Beginner's Guide to Ultrasonic Level Transmitter

Introduction

"Sonic" refers to sound we can hear,  "ultrasonic" refers to sound above the human hearing range. Humans can hear frequencies up to 20 kHz, while ultrasonic frequencies are above 20 kHz. These sound waves are used in industry to measure the level of liquids and solid objects. Ultrasonic level measurement is a contactless method, making it suitable for hot, corrosive, and boiling liquids. The typical frequency range for ultrasonic level measurement is between 40 and 200 kHz.

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Principle of Ultrasonic Level Measurement

Ultrasonic waves detect objects similarly to radar. While radar uses radio waves, ultrasonic devices use sound waves. When an ultrasonic pulse is directed at an object, it bounces off the surface, and the echo returns to the sensor. The travel time of the pulse is measured, and the object's distance is determined. This is possible beccause we know how fast the ultrasonic sound wave travels through air. The simple formula used to measure the level is :-

Formula:
Level = (Speed of Sound in Air × Time to travel from the sensor, bounce off the level and back again) / 2

Challenges in Ultrasonic Level Measurement

 In real-world there are other factors that present some technical challenges:

• Temperature Variations: The speed that ultrasound travels though air changes with air temperature, requiring us to measure the temperature at the sensor and to compensation for any variation.

• Interference Echoes: Structures like mixers, ladders, dip tubes and even welded joints can produce multiple relected waves, this has to be filtered our using echo suppression software.

• Calibration Accuracy: Correctly setting the empty distance (A) and measurement span (D) is crucial.

• Blocking Distance: When the ultrasound is transmitted from the sensor, it also vibrates the reciever. This means that any reflected sound would be "blocked" by these vibrations. To solve this issue the ultrasond is turned off after the initial pulse, and a period of silence allowed before the receiver listens for the returning pulse of ultrasound. if we calculate how long this silence needs to be for the receiver to be silent, then we can also calculate which levels would be blocked if they were so close to the sensor, that their reflective waves would be blocked by the vibrations from the transmitter. 

Structure of an Ultrasonic Transducer

An ultrasonic level transmitter consists of a sensor that converts electrical energy into ultrasound waves. This process relies on piezoelectric crystals, which vibrate at high frequencies when energized. These crystals can also convert received ultrasound echoes back into electrical signals for processing.

Functional Block Diagram of an Ultrasonic Level Transmitter

A typical ultrasonic level transmitter consists of:

• Microcontroller-Based Control Circuit: Manages the measurement process.

• Pulse Transmission and Reception Circuits: Generate and receive ultrasonic signals.

• Timing Generator Circuit: Synchronizes functions.

• 4-20mA Output Signal: Converts measured levels into standard industrial signals (4mA = 0% level, 20mA = 100% level).

Advantages of Ultrasonic Level Transmitters

• Non-Contact Measurement: Ideal for corrosive, boiling, or hazardous liquids.

• No Moving Parts: Low maintenance and long-lasting.

• Consistent Accuracy: Unaffected by changes in liquid properties such as dielectric constant.

Limitations of Ultrasonic Level Transmitters

• Weak Echo in Deep Tanks: Performance declines with increased tank depth.

• Material Dependency: Not suitable for sound-absorbing materials.

• Environmental Limitations: High-density moisture or smoke can disrupt readings.

Conclusion

Ultrasonic level transmitters provide a reliable, non-contact method for measuring levels in industrial applications. While effective, considerations such as calibration, environmental conditions, and sensor placement are crucial for optimal performance.

For more details, explore our Ultrasonic Level Transmitter Range.

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