Wide bandwidth air-coupled ultrasonic testing of food containers in air

Air-coupled NDE has been used to perform measurements on food containers. This relies on the broad bandwidth available from polymer-filmed capacitive transducers, combined with pulse compression techniques. The first experiments involve liquids within cylindrical polymer containers. It will be demonstrated that transmission through the drinks bottles can be used to measure liquid level, either from monitoring the through-transmitted signal directly, or by observation of a reflection from the liquid surface. This can be achieved without contact to the container. By scanning the transducers around the container, it is also possible to collect tomographic data. It will be demonstrated that this can be used to reconstruct air-coupled cross-sectional images of such containers, so that contaminants can be located. It is also shown that the temperature of a liquid can be estimated successfully, using time-of-flight measurements. The result is a powerful method for the NDE of such materials, which could be applied to production-line situations.