This is a makedo airlock; I couldn't make it shorter as the loop bends itself and air couldn't pass through.
It will probably be OK. The smaller the bore of the tube, the greater the tendency will be for any bubbles to push the liquid right out. You could just make a blow-off instead by submerging the venting end of the tube in a vessel full of water.
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I second the water vessel idea. That prevents insects like fruit flies from entering. – Robert May 7 '18 at 2:07
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1I agree. Just 1/2 fill the bottle and submerge the tube. Using a loop can easily blow out and go dry. – Evil Zymurgist May 7 '18 at 2:59
