Ultrasonic flowmeters measure water or other liquids, and, in some cases, gases, flowing through pipes. These meters can measure both the velocity and the volumetric flow of a liquid. They are simple to use because, unlike most types of meters, ultrasonic flowmeters clamp onto the pipes rather than being installed inside them, so the flow does not have to be interrupted to install one.
There are two main types of ultrasonic flowmeter — transit-time flowmeters and Doppler flowmeters. Both types clamp onto the outside of the pipes, but they differ in the method used to measure the flow, and each method has its own advantages and disadvantages. Transit-time flowmeters work better for clean liquids, which do not have many air bubbles or solid particles. Doppler flowmeters, by contrast, require that the liquid has bubbles or particles in it in order to measure the flow, but they also require the liquid to be moving at or above a certain speed.
Transit-time ultrasonic flowmeters, also called transmission flowmeters, use a principle called phase shifting. Two transducers, which both send and receive ultrasonic energy beams, are positioned some distance apart from each other on the pipe. The meter measures the amount of time it takes the energy to travel in each direction, and finds the difference between the two times. This difference is directly proportional to the velocity of the liquid, which is calculated by the flowmeter each time it takes a measurement.
Doppler ultrasonic flowmeters make use of the Doppler effect, which states that the frequency of sound waves depends upon how both the source and the observer of the sound are moving. Like transit-time meters, Doppler varieties use two transducers, but one transducer only sends and the other only receives. The sending transducer sends out ultrasonic energy beams, and particles or bubbles in the liquid cause the frequency of the beams to change. This shift in frequency is measured by the receiving transducer, and it is proportional to the velocity of the liquid.
Ultrasonic flowmeters are versatile, as they can measure the flow of several different liquids, including not only water but sewage water, oils, and liquids that are produced by industrial processes. These meters are most commonly used to measure flow in pipes, but they can also be used to measure it in open channels. This is done by measuring the water level in the channel, which can then be related to the volumetric flow.