A sensor mounted above the flow stream transmits a sound pulse that is reflected by the surface of the liquid. The elapsed time between sending a pulse and receiving an echo determines the level in the channel. Because the speed of sound changes with air temperature, an ultrasonic system must compensate for changes in air temperature, usually with a temperature probe built into the ultrasonic sensor.

However, ultrasonic systems may be affected by wind, steam, and air temperature gradients, and may provide inaccurate results in channels with turbulence or floating foam or debris. In addition, ultrasonic sensors require space above the flow to mount the sensor, and are usually not suitable for use in very narrow channels.

Because ultrasonic sensors compensate for changes in air temperature, exposure to sunlight can impact performance by artificially heating the sensor and introducing temperature errors to the internal temperature compensation. As a result, it is more suitable to use such devices in applications that are sheltered rather than in an exposed outdoor type of application. ​

An ultrasonic sensor is easy to install and, because it does not contact the liquid, requires minimal maintenance and is not affected by grease, suspended solids, silt, and corrosive chemicals in the flow stream, and liquid temperature fluctuations.

There are a variety of methods available for installing a flow sensor in an open channel. Ultrasonic level sensors are relatively easy to install because they are mounted above the flow stream. Ultrasonic sensors are usually installed using a stainless steel bracket attached to a wall or pipe near the channel. Most ultrasonic sensors also have pipe threads on the end from which the cable exits the sensor. This allows the sensor to be connected directly to conduit.

In addition, some ultrasonic sensors can simply be suspended by their cable in portable flow monitoring applications. In this case, a weight is usually added to the sensor to ensure that it maintains its alignment above the flow stream. An ultrasonic level sensor will not work properly if condensation causes water droplets to collect on the transducer surface. If high ambient humidity is a problem, try mounting the ultrasonic level sensor horizontally and aiming it at a 45° angled reflector.​

​Frequently Asked Questions

How accurate is an ultrasonic flow meter?

Ultrasonic flow meter accuracy is quite high. Typical ultrasonic flow ​meter accuracy ranges from 0.7% to 1%.

What is the minimum pipe size for an ultrasonic flow meter?

 Typically, flow meter manufacturers specify a minimum of 10 to 15 diameters of straight pipe. This means that for a 4″ diameter pipe, you would need 10 x 4 = 40″ of straight pipe, or for a 2″ pipe you would need 10 x 2 = 20″ of straight pipe. However, for more severe flow distortions, you may require more.

Do ultrasonic flow meters need calibration?

Flow calibrating the ultrasonic flow meter is a vital part of the process. Most users require flow calibrations to improve flow meter performance and overall measurement uncertainty.