An apparatus comprises a detector, a pressure sensor and a processor. The detector is operable to detect light that is scattered by an aerosol that is associated with a pressure. The pressure sensor is operable to measure the pressure. The processor is coupled to the detector and to the pressure sensor, and is configured to receive at least a signal from the detector and the pressure sensor. The processor is further configured to use the received signals to calculate a volume of the first aerosol, and to output an output signal associated with the calculated volume. The various measurements can be repeated and compared, and the output signal can be a feedback signal for metering subsequent amounts of the aerosol, based on the comparison.