Medical Flowmeter - The Spirocell uses ultrasound to measure the velocity of gas travelling through the device. Bursts of ultrasound are transmitted upstream and downstream in the gas flow between the two sensors. The time of flight in both directions is measured and the distance between the sensors is known. Therefore, by using the equation opposite, the flow rate of the gas can be calculated. The tube in which the gas is flowing has a known cross sectional area, so for a given period of time the volume of the gas flow can also be measured. The speed of sound in a gas is dependent on factors such as temperature, pressure, humidity and composition.