Patent ID: 8561460

Claim:
A volume sensing system for use in determining a volume of a variable volume reservoir, the system comprising: a plunger assembly, including: a lower plunger having a first end and an opposite second end, the first end adapted to move with a bottom of the reservoir; an upper plunger having a first end and an opposite second end, the upper plunger slidable relative to the lower plunger; a lower biasing member positioned substantially between a base of the reservoir and the upper plunger; an upper biasing member adapted to be positioned between the upper plunger and an interference member; wherein the upper biasing member is configured to have a predetermined stiffness relative to the lower biasing member such that upon movement of the lower plunger in a first direction, the upper plunger is configured to move in the first direction at a predetermined fraction of the amount of movement of the lower plunger, and wherein a position of the upper plunger is adapted to be correlated to a corresponding volume of the reservoir; a first sealing member carried by the lower plunger; and a second sealing member carried by an outer surface of the upper plunger.