Ultrasonic Level Sensors - Operating Principle ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Ultrasonic Level Sensors - Operating Principle

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Ultrasonic level sensors measure level by measuring the distance from the transmitter (usually located at the top of a vessel) to the surface of a process material located further below inside the vessel or tank. The time for a sound wave to travel back and forth the process material surface is used to calculate this distance, and is interpreted by the transmitter electronics as process level.

The transmitter electronics module contains all the power, computation, and signal processing circuits and an ultrasonic transducer. The transducer consists of one or more piezoelectric crystals for the transmission and reception of the sound waves. When electrical energy is applied to the piezoelectric crystals, they move to produce a sound signal. When the sound signal is reflected back, the movement of the reflected sound wave generates an electrical signal; this is detected as the return pulse. The transit time, which is measured as the time between the transmitted and return signals, is then used to infer the level of a vessel. The basic design of an Ultrasonic level instrument is shown below:

Ultrasonic level transmitters are designed to output a signal corresponding either to the Fillage –fullness of the vessel or the Ullage – the amount of empty space remaining at the top of the vessel.
Most Ultrasonic level instrument are designed to measure Ullage. In this measurement mode, the time of travel of the sound wave is then proportional to how much empty space exists between the liquid surface and the top of the vessel.

In the diagram above,
Tank Height (H) = Ullage(U) + Fillage(F)
Therefore Ullage, U = H – F  or
                 Fillage, F = H – U
For an Ultrasonic level transmitter configured to measure Ullage, it can easily measure Fillage by simply subtracting the Ullage from the height of the vessel which must be programmed into the device in order to be able to measure Fillage.
Fillage can also be measured directly with the ultrasonic transmitter if it is mounted at the base of the vessel. In this case, the time of flight of the sound wave to and from the surface of the process material is then used to infer the Fillage or Ullage.

Advantages of Ultrasonic Level Sensors
  • Non contact with product
  • Suitable for wide range of liquids and bulk products like powders and grains
  • Reliable performance in difficult service
  • No moving parts
  • Measurement without physical contact
  • Accuracy of 0.25% with temperature compensation and self-calibration
Disadvantages of Ultrasonic Level Sensors
  • Product must give a good reflection and not absorb sound
  • Product must have a good distinct layer of measurement and not be obscured by foam or bubbling.
  • Not suitable for higher pressures or in a vacuum
  • The temperature is limited to 170 degree C

You can consult Level Measurement to read more about other technologies used in measuring level.

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