This type of instrument uses “time of flight” principle. An ultrasonic level measuring device uses reflected sound to determine the height of a substance in a vessel. The time required for a sound wave to be transmitted, reflected, and returned to the device may be translated directly into a level indication. A high frequency sound signal is used for this purpose, normally in the range of 20KHZ to 200KHZ.
The timing circuit of the device is started when the transmitter sends out the sound wave. It is stopped when the receiver detects the return of the reflected signal. The device is calibrated in an empty vessel; therefore, as the level in the vessel increases, the time required for the reflected signal to return decreases.
Although this principle is very accurate and used in many industries, it is not widely used as an indicator in power plants. The device is generally fragile and affected greatly by outside forces. Because of this, it requires frequent calibration to maintain its accuracy. It is, however, used to some extent as a level switch. A typical example is illustrated in Figure .
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| ultrasonic level measurement |
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