Originally Posted by Brew_Master
Well that's the point of an AIRLOCK to make the fermenter airtight, and yeah I know it can't be as reliable as an SG reading but you have to wait a day or two between readings to tell if activity has ceased but usually if there are no bubbles that's an instant hint that fermentation has stopped or is really really slow
Exactly. Its a hint. Not a guarantee of anything, and that's the point people are trying to make when they say the airlock is an unreliable sign of fermentation.
In order to see airlock activity for a given fermentation, all of the following must be true:
1) fermentation occurs, producing CO2
2) fermenter is sufficiently sealed so that CO2 has no significant escape path other than through the airlock/blowoff tube
3) you happen to check the airlock for activity frequently enough/long enough/at the right time to witness the release of CO2
So if you didn't quite seal your fermenter, or you just happen to check at the wrong times, you can certainly go through a completely successful fermentation without ever seeing a bubble.
In addition, there are a number of things that could cause airlock activity outside of active fermentation, the most common of which are probably:
1) your fermenter warmed up a little, and the gas in the vapor space expanded, building pressure and causing gas to vent
2) your fermenter warmed up a little, resulting in gas coming out of solution, since gas is less soluble at high temperatures.
So yes, airlock is a hint. If you had two fermenters sitting next to each other and one had a bubbling airlock and one didn't, and you had to guess which one was actively fermenting, you would guess the one with the bubbling airlock, for sure. But airlock activity is not a sure fire sign one way or the other, and the reason this gets stressed on this board is because of the large number of "OMG, my beer isn't fermenting" threads that end up with "oh, it turns out everything was fine. :-D