Principles of ultrasonic Pulmonary Function Testing device The ultrasonic time difference method uses a pair of ultrasonic transducers to alternately (or simultaneously) transmit and receive ultrasonic waves. By observing the difference in the propagation time between the upstream and the backward flow of the ultrasonic in the medium (gas or liquid), the fluid velocity is measured indirectly, and then calculate the flow data based on fluid velocity. The flow measurement based on the ultrasonic principle is only influenced by the flow and geometry, and accuracy will not be affected by temperature, gas composition, humidity or pressure. So it is a perfect alternative to differential pressure and turbine principles, and is the best choice for Pulmonary Function Testing device.