An implantable pump (10) includes a base plate (14) and a can (13) that are parts of the pump housing (12). The base plate bottom divides the housing into a first electronics chamber and a second chamber. A bellows mechanism (20) is connected to the base plate and is disposed within the second chamber. The bellow mechanism has an expandable sidewall and a bottom plate. The bellow mechanism divides the second chamber into a medicament receiving portion (26) and a non-medicament-receiving portion (28). The bellow mechanism has an intermediate plate (32) disposed within the medicament-receiving portion. A coil (34) is disposed in a recess on the lower surface of the base plate, and the coil is spaced from an internal wall of the recess. The amount of fluid remaining in the implantable pump can be monitored by energizing the coil. A primary magnetic field is generated by the energized coil. A secondary magnetic field, which is dependant on the position of the bottom and intermediate plate of the bellow mechanism, couples back to the primary field affecting the electrical characteristics of the coil. The amount of fluid remaining in the reservoir is determined based on the resulting changes in the electrical characteristics of the coil.
