Most leak test systems use an elastomeric material as the test part seal. The elasticity may vary of course but they are all to a degree flexible and as the part fills with air the seal will yield or move in response to the force of the increasing pressure.

Any movement of the seal will create slightly larger volume and thus a pressure drop, possibly resembling a leak. For this reason the leak tester features a step in the cycle called the stabilization step. Since many medical devices are to some extent elastic, being made of plastic or featuring silicone, the importance or recognizing elastic creep must be stressed. Uson has new blog post at which discusses the effects of elastic creep and how they may be overcome.