# Why is counting airlock bubbles a poor method of determining whether or not fermentation has slowed down or stopped?

I am eager to figure out how to measure the specific gravity of my wort using a Raspberry Pi, Arduino, or other device, but I do not wish to spend \$250 for a BeerBug. People on the internet have used the RPi to count bubbles per minute in the airlock and charted them on a graph.

My sole purpose of this is to determine when my RPi should tell my Fermwraps to turn on and increase the heat of the carboy. On this stackexchange I've been told it is a good idea to warm up the carboy a bit when fermentation has slowed down or stopped.

If yeasts consumes sugar and produces alcohol and CO2, and the only way for the CO2 to escape is through the airlock, then once C02 stops escaping through the airlock, no more C02 is being produced. Therefor, counting air bubbles in the airlock is a reasonable method of determining if fermentation has stopped.

What is wrong with this syllogism?

• Counting bubbles may tell you when fermentation has stopped, but that won't tell you the SG. Fermentation could stop for lots of reasons besides no more sugar. Apr 7, 2014 at 14:10
• You have a solution in search of a problem. Apr 7, 2014 at 16:11
• @DennyConn I thought I worded the question well enough to avoid that accusation :) -- but yes, you are right. Apr 8, 2014 at 3:49

The other compounding factors are temperature changes and atmospheric pressure changes:

• as temperature changes the pressure inside the headspace will change, causing fewer if the temperature is lower or more bubbles if the temperature is higher to be released
• atmospheric pressure: changes in atmospheric pressure will cause more or less bubbles to be released, similar to temperature changes, but in reverse - higher atmospheric pressure will cause fewer bubbles to be released until the internal pressure matches the atmospheric pressure.

How significant these changes are is not clear to me, but to build a complete model I would think it's necessary to monitor both temperature and internal/external pressure. Then you can decide if the bubble rate changes due to changes in pressure can be some how accounted for or ignored (obviating the need to sense pressure.)

I built a simple proof-of-concept wifi enabled-bubble counter from a spark core a few months back that used conductance rather than a IR sensor. I've not put it to use yet in my brewery.

• Would it be fair to say that if I account for temperature changes, and internal/external pressure changes, then counting airlock bubbles would be precise? Apr 7, 2014 at 8:35
• Forgive my ignorance, but why would external or internal pressure change happen in the first place? I understand temperature changes, given its palatable nature, but do not understand the pressure issue at all. Apr 7, 2014 at 8:51
• Damn, I was excited to read your POC but after reading it I must admit I couldn't follow any of it. Do you happen to have a summary article you could link me to explaining the difference between conductance vs IR for this purpose? I was under the impression that those who have done this use an audio sensor to detect the bubbles. This appears to be more precise, from what I can understand. Apr 7, 2014 at 8:53
• With conductance you physically detect the bubble since the conductors go from being immersed in water, to not, and then being immersed again. I imagine there's potential for an audio device to be tripped by any ambient noise, and an IR sensor doesn't always pick up the bubble. But really every single bubble isn't important - you just need to look at the rate of change of bubbling - the absolute bubble frequency is less important.
– mdma
Apr 7, 2014 at 11:43
• Do you favor a particular sensor you use which I could google and purchase? Apr 8, 2014 at 3:53