Above: A typical ultrasonic level probe (aka level "switch"). This one is from Omega Engineering.
The sonic-level sensor takes advantage of the principle that the speed of sound waves traveling through air or a gas can be measured and timed. The longer the distance the sound waves must travel, the longer the time it will take for them to be sent and reflected. In this type of application, the sonic transmitter is mounted in the top of a tank. The sensor has a transmitter and receiver mounted in the same head. An electrical pulse is applied to a crystal that causes it to vibrate against a diaphragm. When the diaphragm vibrates, sound waves are produced and they are directed at the level of the liquid or solid that is being measured. When the sound waves reach the liquid or solid, they will be reflected back to the sensor. The receiver in the sensor will detect the reflected sound waves and the electronic circuitry measures the amount of time between the instant the sound waves are sent and when they are received. The amount of time is accumulated digitally in the circuit in the form of a binary-coded decimal (BCD) number. The number is used in a calculation to convert the amount of time to distance. The answer to the calculation is in a BCD format so a digital-to-analog converter is used to provide a 4-20 mA signal that represents the level of the liquid or material. The sonic type of level sensor is usable in applications for measuring material such as pitch blend and carbon black, which tend to be dirty, sticky, and gummy, or other materials that are corrosive or abrasive that may damage a probe if it came into contact with these substances. Some substances such as powders and substances that are foamy may not be suitable for this type of level probe because they tend to absorb sound waves rather than reflect them (see ill. 1 below).
Above: ill. 1: Ultrasonic-level sensor in use.
The sensor has a transmitter and receiver in its head. Sound waves are
transmitted to the liquid and the time it takes for them to be reflected and received by the sensor is detected. This time is converted to distance
so the sensor can determine how far the level is from the probe.
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