Reservoir volume in a drug delivery device is sensed by providing a capacitor, the capacitance of which varies with bellows position or, alternatively, with the amount of propellant liquid absorbed in a dielectric material. In one embodiment, a capacitance is provided between a surface of the bellows,...http://www.google.com/patents/US6542350?utm_source=gb-gplus-sharePatent US6542350 - Reservoir volume sensors
