If there is a leak in the bottle, gas escapes from the headspace, and the fill level rises and is detected.
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Heuft USA (Downers Grove, IL) has released a leak detector for PET containers for liquids. The Heuft Squeezer guides containers by a belt drive that exerts pressure on the PET bottles.
If there is a leak in the bottle, gas escapes from the headspace, and the fill level rises and is detected.