Truma has invented this very practical device that tells you how full a gas bottle is, and it uses ultrasound technology to check the level.
The Truma levelcheck is held horizontally against the gas cylinder and a status LED turns green if there is gas at the measuring point on the cylinder, or red if there is no gas. This means it's very easy to establish how full the gas cylinder is. The ultrasound measurement makes it very precise and reliable.
Whilst the device is determining the gas level, the LED display flashes orange, the start and end of the measuring process is indicated by a beep tone.
The practical LED torch that is integrated into the device is an added benefit