I would guess one inch below the shoulders of the bottle??

The small fermentation bottle pictured above is not something I have dealt with before. However I might say this? In general, never let the bottom of the airlock get clogged. Explosions occur! If you are worried about clogging just spray a small piece of plastic wrap with sanitizer and rubber band-it to the top of the bottle instead of the air lock. Not tight mind you. Just like a deflated balloon about to be inflated? Remember as a child we all put baking powder in a bottle and added water or vinegar and then placed a balloon over the mouth of the bottle. Same principle.

Remember that the fermenting beer fills the fermentation bucket or carboy with C02. The Carbon dioxide exerts a constant positive pressure keeping airborne yeasts and bacterias out (for the most part.) After the vigorous fermentation is over and you deem it safe. Replace the plastic wrap with a airlock for the rest of the primary ferment. I would place the bottle in a pan or bowl in case the wort happens. At least you won't have an explosion!

Cheers

I try to keep the headspace at @ 20% of the total volume (I usually ferment 5 gallons in a 6 gallon fermenter) but I still get some messy blow-off sometimes. Bob was wise to caution against allowing the airlock to get plugged with crud, this will cause either a volcanic eruption of krausen which will cover your ceiling and make your wife really mad or it will cause your glass fermenter to explode (I use plastic). Follow Bob's advice and just cover the opening with something loosely (I use aluminum foil) until the majority of the fermentation is complete, then you can install the airlock and let it finish up. CO2 will be produced the entire time fermentation is happening so there is no danger of any "nasties" settling into your beer and once the fermentation slows a bit there will be enough alcohol in the beer to kill any bad bugs anyway.

---Todd

Hi NMP

Another option is a blow off tube. http://www.howtobrew.com/section1/chapter9.html Goes into more detail and even has helpful diagrams. The rest of the site is worth reading to.