Mead Vacuum?

I made my first mead on saturday late afternoon finished around 7pm. My girlfriend's parents gave me 8# of honey for a birthday gift, and i was going to make a 1/2 batch (since i have only had mead once and it was overly spiced and didnt taste of honey at all). I used the recipe in the "Complete Joy of Homebrewing 3rd ed" After i was done pitching the yeast (I used Vierka Mead Yeast, which didnt look anything like the dried yeast i have used before for my beers, there was other stuff in it...i dont know and i will probably never use it again, i'll use white labs) I put the air lock on it.

About 4 hours later i went to check on it, and the airlock showed it pulling a vacuum! The water in my airlock was pulled tward the carboy side instead of being pushed to the atmospheric side of the air lock. I pulled it off, and re-corked it releasing the vacuum. The next morning, i checked it again and it pulled another vacuum.

Today when i got home from work, it was finally pressurized and acting normally.

The only thing i would think that would cause a vacuum would be is if i dint cool the liquid and it cooled in the confined space causeing a vacuum. Has anyone ever seen this phenomenon? What caused this? Thanks

You answered your own question. If your mead, wort, must, or anything is warmer than ambient temperature when you put the airlock on it, it can create a vacuum by cooling. A good reason to fill the airock with sanitizer instead of tap water. I've had the thing completely empty before.

Definitely want to keep that airlock full at all times, especially if you have fruit flies. Fruit flies + mead = tasty vinegar

It pulled a 2nd vacuum after the fist was released after 4 hours, so it is not a cooling thing. (plus i cooled the mead wort to approx 80°F for pitching, and i am fermenting at 70°F so a 10° difference will not create that much vacuum.) Im still confused...

Thanx eviltwinofjoni, I'll keep an eye on the air lock. (I've never fermented something as long as this will be in the fermenter)

It might have been absorbing oxygen from the air as the yeast used up what was in the mead. An airlock (specifically the one piece ones) don't have much volume, so minor changes in temperature, air pressure and oxygen can make a difference.

If you made 2.5 gallons in a 5 gallon carboy, each of these impact the airlock level much more than 5 gallons in a 6.5. A two millibar increase in air pressure would drain the airlock.

Reading this RE-reminded me to check the airlock on my lager in the fridge in the garage...all's well. Thanks for the reminder though...