Ultrasonic Transducers

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Level measurement can be performed via ultrasonic or sonic technology too. Ultrasonic level measurement devices basically employ sound waves for detection of liquid level. They usually work over the frequency range between 20 kHz to 200 kHz.

Operating Principle

The working principle of a typical Ultrasonic level sensor is illustrated in the figure below.
Ultrasonic Level Sensor 
In this design, the level sensor is located at the top of the tank in such a way that it sends out the sound waves in the form of bursts in downward direction to the fluid in the tank under level measurement. As soon as the directed sound waves hits the surface of the fluid, sound echoes gets reflected and returned back to the sensor.
The time taken by the sound wave to return back is directly proportional to the distance between the piezo electric sensor and the material in the tank. This time duration is measured by the sensor which is then further used to calculate the level of liquid in the tank. The speed of the sound waves can sometimes be affected due to variations in temperature for which appropriate compensations need to be provided in the sensor design. In general, the medium over the fluid’s surface is air. However, one can employ a blanket of nitrogen or any other vapor also.

Types of Ultrasonic Level Sensors

Ultrasonic level sensors are available in two basic types which are explained below:
Non-Contact Ultrasonic Sensors
This category of ultrasonic level sensors consists of:

  • An analog signal processor
  • A microprocessor
  • Binary coded decimal i.e. BCD range switches
  • An output driver circuit

In these devices, the microprocessor generates the gate signal and pulses and directs them to the ultrasonic sensor via the analog signal processor. The sensor then transmits a beam of ultrasonic waves to the surface of the fluid. It also receives the reflected echoes from the fluid surface and sends them back to the microprocessor. The microprocessor keeps on receiving echoes of sound waves and performs calculations to determine distance between the sensor and the fluid surface and hence detects the fluid level.
Contact Ultrasonic Sensors
These types of ultrasonic sensors are primarily used to detect fluid level at a specific point only. These level measurement devices consume less energy and basically include:

  • A sensor installed in the process field and
  • An integrated solid state amplifier

They consist of no movable parts and hence do not need any calibration. “Typically, they are equipped with terminal blocks for connection of a power source and external control devices. The ultrasonic signal crosses a one-half inch gap in the sensor, controlling relay switches when the gap contains liquid. The sensing level is midway along the gap for horizontally mounted sensors, at the top of the gap for vertically mounted sensors.”[1] When the level of the fluid drops below the sensing level, the strength of the ultrasonic signal gets reduced. This eventually brings the relay to its former position.
These ultrasonic sensors find their application in vessels or pipes where they are used for automatic action of pumps, solenoid valves, and high or low level alarms. In these areas, two sensors need to be employed: one for filling and emptying tanks and the other one for measuring fluid volumes. They are suitable for use with mostly all kinds of fluids. Their performance does not get easily influenced by coatings, clinging droplets, foam or vapor etc but sometimes, highly aerated or viscous fluids can create trouble by choking the sensor gap.

Main Features

Key features of ultrasonic level measurement devices are listed below:

  • “These sensors use frequencies in the tens of kilohertz range; transit times are ~6 ms/m. The speed of sound (340 m/s in air at 15°C (1115 fps at 60°F) depends on the mixture of gases in the headspace and their temperature.”[2]
  • The speed of sound waves traveling via the medium which is normally air is prone to get affected by changes in the working temperature. In order to compensate for these changes in temperature and resulting changes in sound wave speed, the level measurement system must include a temperature sensing device. This will help in correct distance calculations and hence accurate level detection results.
  • In cases where heavy foam is found on the surface of the process fluid, the use of ultrasonic level measurement techniques are usually avoided since this foam work as a sound absorbent. Consequently, the sound wave will get scattered resulting in non reception of the exact signal by the sensor. This will cause improper functioning of the measurement system.
  • Excessive surface turbulence of the fluid can result in wide fluctuations in level measurement results. To avoid this issue, one may employ a damping correction or a response delay with the device.
  • Good level measurement requires that the reflected echo from the fluid surface returns back in a straight line to the sensor. Besides, it calls for proper installation of ultrasonic transmitter over the tank. The transmitter should be mounted in such a way that the inner composition of the vessel or tank doesn’t get in the way of the signal.
  • In level measurement fields where sound waves get influenced by factors like foam and vapor etc., one can connect a beam guide to the sensor for improving performance of these devices.
  • Ultrasonic level measurement technique proves to be quite costly when employed for point level measurement applications.
  • In case of fluids which are less viscous, one can execute point level measurement via a technique called ultrasonic gap technique. In this method, “a transmit crystal is activated on one side of a “measurement gap” and a receive crystal listens on the opposite side. The signal from the receive crystal is analyzed for the presence or absence of tank contents in the measurement gap.”[3]
  • To overcome barricades encountered in the vessel or tanks, a technique called tank mapping has been introduced. “Tank mapping lets the operator take a "sonic snapshot" of an empty vessel. The transducer transmits a sound burst and the echo is recorded as a signature of the tank. Any obstructions in the vessel will send an echo and create a profile. Later on, this signature or profile is locked into the ultrasonic unit’s memory so it will not respond to echoes created by these obstructions.”[4]


Major advantages offered by ultrasonic level measurement technique are mentioned below:

  • Ultrasonic level sensors are usually non contact type i.e. they do not make any contact with the process fluid under level detection.
  • Besides, they consist of fixed components only hence require less maintenance
  • They are usually mounted at the top of the vessel due to which they are less likely to offer leakage problems as compared to entirely wetted means.


Ultrasonic level measurement technique can not be suitably applied in all fields since use of ultrasonic level sensors includes few setbacks too. Many factors exist which have the tendency to influence the returned echo signal back to the sensor. Some of them include:

  • Materials like powders etc.
  • Heavy vapors
  • Surface turmoil
  • Foam
  • Ambient noise and temperature

Besides, ultrasonic level measurement devices do not work satisfactorily in areas involving vacuum or high pressure conditions.

Ultrasonic Transducers
Generating, detecting & processing ultrasonic signals
Ultrasonic Transducers Seminar Reports
This page is provided to provide a few handy hints to project students grappling with ultrasonics
Last Update 27/5/08
Ultrasonics is the production of sound waves above the frequency of human hearing and can be used in a variety of applications such as, sonic rulers, proximity detectors, movement detectors, liquid level measurement. Ultrasonics is used in medicine and robotics, security devices, laboratory and industrial cleaners and a host of other applications. Many in-depth studies of ultrasonic transducer equivalent circuits have been produced but these studies are far beyond the scope of these pages.
Ultrasonic Transducers Seminar Reports
Fig 1
A simplified model of the series and parallel equivalent circuits of an ultrasonic transducer.
There are two main types of transducers used to transmit ultrasonic signals. They are the Piezo type and the electrostatic type. It is even possible to send ultrasonic signals using a conventional high frequency electromagnetic speaker (tweeter). This discussion will centre around the piezo type of transducer. The electrostatic type may be included at a later stage and we will look at using conventional speakers as well.
Below is a specification for a typical piezo type ultrasonic transducer Tx/Rx pair. Note that the main difference between a receiving and transmitting transducer is the impedance. The transmitter generally has a low impedance and a receiver has a higher impedance. Some ultrasonics transducers can be used as both receiver and transmitter however, when designing circuits for these devices take careful note of the impedance and design the circuitry to suit.

Ultrasonic Transducers Seminar Reports


Fig 2


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