 Welcome to the time of flight flow meter. A time of flight flow meter uses ultrasonic sound waves to measure fluid velocity. This kind of meter consists of two transducers, one on each side of the pipeline. Each transducer is capable of transmitting and receiving ultrasonic sound waves. At sea level, an ultrasonic wave travels through air at the speed of sound. 1,100 feet per second or 340.29 meters per second. The time of flight flow meter operates on the principle that the speed of an ultrasonic sound wave will increase when transmitted in the direction of flow movement. This flow meter also operates on the principle that the speed of an ultrasonic sound wave will decrease when transmitted against the direction of flow movement. An analogy is that an airplane can fly faster when traveling in the direction of a prevailing wind, but flies slower when flying against the wind. Here is how the time of flight meter works. An ultrasonic wave is sent downstream from transducer 1 toward transducer 2. As the wave travels through the flow stream, the natural velocity of the wave is increased by the speed at which the fluid flows. The time from when the signal is launched by transducer 1 to the time it is received by transducer 2 is recorded. Once recorded, transducer 2 launches an ultrasonic wave upstream. By moving against the direction of flow, the wave travels at its natural speed minus the velocity of the fluid. When transducer 1 receives the signal, the time is recorded. If the fluid flow increases, the ultrasonic wave going downstream from transducer 1 will take less time to reach transducer 2, and the wave going upstream from transducer 2 will take more time to reach transducer 1. The difference in the time that it takes the ultrasonic waves to move in both directions is what is being measured. The greater the flow rate, the greater the time difference. The time difference is a direct measure of time and is electronically converted to volumetric flow rate. Watch as we show slow flow, medium flow, and fast flow. A requirement for this type of flow meter is that the liquid being measured must be relatively clean. Particles in the fluid may absorb or scatter the ultrasonic waves and make the reading inaccurate. Now, let's check your understanding. Please pause the video if you need more time to answer the questions. Sound waves are accelerated when they travel in the same direction of a fluid they are passing through. A time-of-flight flow meter obtains its measurements by taking the difference in the time it takes for the signal to move in each direction. Congratulations! You have successfully completed this learning object.