, post results here!
On my next one. I'll start something over the holidays and track it. I've got an empty carboy.

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Yes. After sufficient CO2 has evolved to expel the air from the fermentor headspace it contains CO2 and water vapor and the mix passes out through the airlock. But thinking about it for a minute this probably isn't a major factor. The vapor pressure of water at 20 °C (which is high for fermentation) is less than 17 mmHg and water vapor is less dense than CO2.
Wouldn't it's lower density cause it to rise to the top of the headspace and be the first to be forced out? In any case, I think it's a more of a factor when measuring overall fermentation than when taking point readings.
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Old 12-20-2011, 01:41 AM   #16
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I started a batch. A white wine. I managed to get a hydrometer from the neighbor so I'll be tracking both gravity and CO2 production. I'll graph it as % alcohol. I'll also include ambient temperature, which should be just under 70 for the whole run but I'll include it anyway. Please note that these calculations are rudimentary and only approximate. The trend data is really the only useful information that I'll be able to show.

Anyway, initial gravity is 1.098, so there's plenty of sugar.

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Old 12-20-2011, 12:36 PM   #17
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Wouldn't it's lower density cause it to rise to the top of the headspace and be the first to be forced out? In any case, I think it's a more of a factor when measuring overall fermentation than when taking point readings.
Let's suppose that the water vapor did rise faster than the CO2. Then the partial pressures of water vapor would be lower than the equilibrium value for the given temperature and more would leave the beer to balance the chemical potential. I don't think it does rise faster, however, because I think brownian motion keeps it mixed. I think it stays mixed and gets swept out with the CO2.
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Old 12-20-2011, 12:42 PM   #18
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I ferment with a balloon as my airlock. The fun advantage of this is it allows you to roughly measure the rate of fermentation. If it takes roughly twenty seconds to fill a ~75ml balloon with CO2, then you can crunch the numbers and figure out that roughly 93.3 Quintilian (9.33x10^19) molecules of CO2* are being produced every second.

We also know that 2 molecules of CO2* are made from every sugar molecule. If the population size is known, you can then calculate the rate of fermentation per cell (since yeast are unicellular organisms). The hard part is counting them all.

If you also know the maximum alcohol tolerance of the specific yeast you are using, then you can estimate the appropriate amount of sugar to add to achieve a desired result, be it dry or sweet. This would be useful for recipies you've not yet tried before. Tracking the rate of fermentation at several hour intervals also offers insight into the alcohol tolerance of certain yeasts, assuming abundant sugar and appropriate temperature.

Footnotes:
* - and ethanol
I think this is pretty cool.
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Old 12-20-2011, 03:47 PM   #19
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Let's suppose that the water vapor did rise faster than the CO2. Then the partial pressures of water vapor would be lower than the equilibrium value for the given temperature and more would leave the beer to balance the chemical potential. I don't think it does rise faster, however, because I think brownian motion keeps it mixed. I think it stays mixed and gets swept out with the CO2.
Ok. Right. I see what you're saying now. Plus the CO2 coming off the liquid so quickly would also probably help to keep things stirred up. Either way, there's nothing that can be done about it. Not with turkey basters and balloons for lab equipment
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Old 12-22-2011, 03:25 PM   #20
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Coming along here. Some surprising results. Here's a taste. An almost entirely linear increase in ABV over time. The CO2 readings would not suggest this. If you look closely though, you can see that the latest reading is a little below the line of best fit. Nearing completion now, so stay tuned.

untitled.jpg  
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