Level Measurement – Ultrasonic Type Working Principle Application Advantages and Disadvantages

Ultrasonic Level Detectors

Ultrasonic sound waves with frequencies of 1 to 5 megahertz can be used to detect liquid or solid levels.

Ultrasonic are sound waves but are at higher frequencies that cannot be detected by the human ear.

ultrasonic level measurement working principle

The most common kind of ultrasonic transducer consist of a piezoelectric crystal.When a voltage is applied to the plates the piezoelectric crystal expands or contracts.If the voltage is alternating at an ultrasonic frequency,the crystal expands and contracts at same the same ultrasonic frequency.The crystal vibrates and these vibrations can be transferred to a diaphragm to produce ultrasonic sound waves.

Piezoelectric device can be mounted in the bottom or in the top of a vessel.The liquid surface acts as a reflector and the transducer receives the reflection of its transmitted pulses.The transducer is connected to a transmitter and to a receiver,into which the echo is fed.The transmitter and receiver are both connected to an echo timer which measures the amount of time between the emission of the sound wave and the reception of echo.The elapsed time can be converted into units of level of liquid.
Application of Ultrasonic Level detector
 
For situation where it is impractical or undesirable to install an instrument inside a tank ,non penetrating ultrasonic sensors are available.
Advantages of ultrasonic level detectors
1.Can be used in any tank size
2.Can be used in a vacuum
3.Can be used under high pressure
4.Relatively easy to maintain because they don’t have no moving parts.
Disadvantages of Ultrasonic level detectors
 
1.Expensive
2.sensitive to wide range of temperature variations.
Selection consideration – Ultrasonic sensor
 
1.Choose a sensor range that at least as tall as the tank,doubling sensor margin to add margin against higher temperatures,condensation and turbulence.
2.Tank height is defined as the distance from the installed face of the transducer down to the bottom of the tank.
3.Riser height is the distance from the face of the transducer to top of the tank.
4.Fill height is defined as the distance from the bottom of the tank,upto the maximum desired liquid height.
5. Deadband is defined as the minimum distance from the face of the transducer from which the sensor can measure.
how to select an ultrasonic level sensor
Related Articles

Tips on sensor selection

 

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When identifying the short list of sensors to sample, make sure the set—based on the manufacturer’s data sheet—meets the basic operating conditions of the application.

Here is our list of the top six operating condition requirements:

  • Temperature range
  • Size
  • Protection class
  • Voltage range
  • Discrete or analog output
  • Answering the question: Will it be beneficial to be able to change parameters? If the answer is yes, then an IO-Link enabled sensor should be considered.

Here are an additional six requirements for more specific considerations:

  • Response speed
  • Sensing range
  • Repetition accuracy
  • Electrical connection
  • Mounting type
  • Answering the question: Is on-sensor visual display required?

The following are the most common types of sensors used in manufacturing with tips and insights for each.

Proximity sensors

A proximity sensor detects the presence of nearby objects without physical contact. Presence sensors are discrete output devices. Typically, a magnetic proximity sensor is used to detect when an actuator reaches a specific position by sensing a magnet located in the actuator.img-2

It is not a good idea to purchase actuators from one company and magnetic proximity sensors from another. While the sensor manufacturer may say the sensor is compatible with X, Y, and Z actuators, the reality is variations in magnets and mounting positions can cause sensing issues. For example, the sensor may activate when the magnet is not in the correct position or it may not activate at all. If the manufacturer of the actuator offers a matched proximity sensor, it should be the first-choice sensor.

Transistor-based proximity sensors have no moving parts and long service lives. Reed-based proximity sensors use a mechanical contact and have shorter service lives and cost less than transistor models. Reed sensors are best applied in high-temperature applications and applications where ac power supply is needed.

Position sensors

Position sensors have analog outputs indicating the position of the actuator based on the position of the magnet on that actuator. Position sensors provide flexibility from a control standpoint. The control engineer can determine a range of set points to conform to component variations. Since these position sensors are based on magnets, like proximity sensors, it’s a good idea to purchase the sensor and actuator from the same manufacturer if possible. Position sensors can be acquired with IO-Link functionality, which also can simplify control and parameterization.

 

 

Inductive sensors

Inductive proximity sensors utilize Faraday’s law of induction to indicate presence of an object or an analog output position. The most critical aspect of selecting an inductive sensor is determining what type of metal the sensor is detecting because that determines sensing distances. Nonferrous metals can reduce the sensing range by more than 50% compared to ferrous metals. Sensor manufacturer data sheets should provide the necessary information for sample selection.

Pressure and vacuum sensors

Make sure the pressure or vacuum sensor will accommodate the pressure range required as measured in pounds per square inch for imperial measurement and Bar for metric. Specify the form factor most suitable for the allotted space. Consider whether machine mounted sensors should have indicator lights or a display screen as an aid for operations personnel. If changing setpoints quickly is necessary, investigate IO-Link enabled pressure and vacuum sensors.

Flow sensors

Like pressure and vacuum sensors, flow sensors are specified by flow range, size, and setpoint variability. They can be ordered with on sensor display options. Flow sensors can be specified for relatively low flow rates for one area of the machine and for whole machine applications.

 

 

Optical sensors

The most common optical sensor options are photoelectric—diffuse, reflective, and through beam. Laser sensors and fiber-optic sensing units also fall under the optical sensor category. Photoelectric sensors are mostly presence sensors.

Photoelectric sensors detect the presence of an object via reflected light or an interrupted beam of light. These sensors are among the most applied sensors in manufacturing due to their low cost, versatility, and reliability.

 

Diffuse photoelectric sensors do not require a reflector. They are used for sensing the presence of nearby objects and are inexpensive sensors.

opt sensor

Through beam offers the longest sensing range and is installed at two points with an emitter unit and receiver unit. Garage door safety sensors are through beam sensors. Presence is indicated when the beam is interrupted. One interesting variate of the

through beam is the fork light sensor that features an emitter and receiver in one compact unit. Fork light sensors are used for sensing the presence and absence of small parts.

Reflective photoelectric sensors have a sensor and a reflector and are used for mid-distance presence sensing. For accuracy and cost, they sit midway between diffuse and through beam.

Fiber-optic sensing units are used for presence and distance sensing. Parameters on these versatile sensors can be adjusted to detect various colors, backgrounds, and distance ranges.

Laser sensors are used for long distance presence sensing and are the most accurate in short distance measurement applications

Vision sensors

Vision sensors can be used for bar code reading, counting, shape verification, and more. Vision sensors are a cost-effective use of vision where camera systems would be too costly and complex. Vision sensor bar code reading can be used for tracking individual components and applying the processes identified for that component. In terms of counting, the sensor can verify, for example, the exact number of features present on a part.

vision sensor1

A vision sensor can ascertain whether a specified curve or other shape has been achieved. Since these sensors are dealing with light, it is vital to test the sensor in as close to the operating environment in terms of ambient light and background reflectivity as possible. In most applications, it is recommended to place the vision sensor in an enclosure to isolate it from external sources of light. It is a good idea to enlist the aid of a vision sensor manufacturer in sensor testing. Make sure the right fieldbus is specified.

Signal converter

The signal converter changes the analog output signal from a sensor into switching points on the signal converter, another option is to convert to IO-Link process data.

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Edited by Betty, info@kleevusa.com, http://www.kleevusa,com.

What do you mean by a flow meter and a flow transmitter?

Most diverse substances like liquid, gases, steam are transported and distributed in piping systems in every single day. The fluid going through the pipes often have completely different properties, therefore different principles for their measurements are required. A mathematician found that there is a direct relationship between pressure and speed of a fluid flowing in a pipe.

image 1

What is a flow meter?

The device which is used to measure the flow of liquid or gas in a pipeline is called a flow meter. There are many types of flowmeters. Most commonly used ones are orifice plate, venturimeter, flow nozzle, rotameter etc. All these measures the flow rate in different ways. Generally, the flow meters can display only the value of flow rate.

Here is how the measurement method works:

  • The differential pressure flow meters have an artificial restriction integrated into the measuring tube. For example, an orifice plate illustrated in the figure. Two holes are located in the pipe wall, one before and after the orifice plate.

  • Two separate tubes connect these holes to a chamber separated by a diaphragm who measures the differential pressure. The tiniest differential pressure of the flowing fluid can be sensed.

  • Of the fluid is not flowing, the pressure before and after the orifice is identical.

  • When the fluid is flowing, the velocity of the fluid around the orifice plate increases significantly because of the restriction across the cross-section. According to the fluid mechanics, the static pressure at this point decreases. Thus the diaphragm senses a higher pressure before and a lower pressure after the orifice plateimage 2

What is a flow transmitter?

The flow transmitter is a flow meter with internal electronic circuits which gives an electrical output in a current (4 to 20 mA) or voltage (1 to 5V). According to the figure given above, the output from the flow transmitter is given to the control valve through a controller. Thereby, flow control and monitoring are made possible.

image 3

Measurement method is:

  • The primary element like orifice, venturi, pitot tube flow nozzle etc is designed to create a pressure difference. The secondary element is the differential pressure transmitter is designed to measure the differential pressure caused by the primary element.

  • The differential pressure causes deflection in the diaphragm. As a result, the capacitive sensor senses the change in capacitance which is then converted into an electrical signal(4-20mA).

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