System, method, and apparatus for estimating liquid delivery

A pump, comprising:

• a reservoir configured to deliver a liquid;
• a port coupled to the reservoir and configured to discharge the liquid;
• a plunger having a piston coupled to a shaft, wherein the piston is disposed within the reservoir in sliding engagement with an inner surface of the reservoir, wherein the piston defines a liquid side of the reservoir and a non-liquid side of the reservoir whereby movement of the plunger towards the liquid side of the reservoir discharges liquid through the port;
• and
• a reference-volume assembly coupled to the reservoir, wherein the reference-volume assembly comprises:
• a reference-volume chamber in acoustic communication with the non-liquid side of the reservoir;
• a speaker disposed within the reference-volume chamber;
• and
• a reference microphone disposed within the reference-volume chamber,
• wherein the reference-volume assembly further comprises a conduit configured to receive the shaft, wherein the shaft is in sliding engagement with the conduit.