INTRODUCTION
Ultrasonic makes use of sound waves in the 20 – 200 kHz range (above the range for human hearing). A transducer mounted in the top of a tank transmits sound waves in bursts onto the surface of the material to be measured. Echoes are reflected back from the surface of the material to the transducer and the distance to the surface is calculated from the burst-echo timing.Ultrasonic level measurement is one of the modern level measuring techniques.
Principle Of Operation
Consists of basically a transmitter receiver arrangement working in ultrasonic frequency.
Transmitter sends an ultrasonic pulse of required frequency and the detector detects the same pulse,which gets reflected back from the liquid surface.
Timer in the circuit would monitor the time delay between transmission and detection ,from which the distance can be calculated
The key points in applying an ultrasonic transducer are:
- The speed of sound varies with temperature. If the transducer does not use temperature compensation and the temperature of the air space in the vessel varies your level readings will not be correct.
- Heavy foam on the surface of the material can absorb the sound wave bursts resulting in no echo or an echo that is too weak to process.
- An irregular material surface can cause false echoes resulting in irregular readings.
- Heavy vapor in the air space can distort the sound waves resulting in false reading.
Advantages of Ultrasonic level Measurement
Essentially no moving parts
Utilizes solid-state circuitry requiring little maintenance
Accuracy good, where application is suitable
Applicable to some difficult-to-measure streams such as powders, solids, solids-containing fluids and slurries
Easy to install
Disadvantages of Ultrasonic level Measurement
Insufficient application data
Tendency to bridge for some sensor typesand for some materials
Relatively high cost
Difficulty in fixing distance between transmitting and receiving units in two-element systems this objection is overcome by fixed distances in some models
