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
