Originally Posted by Cervezero1960
Bubbles, are CO2 and CO2 is a direct by-product of fermentation, so don't tell me that airlock bubbles and fermentation are two different things
While you're right that the bubbles are CO2, the issue is that there are multiple reasons for why
that CO2 is coming out of the airlock rather that just staying in the fermenter.
During fermentation, the CO2 exits through the airlock because the pressure inside the fermenter exceeds the outside air pressure. So the CO2 gets "pushed" out the airlock.
But the mistake you're making is in assuming that fermentation is the only
cause of a pressure differential. If the pressure outside the fermenter decreases (such as when a low pressure weather front passes over your area), then the pressure inside the fermenter will be relatively higher than the air outside of it. CO2 will vent out the airlock to equalize the pressures again, even though no fermentation is actually occurring.
Likewise, if the temperature of the room containing the fermenter warms up, this will produce a drop in air pressure (warm air is less dense than cold air). Again, CO2 will vent out of the airlock until the pressures inside and outside of the fermenter are equal again.
So, yes, a gravity reading is the only way to know whether or not fermentation is still occurring. A bubbling airlock does not necessarily mean the yeast are producing CO2.