Big reason I'm not liking the new Big Mouth Blubbers for aging... too much head space in the big necks. had a couple batches get nasty bacteria on top. Hopping to save them with good 4x meta solutionI’m making much better effort now to keep it all topped up with absolute minimal headspace even though I also use headspace eliminators on the carboys.
I think it alos has to do with wall thickness of the glassI've racked from one 5 gallon carboy to another and had it overflow by a significant amount. They vary within an few % manufacturer to manufacturer.
No joke, I use to top mine right up to the bottom of the cork, or within a 1" of it. Not anymore. With heat they expand and will rise up into your airlock.But liquid volume is not constant. It will change by temperature. So when you state that something holds 3 gallons at what temperature is that volume accurate? And does all liquid expand and contract at the same rate when heated or cooled?