This is a makedo airlock; I couldn't make it shorter as the loop bends itself and air couldn't pass through.
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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
