Estimation of fermentation from bubbles in airlock

[luis.ga...@gmail.com]

I have a question I would like to adress to the more theorist of the group.

Is there a model that describes the fermentation level (in terms of speed of fermentation) by the speed of CO2 release in a carboy (number of bubbles expeled from the carboy per minute for example)?

I assume, has I never seen such a graph/formula/model, that it doesn't exist but I think this could be quite interesting. Even if it isn't as accurate has a density drop reading, it could give the ciderist a broad idea of when an action should be taken to improve the process (lower/increase temperature, rack, etc.).

Could such a model be theorized, knowing the approximative amount of CO2 in a bubble coming from an airlock?

Can't wait to see your opinion about this!

Louis

[Claude Jolicoeur]

Le dimanche 6 novembre 2016 22:09:24 UTC-5, luis.ga...@gmail.com a écrit :

Is there a model that describes the fermentation level (in terms of speed of fermentation) by the speed of CO2 release in a carboy (number of bubbles expeled from the carboy per minute for example)?

I assume, has I never seen such a graph/formula/model, that it doesn't exist but I think this could be quite interesting. Even if it isn't as accurate has a density drop reading, it could give the ciderist a broad idea of when an action should be taken to improve the process (lower/increase temperature, rack, etc.).

Could such a model be theorized, knowing the approximative amount of CO2 in a bubble coming from an airlock?

It is fairly easy to calculate the amount of CO2 produced...

A density drop of 1 point of density means approximately 2.13 g/L of sugar has fermented.
Approximately 47% of this fermented sugar is transformed into CO2, hence 1 gram of CO2 per liter of cider for each point of density drop.
Volumic mass of CO2 is 2 g/L under standard conditions (i.e. 20C), then this 1 g of CO2 is 500 mL, but this will vary with temperature.

Where it becomes more difficult is to evaluate the volume of one bubble. I would think this will vary from one airlock to another, and also depend on the amount of liquid in the airlock. Anyway, for the fun of it, let's assume 0.5 mL per bubble.

So let's calculate the production of CO2 per hour, taking FSU as density drop per 100 days, and V the volume of cider
Density drop in points per hour is FSU / (100 days x 24 hours per day) = FSU / 2400
Production of CO2 in mL per hour is (FSU x V x 500 mL) / 2400

So, let's say a carboy of 20 L, FSU of 50, this would produce 208 mL per hour, at .5 mL per bubble, approximately 400 bubbles per hour, or 7 per minute, or about 9 seconds per bubble. Would this make sense?

[luis.ga...@gmail.com]

Wow, it is a very interesting model.

The only ''random'' variable is the volume of a bubble coming from the S shaped airlock....

Assuming a bubble effectively has a volume of 0,5 ml, it means the initial FSU of my cider is quite important! A wild ferment, when it began to vigourosly ferment at 15 degree, expeled an average of about 120 bubbles per minute in my 23L carboy, for a FSU of approx. 750! I then cooled it down to 12 deg. for a decrease of fermentation to about 30 bubbles per minute, for a FSU of circa 200...

By taking a record of the fermentation every day or so (in bubbles per minute), with gravity readings, it could even by possible to get a clearer idea of the volume of co2 contained in a bubble.

Thank you Claude for the input!

Louis

[Francis Bonenfant]

Well, the easiest way to measure the amount of CO2 coming from a fermentation is to rig a three-piece airlock with a flexible plastic pipe, which you then stick in a graduated cylinder that's upside down in a beaker of water. You can then measure the volume of generated gas, as it will displace water from the bottom of the cylinder. It's not super precise, but better than counting bubbles. Assume that the gas is mostly CO2 and follows the idea gas law and you can calculate the mass and moles of evolved CO2. 

[luis.ga...@gmail.com]

It is a bright idea!

Thanks for the input.

Louis