An ultrasonic flow meter (a doppler flowmeter) uses ultrasonic transducers to measure the velocity of a liquid or gas flowing through a pipe. It does this by focusing on bubbles or other particulates (called discontinuities) within the flow stream, so ultrasonic flowmeters generally don't work with distilled water or drinking water because it lacks aeration. People most often use ultrasonic flowmeters for dirty liquids such as wastewater applications in addition to slurries, bubbly liquids (think Perrier water), gases with sound reflecting particles, or "single phase turbulent clean liquids."

The flow rate measurement results are affected by the density or viscosity of the gas or liquid which in turn is usually affected by temperature and weather environment. Many flow meters have clamp-on designs with transducers that measure flow rates from the outside. The benefit to these devices is that the installation doesn't require breaking the line nor interrupting the flow. The flow meter is placed near the application and two clamp-on transducers are placed on sides of the pipe. Acoustic waves are generated betweent he sensors, measured, and the speed of flow is then calculated.

There are no moving parts to ultrasonic flow meters so they are very easy to maintain, which is one of their main benefits. They are typically used where there is chemical compatibility, and low pressure drop. Some are also equipped with the capability of measuring liquid levels so that together with the pipe size, in turn the flow rate and total discharge can be calculated. Two measuring systems are used to calculate flow rates: Doppler and Time-transit. The Doppler method measures the change in frequency from one sensor to the other and converts the measurement into a flow rate whereas the Time-transit method depends upon sound waves bouncing within the pipe; the transducer measures the time it takes for the sound wave to return, and calculates the flow rate from this measurement.

The basic principle of operation depends on the fact that a sound wave changes its frequency when it is reflected by moving particles in a flowing liquid. The meters therefore employ the frequency shift (Doppler Effect) of an ultrasonic signal when reflected back to a measuring device (an ultrasonic transducer) by either bubbles or suspended particles in a gas or liquid.

First an ultrasonic signal is transmitted into a pipe with the flow material, and the bubbles or particulates reflect back the ultrasonic wave. The measured frequency that is recorded is proportional to the rate of flow, and that is how the flow rate is determined. The most modern devices require that the gas or liquid contain at least 100 ppm of suspended particles or bubbles. The discontinuities must be 100 micron or larger in size. Fast Fourier transform (FFT) signal processing is often used in flow meter design to help measure and process calculations.