How dies airlock keep oxygen out

[anico4704]

I am wondering if I use an s shaped airlock and it sucks back the vodka into the first chamber, would this be introducing oxygen into the beer even if the vodka doesn't get sucked all the way through into the wort? Isn't this still air being sucked in or am I missing the point of the airlock? How does it keep oxygen out?

 

[zachattack]

If you have negative pressure in the carboy (temp dropping is generally the only thing that causes this), then yes, it will suck air in through the airlock. In a 3-piece airlock the sanitizer/vodka will get sucked in with the air, in an S-shaped airlock it does not. If you're going to be dropping the temperature a lot after fermentation is over (ex. cold crashing, lagering) you can replace the airlock with a solid bung.

 

[unionrdr]

It vents the excess co2 without letting air in. During a suck back,the liquid gets in,so use something that'll kill the nasties. Vodka or Starsan are most common. A little air getting in is ok,but bleed off some pressure now & then while it's still bubbling a little,off gassing co2. I like the 3 piece airlocks better for this. If you control the temp,you can lessen the suck back effect a little.

 

[AlCophile]

Follow up the bubbler tube with a tubular filter containing clean cotton. This will keep particulates(fruit flies!) out. If you place a length of tubing between the bubbler and cotton filter, and let the ferment fill it with CO2, it should remain in the tubing for a long time. With an obstruction at each end, the CO2 should stay put. Or, slap on a second bubbler.
Also, no rule against using CO2 from a later batch. I gotta do EVERYTHING for you folks?