A method includes calculating a volume of fluid in a pumping chamber when the chamber is not full or empty based on pressure and temperature measurements taken during a negative full-volume state, a positive full-volume state, and an empty-volume state, and an infusion system includes a controller configured to carry out such calculation. Such method, or a separate method, may measure temperature and pressure of a common volume and a pumping chamber, wherein the pressure is measured using a pressure sensor and the temperature is measured using a temperature sensor, optionally one that measures the temperature of the pressure sensor, and calculate a volume of fluid in the pumping chamber based on ideal gas law and the pressure and temperature measurements, and such infusion system, or a separate system, includes a controller configured to carry out such measurement and calculation.