There are two main types of ultrasonic flowmeters, transit time and Doppler:
Transit time
A transit time ultrasonic flowmeter has both a sender and a receiver. It sends two ultrasonic signals across a pipe at an angle: one with the flow, and one against the flow. The meter then measures the “transit time” of each signal. When the ultrasonic signal travels with the flow, it travels faster than when it travels against the flow. The difference between the two transit times is proportional to flowrate.
Transit time ultrasonic flowmeters are distinguished according to the number of “paths” they have. A path is simply the path or track of the ultrasonic pulse as it travels across the pipe and back again. Many ultrasonic flowmeters are single or dual path, meaning that they send either one or two signals across a pipe and back. Typically, there are two transducers for each path; one is a sender and one is a receiver.
Doppler
Doppler flowmeters also send an ultrasonic signal across a pipe. Instead of tracking the time the signal takes to cross to the other side, a Doppler flowmeter relies on having the signal deflected by particles in the flowstream. These particles are traveling at the same speed as the flow. As the signal passes through the stream, its frequency shifts in proportion to the mean velocity of the fluid. A receiver detects the reflected signal and measures its frequency. The meter calculates flow by comparing the generated and detected frequencies.
Doppler ultrasonic flowmeters are used with dirty liquids or slurries. They are not used to measure gas flow.