Level Measurement

What is an ultrasonic level sensor?

The ultrasonic sonic sound is above the human hearing range and it is used to do the level measurement. The sound frequencies which are above 20 kHz are ultrasonic frequency, ultrasonic waves are used to measure the level of solids and liquids in industries. This device can be used to do continuous or single point level measurement. The operation of this device is based on the reception of the acoustic energy, it travels from source to the receiver by the attenuation of the vibrating diaphragm face. This type of level sensor would mostly measure the distance between contents and the top of the vessel.

What is the principle of the ultrasonic sensor?

The working principle of the ultrasonic sensor is by transmitting sound waves from a transmitter to the tank content. This device can measure the level by determining the time taken for the reflected sound waves to return to the transducer, in order to do a proper level measurement the sound waves must reflect from the process material in a straight line back to the transducer. The ultrasonic sensors use ceramic discs which are basically piezoelectric crystals, in order to transmit and receive the ultrasonic energy. The transit crystal will be placed on one side of the transducer gap and the receive crystal is positioned on the opposite side. The transducer will emit short ultrasonic impulses. Bursts would be created by the transducer by applying electrical energy to the piezoelectric crystals which is situated inside the transducer. Then after this process, the transducer will create a sound wave.

In case if the measurement is to be done for longer ranges then the lower frequency and higher amplitude are required to create sound waves that can travel a longer distance. If the measuring range is long then the transducer will be larger. So the transmitter would transmit the sound waves and these waves will be reflected from the material that is measured, so the level is determined by the time taken by the wave to reach back if the level is higher then it would take more time for the wave to reach back.

How to select an ultrasonic level sensor?

We must consider the vessel or tank height and also the headroom should be considered to select an instrument with a suitable minimum and maximum range. The ultrasonic sensors must be made up of material that is suitable for the measured medium.

  • We must consider the distance that is to be measured
  • The surface of the material must be considered, like if there is a layer of foam in the liquid surface
  • We must consider the environmental conditions
  • Acoustic noise
  • Pressure
  • We must consider the type of mounting
  • The speed of the sound would vary for different gases so this must be considered

How to install an ultrasonic level sensor?

In this type the sensors must be mount on the top of the vessel, the beam path must not be affected by any filling streams. The sensor must not be positioned at the middle of the tank because it could cause multiple reflections. Beam angle must be considered while installing. We must consider the vessel geometry and nozzle location and size. We must also consider certain internal obstructions like baffles, standpipes while installing the sensor.

What are the advantages of ultrasonic level sensors?

  • There is no physical contact with the process material
  • There are no moving parts
  • The measurement is not affected by the changes in moisture, density, electrical conductivity, or the dielectric constant
  • Easy to install
  • Quick response
  • Less maintenance

What are the disadvantages of the ultrasonic level sensors?

  • The reflective properties of the process surface could result in errors
  • The reception of the waves could be affected by dispersion or absorption
  • It is affected if the vapor space is dusty
  • Measurement is affected if there is a change in the angle of repose
  • Because it works on the reception of the sound wave signal, it cannot be used for lightweight material
  • Dust, heavy vapors, surface turbulence, foam all of this could affect the measurement.
  • It is expensive
  • It cannot be used for high-pressure applications

What are the applications of the ultrasonic level sensor?

  • Suitable for liquid and solid level measurements
  • It can be used as an open channel flow monitors
  • Proximity sensing
  • Wet-well installations
  • Open channel flow
  • It can be used in systems that need remote monitoring

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