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Basics of Ultrasonic Meter

There are various types of flowmeters:  Coriolis Mass Flowmeters, Positive Displacement Flowmeters such as Helical Turbine Flow Meter, and Volumetric Flowmeters like Ultrasonic Flowmeter just to name a few.

This article will show that the ultrasonic meter requires the use and understanding of physical, mechanical, and electrical principles to accurately measure the flow rate over a wide range of flow conditions

Working Principle

When a fluid flows through a pipe, it flows in one direction at a velocity that is influenced by many factors such as temperature, pressure, fluid viscosity, and pipe size. The higher the pressure differential upstream of a point in the pipe and the downstream from that point, the higher the flow of velocity. Increasing the temperature usually leads to increased flow, all other factors remain the same.

Some fluids are clear and some contain particles and bubbles. Some fluids are very viscous like heavy oils and do not flow easily as less viscous fluids like water. Sound waves move through the fluid based on the vibrations of the fluid, these mechanical oscillations between molecules of the flowing fluid are passed to the adjacent molecules thereby transferring the wave to those adjacent molecules.

Just like a car horn is heard because a rapidly vibrating diaphragm transmits high-frequency waves through the air, the fluid of transmission in this case. Humans are able to hear the sound because the vibration frequency is in the audible range i.e., between 20 Hz and 20kHz

Ultrasonic flowmeters are operating at the vibration frequency outside of the audible range at 25kHz or above to have an accurate measurement.

How Does an Ultrasonic Flowmeter Generate Sound Waves?

The rapidly vibrating piezoelectric electric crystal. The special crystals deform when an electric current is apply. Through quick changing the electrical signal, the crystal will deform in particular one direction and then in the other, which then generates a high-frequency wave.

Putting these three principles together. An ultrasonic flowmeter consists of a “Sensor” and “Transducer pair”, each member of the pair connects both as a “Transmitter” and “Receiver”. When in transmit mode an oscillating electrical current creates a vibration in the piezoelectric electric crystal and the ultrasonic wave is sent through the flowing fluid. When in receive mode an ultrasonic wave passing through the fluid creates a vibration in the piezoelectric electric crystal and an electrical impulse is generated.  There are various factors on which the time taken by the ultrasonic wave to pass from the transmitting element to the receiving element is dependent. The fluid velocity will affect the time it takes for the signal to reach the receiving element if the two elements are placed at separate locations along the pipe.

The Concept of Time of Flight Flow Meter

The vibrations are passed from the transmitter to the receiver in a still fluid in a measurable amount of time. The ultrasonic pulse generated in the opposite direction should take the amount of time. The measurement environment is constant. But due to the mechanical properties of ultrasonic waves, the time it takes to move from transmitter to receiver is influenced by the fluid velocity.

A pulse that travels from upstream element to downstream element or with the flow takes less time to make the trip than the pulse returning in the opposite direction or against the flow. The difference in the time of flight between the two pulses is directly proportional to the flow velocity. Since the volumetric flow rate is equal to velocity times the cross-sectional area of the pipe which is fixed at the flow meter, this measurement yields the flow rate. This type of Ultrasonic Meter is appropriately named “time of flight flow meter”. In order to eliminate the differences in the flow profile across the pipe, additional pairs of sensors are added to ensure accurate measurement. Each pair of transmitter and receiver forms, what is called a “chord”. Each chord is placed to provide measurements at different locations along the pipe.

There are many ways in which the sensors are mounted in a “time of flight flow meter”. The sound waves pulses can travel directly from transmitter to receiver. The sensors can be wedded or built into the flow meter such that the sensors contact the fluid or sensors can be strapped to the exterior of the pipe. But the non-contact sensors are not that accurate.

Doppler Effect Measure

Another type of Ultrasonic flowmeter uses the Doppler effect to measure the flow rate. In this method, air bubbles are used to reflect the sound waves from a transmitter to a receiver to determine the flow rate.

Conclusion:

Now that you got aware of how an Ultrasonic flowmeter operates, you may ask where are these used? These are an excellent choice for high pressure and high flow applications like natural gas pipelines. For such fluid streams, ultrasonic is the best choice.

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