An ultrasonic flow meter is a type of flow meter that measures the velocity of a

fluid In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...

with

ultrasound Ultrasound is sound with frequency, frequencies greater than 20 Hertz, kilohertz. This frequency is the approximate upper audible hearing range, limit of human hearing in healthy young adults. The physical principles of acoustic waves apply ...

to calculate volume flow. Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow or by measuring the frequency shift from the Doppler effect. Ultrasonic flow meters are affected by the acoustic properties of the fluid and can be impacted by temperature, density, viscosity and suspended particulates depending on the exact flow meter. They vary greatly in purchase price but are often inexpensive to use and maintain because they do not use moving parts, unlike mechanical flow meters.

Means of operation

There are three different types of ultrasonic flow meters. Transmission (or contrapropagating transit-time) flow meters can be distinguished into in-line (intrusive, wetted) and clamp-on (non-intrusive) varieties. Ultrasonic flow meters that use the Doppler shift are called reflection or Doppler flow meters. The third type is the open-channel flow meter.

Principle

Time transit flow meter

Ultrasonic flow meters measure the difference between the transit time of ultrasonic pulses propagating with and against the flow direction. This time difference (TTFM = Transit Time Flow Measurement) is a measure for the average velocity of the fluid along the path of the ultrasonic beam. By using the absolute transit times

t_

and

t_

, both the averaged fluid velocity

v

and the speed of sound

c

can be calculated. Using these two transit times, the distance between receiving and transmitting transducers

L

and the inclination angle

\alpha

, if we assume that sound has to go against the flow when going upstream and along the flow when going downstream, then one can write the following two equations from the definition of

velocity Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...

c - v \cos \alpha = \frac

and

c + v \cos \alpha = \frac

By adding and subtracting the above equations can solve for

v

and

c

v = \frac\;\frac

and

c = \frac\;\frac

where

v

is the average velocity of the fluid along the sound path and

c

is the speed of sound.

Doppler shift flow meters

Another method in ultrasonic flow metering is the use of the Doppler shift that results from the reflection of an ultrasonic beam off sonically reflective materials, such as solid particles or entrained air bubbles in a flowing fluid, or the

turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...

of the fluid itself, if the liquid is clean. Doppler flowmeters are used for slurries, liquids with bubbles, gases with sound-reflecting particles. This type of flow meter can also be used to measure the rate of blood flow, by passing an ultrasonic beam through the tissues, bouncing it off a reflective plate, then reversing the direction of the beam and repeating the measurement, the volume of blood flow can be estimated. The frequency of the transmitted beam is affected by the movement of blood in the vessel and by comparing the frequency of the upstream beam versus downstream the flow of blood through the vessel can be measured. The difference between the two frequencies is a measure of true volume flow. A wide-beam sensor can also be used to measure flow independent of the cross-sectional area of the blood vessel.

Open channel flow meters

In this case, the ultrasonic element is actually measuring the height of the water in the open channel; based on the geometry of the channel, the flow can be determined from the height. The ultrasonic sensor usually also has a temperature sensor with it because the speed of sound in air is affected by the temperature.

References

*Lipták, Béla G.: Process Measurement and Analysis, Volume 1. CRC Press (2003), (v. 1)
Ultrasonic Acoustic Sensing
Brown University *Lynnworth, L.C.: Ultrasonic Measurements for Process Control. Academic Press, Inc. San Diego.

External links

at MathPages

* ttps://ogsi.ae/ultrasonic-flowmeters Advantages of Ultrasonic Flowmeters {{DEFAULTSORT:Ultrasonic Flow Meter Ultrasound Flow meters