For the primary fermentor, your yeast are actively creating CO2 and displacing the existing oxygen in the container, so you don't have to worry about purging it. However, in a secondary fermentor, a CO2 purge would minimize the risk of oxidization of your beer and contamination.
We learned a cool trick from our local homebrew store using a funnel and dry ice, and thought we'd share it here!
- Remember to use good gloves and/or tongs when handling dry ice!
- You only need a small block--maybe the size of a full wallet or a deck of cards.
- Check your dry ice for dirt, and use a sink brush or sponge to quickly clean it.
Check your funnel for a small notch in the stem--this method will only work if there is a way for your purged gas to exit.
Place the sanitized funnel into your carboy, and carefully place a small amount of dry ice in the funnel.
Since CO2 is heavier than air, the CO2 will sublimate off the dry ice, travel down the funnel, and displace the air out of the carboy through the stem notch. Let the sublimating CO2 purge until you see the condensed air (what looks like fog) come out of the stem notch.
Be careful when removing the funnel: water frost will form on the outside of the funnel, and you don't want to accidentally shake or knock this into your fermentor!