A pump includes a reservoir, a port, and a plunger. The reservoir delivers a liquid by discharging the liquid through the port coupled to the reservoir. A piston of the plunger 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. The pump also includes a reference-volume assembly and/or a linear position sensor. The reference-volume assembly is coupled to the reservoir at an opposite end of the reservoir relative to the port and includes 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. The pump estimate the amount of liquid discharged from the reservoir.