Is the keg empty? Acoustic beer keg monitor

Using a Raspberry Pi to roughly detect when the fill level of a liquid in a closed steel container (Otherwise known as a beer keg).

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Quantity		Component name
1	×	Raspberry Pi B+
1	×	Witty Pi
1	×	Adafruit PowerBoost 1000 Charger
1	×	Transducer
1	×	Adafruit Stereo 3.7W Class D Audio Amplifier - MAX98306