Ultrasonic Level Sensors

Ultrasonic level sensors (sometimes called sonic) are ideal for non-contact level sensing of highly viscous liquids such as heavy oil, grease, latex, and slurries as well as bulk solids like cement, sand, grain, rice, and plastic pellets They are also widely used in water/waste water applications for pump control and open channel flow measurement. The sensors emit high frequency, “ultra” sonic (20 kHz to 200 kHz) acoustic waves that are reflected back to and detected by the emitting transducer.

Since the speed of sound in air fluctuates with moisture level and temperature, ultrasonic level sensors are also affected by changing moisture levels and varying temperatures and pressures inside the hopper or container. But when ultrasonic sensors are used in conjunction with humidity and temperature sensors, or a distance reference, correction factors can be applied to the level measurement making the technology very accurate.

Turbulence, foam, steam, chemical mists (vapors), and changes in the concentration of the process material also affect the ultrasonic sensor’s response. Turbulence and foam prevent the sound wave from being properly reflected to the sensor; steam and chemical mists and vapors distort and/or absorb the sound wave; and variations in concentration cause changes in the amount of energy in the sound wave that is reflected back to the sensor. Stilling wells and wave guides are used to address some of the above constraints.

Proper mounting is important to ensure that sound waves are reflected perpendicularly back to the sensor. Otherwise, even slight misalignment of the sensor in relation to the process material reduces the amount of sound wave detected by the transducer. In addition, the hopper, bin, or tank should be relatively free of obstacles such as weldments, brackets, or ladders to minimise false returns and the resulting erroneous response, although most modern systems have sufficiently “intelligent” echo processing to make engineering changes largely unnecessary except where an intrusion blocks the “line of sight” of the transducer to the target. Since the ultrasonic transducer is used both for transmitting and receiving the acoustic energy, it is subject to a period of mechanical vibration known as “ringing”. This vibration must attenuate (stop) before the echoed signal can be processed. The net result is a distance from the face of the transducer that is blind and cannot detect an object. It is known as the “blanking zone”, typically 150mm – 1m, depending on the range of the transducer.

The requirement for electronic signal processing circuitry can be used to make the ultrasonic sensor an intelligent device. Ultrasonic sensors can be designed to provide point level control, continuous monitoring or both. Due to the presence of a microprocessor and relatively low power consumption, there is also capability for serial communication from to other computing devices making this a good technique for adjusting calibration and filtering of the sensor signal, remote wireless monitoring or plant network communications. The ultrasonic sensor enjoys wide popularity due to the powerful mix of low price and high functionality.