To test the tightness and/or the correct closure of containers which are transported on a conveyor, the containers succeeding each other at brief time intervals and the containers being sealed by attachment of a closure, a characteristic dependent on the internal pressure (internal-pressure characteristic) of the container is measured. Upon attachment of the closures to the containers, the internal-pressure characteristic or parameters of the closures or containers, knowledge which is necessary to measure the characteristic depending on the internal pressure, are recorded. The containers are marked no later than when the closure is attached in a way which permits the values recorded upon attachment of the closures to be allocated to the respective container. The internal pressure of the container is ascertained from the value, measured at a time interval after the attachment of the closure, of the internal-pressure characteristic, and comparison with the value of the internal-pressure characteristic measured upon attachment of the closure or taking account of the parameters recorded. In the case of rigid containers such as glass bottles, the internal-pressure characteristic is typically the oscillation frequency of the closures; in the case of flexible containers such as PET bottles, on the other hand, it is the fill level. The marking can consist of a magnetization of the closure.
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