Description: The ultrasonic sensor is a low cost non-contact water level sensor that measures the distance from the sensor to the water surface by sending a high-frequency pulse (typically between 20kHz to 200kHz), listening for the echo to return. Knowing the speed of sound, the sensor can determine the distance to the object. The reflected acoustic wave is received at the transducer, where it is converted into an electrical signal (analog or digital) and recorded by a data logger. The ultrasonic sensor pulse is transmitted in a cone (up to 22° depending on manufacturer) with a range of about 10m and a typical accuracy around 0.25% full scale. With a 10m measurement range the sensor can never be further than 10m from the surface of the water level to make a reliable measurement. The ultrasonic sensor also has a dead zone which typically is between 30 and 45cm from the sensor. The dead zone is designed to prevent spurious reading from sensor ringing caused by the high amplitude pulse. Ultrasonic sensors are effected by air temperature and air density, thus affecting the speed of sound. Most manufacturers account for this through built in temperature compensation, though the temperature sensor is usually mounted at the sensor, which doesn't give a reliable indication of temperature along the path the ultrasonic takes
Costs USD Lower: $400
Costs USD Upper: $1,000
Accessories: staff gauge
Advantages: Relatively inexpensive compared to the radar (another non-contact solution). Sensor is not located in the channel and is not susceptible to damage from water flow, debris and water pollution. Since the sensor requires mounting above the body of water it is ideally suited for canals and small streams and is not as practical in reservoirs or rivers with shallow slopes.
Disadvantages: The drawbacks of the ultrasonic sensor are its generally limited range of detection which is 10m from the sensor head. The price reflects the capabilities both in measurement range and accuracy (0.25% full scale accuracy compared to the radar accuracy of 0.03% full scale). Also, the ultrasonic sensor must be placed directly over the body of water. Often ultrasonic sensors are mounted to bridges, platforms or booms over the water.
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