A volume of fluid moved by a pump, such as a pump in an APD system, may bedetermined without direct measurementof the fluid, such as by flow meter, weight, etc. For example, a volume of apump chamber (181) (having a movable elementthat varies the volume of the pump chamber) may be determined by measuringpressure in the pump chamber and a referencechamber, both while the two chambers are isolated from each other, and afterthe two chambers are fluidly connected so that pressures inthe chambers may equalize. Equalization of the pressures may be assumed tooccur in an adiabatic way, e.g., a mathematical modelof the system that is based on an adiabatic pressure equalization process maybe used to determine the pump chamber volume. Inone embodiment, pressures measured after the chambers are fluidly connectedmay be measured at a time before complete pressureequalization has occurred, and thus the pressures for the pump and referencechambers measured after the chambers are fluidlyconnected may be unequal, yet still be used to determine the pump chambervolume.
