An ultrasonic flow meter measures the velocity of any liquid or gas through a pipe using ultrasonic transducers. The results get slightly affected by temperature, density or viscosity of the flowing medium. Maintenance is inexpensive because there are no moving parts. Some may be able to measure liquid level as well. With the level measurement and pipe size, flow rate and total discharge can be calculated.

Ultrasonic flow meters work with at least three different types:

• Transmission (contrapropagating transit-time) flowmeters
• Reflection (Doppler) flowmeters
• Open-channel flowmeters

Transmission flowmeters can be distinguished into:

• In-line flowmeters (intrusive, wetted)
• Clamp-on flowmeters (non-intrusive)

The most commonly used ultrasonic flowmeter is the transit-time flowmeter which is used for liquids and gases.  Boocca BF70 series are transit-time type.

Transit-time flowmeters work by measuring the time of flight difference between an ultrasonic pulse sent in the flow direction and an ultrasound pulse sent opposite the flow direction. This time difference is a measure for the average velocity of the fluid along the path of the ultrasound beam. By using the absolute transit time and the distance between the ultrasound transducers, the current speed of sound is easily found. The measuring effect can be adversely affected by many things including gas and solid content.

By using at least 3 transducers, an "ultrasonic anemometer" measures wind speed and direction in open air, with no moving parts.