Measurement Ultrasonic flow measurement of time passing. measuring tools These are ideal for measuring flow. No liquid suspensions campsites By measuring the travel time of the ultrasonic waves from the audio source to the receiver. The installation of the transmitter and receiver are two separate aspects. A simple principle for measuring flow rates in this way is to find time to wave when the fluid is at rest (v = 0) to be used as the default variable to calculate the velocity of the fluid.