A volume sensing system for use in determining a volume of a variable volume reservoir can include a lower plunger that can have a first end adapted to move with a bottom of the reservoir. An upper plunger can be slidable relative to the lower plunger. A lower biasing member can be positioned substantially between a base of the reservoir and the upper plunger, and the upper biasing member can be positioned between the upper plunger and an interference member. The upper biasing member can have a predetermined stiffness relative to the lower biasing member such that upon movement of the lower plunger, the upper plunger can move at a predetermined fraction of the amount of movement of the lower plunger, where the predetermined fraction can be determined at least in part by the predetermined stiffness of the upper biasing member relative to the lower biasing member.
