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.
The working principle of a typical Ultrasonic level sensor is illustrated in the figure below.
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.
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:
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:
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.” 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.
Key features of ultrasonic level measurement devices are listed below:
Major advantages offered by ultrasonic level measurement technique are mentioned below:
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:
Besides, ultrasonic level measurement devices do not work satisfactorily in areas involving vacuum or high pressure conditions.
Generating, detecting & processing ultrasonic signals
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.
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.
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