Experience with ultrasonic flowmeters

Ultrasonic flowmeters have been around since the 1960s and were used for oceanographic and military applications before they were introduced to the process industries. There are several different kinds of ultrasonic flowmeters, but they all work on the principle of frequency shift due to velocity. The purpose of this article is to review experiences, both good and bad, with both Doppler and transit time ultrasonic flowmeters. The most common type of ultrasonic flowmeter is the Doppler meter. The Doppler meter signal is reflected by particles, bubbles, or other discontinuities in the liquid, and the frequency is shifted by velocity of these discontinuities. This frequency shift renders a signal proportional to velocity. Another common type of ultrasonic flowmeter is the ''time-of-flight'' or transit time meter. This meter uses two sensors that are lined up at an angle to the direction of flow, and that pulse alternately. A time-differential relationship proportional to the flow is calculated. In this case, the fluid must be free of entrainment or else scattering of the signal may induce an error.