A number of parameters related to the operation of a fluid delivery device are determined based on a pressure within the device sensed using a pressure sensor. In one example, the volume of therapeutic fluid added to or removed from a reservoir of a fluid delivery device is determined based on a sensed pressure of the reservoir. In another example, the volume of therapeutic fluid added to or removed from the reservoir is determined based on a sensed pressure of a refill port assembly of the device. In another example, an initial temperature of the reservoir as a therapeutic fluid is removed from the reservoir is estimated based on a sensed pressure within the device. In another example, a temperature of a therapeutic fluid added to the reservoir is estimated based on a sensed pressure within the device.