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A number of parameters related to the operation of a fluid delivery device are determined based on pressures within the device sensed using multiple pressure sensors. In one example, the volume of therapeutic fluid within a reservoir of a fluid delivery device is estimated based on a measured pressu

A number of parameters related to the operation of a fluid delivery device are determined based on pressures within the device sensed using multiple pressure sensors. In one example, the volume of therapeutic fluid within a reservoir of a fluid delivery device is estimated based on a measured pressure differential. In another example, the rate at which a therapeutic fluid is added to or removed from the reservoir is estimated based on the measured pressure differential.

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1. A method comprising: measuring a pressure within a therapeutic fluid reservoir of an implantable fluid delivery device with a first pressure sensor;measuring a pressure within a chamber of the fluid delivery device at least partially surrounding the reservoir with a second pressure sensor; andest

1. A method comprising: measuring a pressure within a therapeutic fluid reservoir of an implantable fluid delivery device with a first pressure sensor;measuring a pressure within a chamber of the fluid delivery device at least partially surrounding the reservoir with a second pressure sensor; andestimating a volume of a therapeutic fluid within the reservoir based on a pressure differential between the measured pressure within the reservoir and the measured pressure within the chamber. 2. The method of claim 1, wherein the chamber comprises a propellant gas chamber configured to store a propellant gas configured to regulate the pressure within the reservoir. 3. The method of claim 1, wherein estimating the volume of the therapeutic fluid within the reservoir comprises estimating the volume of the therapeutic fluid within the reservoir based on the pressure differential, a baseline volume of therapeutic fluid in the reservoir, and a pressure sensitivity to volume changes constant for the reservoir. 4. The method of claim 3, wherein the baseline volume of therapeutic fluid in the reservoir comprises a volume of therapeutic fluid in the reservoir when the reservoir is in a substantially unloaded equilibrium state. 5. The method of claim 3, further comprising retrieving at least one of the baseline volume of therapeutic fluid in the reservoir or the pressure sensitivity to volume changes constant for the reservoir from a computer-readable storage medium. 6. The method of claim 3, wherein estimating the volume of the therapeutic fluid within the reservoir comprises: dividing the pressure differential by the pressure sensitivity to volume changes constant for the reservoir; andadding the baseline volume to the pressure differential divided by the pressure sensitivity to volume changes constant. 7. The method of claim 1, wherein estimating the volume of the therapeutic fluid within the reservoir comprises estimating a rate at which the therapeutic fluid is added to or removed from the reservoir based on the measured pressure within the reservoir and the measured pressure within the chamber. 8. The method of claim 7, wherein estimating the rate at which the therapeutic fluid is added to or removed from the reservoir comprises estimating the rate based on a rate of change of the pressure differential over time and a pressure sensitivity to volume changes constant for the reservoir. 9. The method of claim 7, wherein estimating the volume of the therapeutic fluid within the reservoir comprises estimating the volume based on the estimated rate at which the therapeutic fluid is added to or removed from the reservoir and a baseline volume of therapeutic fluid in the reservoir. 10. The method of claim 9, wherein the baseline volume comprises a starting volume of therapeutic fluid in the reservoir when the therapeutic fluid is first added to or removed from the reservoir. 11. The method of claim 10, wherein estimating the volume of the therapeutic fluid within the reservoir comprises: multiplying the estimated rate at which the therapeutic fluid is added to or removed from the reservoir by a time over which the therapeutic fluid is added to or removed from the reservoir; andadding the starting volume to the estimated rate multiplied by the time. 12. The method of claim 1, further comprising generating an alert based on the estimated volume of the therapeutic fluid within the reservoir. 13. The method of claim 12, wherein the alert comprises at least one of a reservoir empty state, a reservoir full state, or a maximum fill rate exceeded alarm. 14. The method of claim 12, wherein the alert comprises at least one of an audible, visual, or tactile alert. 15. The method of claim 1, further comprising displaying an indication of the estimated volume of the therapeutic fluid within the reservoir. 16. The method of claim 15, wherein displaying the indication of the estimated volume comprises displaying a volume gauge on a computing device communicatively connected to the fluid delivery device. 17. A fluid delivery system comprising: an implantable fluid delivery device comprising: a reservoir configured to store a therapeutic fluid;a chamber at least partially surrounding the reservoir;a first pressure sensor configured to sense a pressure within the reservoir; anda second pressure sensor configured to sense a pressure within the chamber; anda processor configured to receive the sensed measurements of the pressure within the reservoir from the first pressure sensor, receive the sensed measurements of the pressure within the chamber from the second pressure sensor, and estimate a volume of a therapeutic fluid within the reservoir based on a pressure differential between the measured pressure within the reservoir and the measured pressure within the chamber. 18. The system of claim 17, wherein the chamber comprises a propellant gas chamber configured to store a propellant gas configured to regulate the pressure within the reservoir. 19. The system of claim 17, wherein the implantable fluid delivery device comprises the processor. 20. The system of claim 17, further comprising an external programmer comprising the processor. 21. The system of claim 17, wherein estimating the volume of the therapeutic fluid within the reservoir comprises estimating the volume of the therapeutic fluid within the reservoir based on the pressure differential, a baseline volume of therapeutic fluid in the reservoir, and a pressure sensitivity to volume changes constant for the reservoir. 22. The system of claim 21, wherein estimating the volume of the therapeutic fluid within the reservoir comprises: dividing the pressure differential by the pressure sensitivity to volume changes constant for the reservoir; andadding the baseline volume to the pressure differential divided by the pressure sensitivity to volume changes constant. 23. The system of claim 17, wherein estimating the volume of the therapeutic fluid within the reservoir comprises estimating a rate at which the therapeutic fluid is added to or removed from the reservoir based on the measured pressure within the reservoir and the measured pressure within the chamber. 24. The system of claim 23, wherein estimating the rate at which the therapeutic fluid is added to or removed from the reservoir comprises estimating the rate based on a rate of change of the pressure differential over time and a pressure sensitivity to volume changes constant for the reservoir. 25. The system of claim 23, wherein estimating the volume of the therapeutic fluid within the reservoir comprises estimating the volume based on the estimated rate at which the therapeutic fluid is added to or removed from the reservoir and a baseline volume of therapeutic fluid in the reservoir. 26. The system of claim 25, wherein estimating the volume of the therapeutic fluid within the reservoir comprises: multiplying the estimated rate at which the therapeutic fluid is added to or removed from the reservoir by a time over which the therapeutic fluid is added to or removed from the reservoir; andadding the baseline volume to the estimated rate multiplied by the time. 27. The system of claim 17, further comprising generating an alert based on the estimated volume of the therapeutic fluid within the reservoir. 28. The method of claim 17, further comprising displaying an indication of the estimated volume of the therapeutic fluid within the reservoir. 29. A method comprising: A computer-readable storage medium comprising instructions for causing a programmable processor to:measure a pressure within a therapeutic fluid reservoir of an implantable fluid delivery device with a first pressure sensor;measure a pressure within a chamber of the fluid delivery device at least partially surrounding the reservoir with a second pressure sensor; andestimate a volume of a therapeutic fluid within the reservoir based on a pressure differential between the measured pressure within the reservoir and the measured pressure within the chamber. 30. The computer-readable storage medium of claim 29, wherein estimating the volume of the therapeutic fluid within the reservoir comprises estimating the volume of the therapeutic fluid within the reservoir based on the pressure differential, a baseline volume of therapeutic fluid in the reservoir, and a pressure sensitivity to volume changes constant for the reservoir. 31. The computer-readable storage medium of claim 29, wherein estimating the volume of the therapeutic fluid within the reservoir comprises estimating a rate at which the therapeutic fluid is added to or removed from the reservoir based on the measured pressure within the reservoir and the measured pressure within the chamber. 32. The computer-readable storage medium of claim 29, further comprising instructions for causing a programmable processor to generate an alert based on the estimated volume of the therapeutic fluid within the reservoir. 33. The computer-readable storage medium of claim 29, further comprising instructions for causing a programmable processor to display an indication of the estimated volume of the therapeutic fluid within the reservoir. 34. A system comprising: a first means for measuring a pressure within a therapeutic fluid reservoir of an implantable fluid delivery device;a second means for measuring a pressure within a chamber of the fluid delivery device at least partially surrounding the reservoir; andmeans for estimating a volume of a therapeutic fluid within the reservoir based on a pressure differential between the measured pressure within the reservoir and the measured pressure within the chamber.