The food packaging industry has for some time now made use of an on-line sensor to monitor the level of liquid in cans or bottles after they have been filled on the production line. Such devices generally employ a source of gamma radiation which propagates through the side of each can as it passes. The attenuation of the radiation is measured by a detector on the other side, thus indicating if the can is full at the level of the detector. This system presents two problems; -- The system only detects the presence of liquid at the level of the sensor. A can filled too low will be detected, but a can filled too high will register as being correctly filled. There is no quantitative measurement of the height of the liquid. The utilisation of a gamma emitting isotope presents health and safety problems in the use of the sensor, as well as in disposal of the unit at the end of its working life. There is a need for an alternative detection system which measures the exact height of liquid in each can as it passes on the production line. The possibility of weighing each can is inappropriate for the speed of production lines, and so novel methods should be developed. The proposed system to be developed within this project will incorporate a small waterjet (a few mm high) which will strike the bottom of each can as it passes. This contact will mediate an ultrasonic pulse, which will travel through the bottom of the can, and through the liquid inside. Air is an effective barrier for ultrasound, and the pulse will reflect off the liquid/air boundary. By measuring the "time of flight" up and down the can, it is possible to calculate the level of liquid inside. Such a system will be considerably safer, more accurate, and much more versatile than the present level detectors.