My airlock keeps becoming empty on one side, is that normal?

Question

This is my first time brewing and I only started yesterday, so I'm still very much a novice.

I have a bubbler airlock on my 33 litre fermenting bucket, like the one in this image:

I filled it up to about 50% at each side and left the bucket overnight, when I checked it around 7 hours later, it had become empty on one side (right side if that matters). I filled it back up to 50% each side and went off to work, when I checked it just now (10 hours later), it's the same as it was this morning, 50% full on the left side and empty on the right side. I'm currently brewing 20 litres (40 pints) of cider and the temperate in the cupboard is around 18c - 20c, fairly consistently.

Is this how the airlock should function? Am I suppose to leave one side empty and have the other half full? Or do I need to keep topping it up with water every 6-8 hours or so?

Thanks for any help you can provide.

Carl.

Answer 1

Totally normal. C02 is SLOWLY pushing the water to the opposite side of the lock. As fermentation starts to really kick in, you'll see much more movement and "bubbling" in the airlock.

Answer 2

You don't need to keep topping up with water. As long as there is some water inside the airlock, there is a barrier to the outside environment allowing the generated carbon dioxide to escape but not allowing unwanted air in.