Ultrasonic liquid level sensors are sensors that emit ultrasonic pulses of sound, well beyond the capabilities of humans to hear (“ultrasonic” defines any soundwave outside of this threshold, roughly speaking any frequency above 20 KHz; ultrasonic sensors typically use frequencies in excess of 40 KHz). Ultrasonic level sensors are capable of providing both continuous and point-level measurement for a variety of industrial applications. While ultrasonic sensors are useful with both liquid and dry media, we will be focusing on the former.
How Ultrasonic Liquid Level Sensors Work
Ultrasonic level sensors use piezoelectric transducers to generate sound waves in the ultrasonic range. The same piezoelectric transducers also generate electrical signals when ultrasonic sound waves are received.
When ultrasonic soundwaves are emitted from the base of a sensor’s transducer and directed at the surface of the liquid media being measured, the sound pulse travels through the air before being bounced back as an echo. The emitted sound pulse travels in a conical shape as it moves away from the transducer. The angle of this shape is known as the beam angle, usually between 8 and 12 degrees.
The pulse’s return echo stimulates an electrical pulse when it is received by the piezoelectric transducer. The sensor measures the amount of time between the ultrasonic pulse being sent and the receipt of the echo pulse and translates it into distance based on the speed of sound. Liquid level is then determined by subtracting the distance between transducer and liquid surface from total tank depth.
Challenges to Consider
While the basic principal of ultrasonic level transmitting is fairly simple, there are numerous application-specific challenges to consider in every case.
Air temperature is one such variable; the velocity of soundwaves change depending on air temperature, as air expands or contracts due to heat and cold. Sensors must be pre-calibrated to account for air temperature, or in the case of applications involving varying temperatures an integrated temperature sensor must be utilized.
Another concern with ultrasonic level sensing is liquid turbulence. Agitated liquids can distort the ultrasonic sound waves, affecting the return echo and leading to inaccurate level measurement. This can be compensated for by installing the transducer with what is known as a still pipe. A still pipe is a hollow tube or pipe that extends below the liquid surface, protecting the interior from the turbulence and agitation outside.