Ultrasonic flow meter

An ultrasonic flow meter can be defined as, a meter that is used to measure liquid velocity with ultrasound to analyze the volume of liquid flow.

Ultrasonic Flow Meters Working Principle

Ultrasonic flow meters calculate the flow rate by utilizing the speed of sound through a fluid created by transducers mounted to the pipe wall. There are two types of ultrasonic meters: Doppler and Time-of-Flight. The Doppler ultrasonic meter requires particles in the fluid to reflect sound waves back to the pipe wall transducers. The difference in frequency between the sent and reflected wave is proportional to fluid velocity. The Time-of-Flight ultrasonic meter requires a clean fluid and works by using opposing transducers mounted to transmit/receive sound waves at an angle across the pipe. The difference in the time required to send a pulse along the path between transducers in the direction of flow vs. against the flow is proportional to the fluid velocity. Some models use multiple pairs of transducers to fully cover the pipe cross-section, which provides a more accurate flow rate than a single-path meter.

There are two principle

Transit time principle
Doppler effect principle

Transit time principle

Ultrasonic flow meters measure the difference of the transit time of ultrasonic pulses propagating in a flowing fluid. This time difference is a measure for the average velocity of the fluid along the path of the ultrasonic beam. By using the absolute transit times both the averaged fluid velocity and the speed of sound can be calculated. This measurement principle is only possibly for large bore tubes, for higher flow ranges.

Doppler effect principle

Another method in ultrasonic flow metering, also not suited for very low flowrates, 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 of the fluid itself, if the liquid is clean.

Types of Ultrasonic Flow Meter

Ultrasonic flow meters available in the market are radar, Doppler velocity, ultrasonic clamp-on, and ultrasonic level.

Doppler velocity type meters use reproduced ultrasonic noise to calculate the liquid’s velocity.
Radar type meter employs microwave technology for transmitting small pulses to reflect off a flowing surface back to the sensor for deciding velocity.
Ultrasonic clamp-on type meter is ideal for applications wherever accessing the pipe is difficult otherwise not possible.
Ultrasonic level type meter is ideal for determining the fluid level in both open & closed channels.

Transit time principle
Clean liquids with little or no solids or bubbles; gases

Doppler effect principle
Slurries with solids (0.2 to 60% concentration, depending on particle size), liquids that are aerated or contain bubbles, gases with sound-reflecting particles, single-phase turbulent clean liquid.

Process Temperature
− 300 to 500 ° F ( − 184 to 260 ° C); higher or lower with special sound-transmitting wedges.

Materials of Construction
Spools or transducer probes: steel, stainless steel, or alloys.

Sizes
0.125 to 120 in. (3 mm to 3 m) diameter ( Transit time )
0.5 to 72 in. (13 mm to 1.8 m) diameter (Doppler effect )

Price
Spool designs in steel, not including options or special features and of the single-path design: $5000 for 4 in. (100 mm), $8000 for 10 in. (250 mm), $14,000 for 24 in.

What is an Ultrasonic Flow Meter?

An ultrasonic flow meter can be defined as, a meter that is used to measure liquid velocity with ultrasound to analyze the volume of liquid flow. This is a volumetric flow meter that needs bubble or minute particles within the liquid flow. These meters are suitable in the applications of wastewater but they will not work with drinking/distill water. So this type of flow meter is ideal for the applications wherever chemical compatibility, low maintenance, and low-pressure drop are required.
These meters will affect the audio properties of the liquid and also impact through viscosity, density, temperature, etc. Like mechanical flow meters, these meters do not include moving parts. The price of these meters will change greatly so frequently it can be used and maintained at a low cost.

How do Ultrasonic Flow Meters work (Time of Flight) ?

They measure the time it takes for an ultrasonic signal transmitted from one sensor, to cross a pipe and be received by a second sensor.

Typical Ultrasonic Flow Meter "clamp-on" sensor arrangements for Time of Flight variety
Transit time or Time of Flight flow meters utilize two transducers which function as both ultrasonic transmitters and receivers. The flow meters operate by alternately transmitting and receiving a frequency modulated burst of sound energy between the two transducers. The burst is first transmitted in the direction of fluid flow and then against fluid flow. Since sound energy in a moving liquid is carried faster when it travels in the direction of fluid flow (downstream) than it does when it travels against fluid flow (upstream), a differential in the times of flight will occur.
The sound's time of flight is accurately measured in both directions and the difference in time of flight calculated. The liquid velocity (V) inside the pipe can be related to the difference in time of flight (dt) through the following equation: V = K * D * dt, where K is a constant and D is the distance between the transducers

Advantages of Ultrasonic Flow Meter

It does not block the path of liquid flow.
The o/p of this meter is different for density, viscosity & temperature of the liquid.
The flow of liquid is bidirectional
The dynamic response of this meter is good.
The output of this meter is in analog form
Conservation of energy
It is appropriate for huge quality flow measurement
It is handy to fit and maintain
Versatility is good
There is no contact to liquid
There is no leakage risk
There are no moving parts, pressure loss
High accuracy

Disadvantages of Ultrasonic Flow Meter

It is expensive as compared with other mechanical flow meters.
Design of this meter is complex
Auditory parts of this meter are expensive.
These meters are complicated as compared with other meters, thus it requires specialists for maintaining and repairing these meters
It cannot measure cement or concrete pipes one they rusted.
It doesn’t work once the pipe contains holes or bubbles in it
Can’t measure cement/concrete pipe or pipe with such material lining

Applications

These meters are used in wastewater and dirty liquid applications
These meters are used wherever chemical compatibility, less maintenance, and low-pressure drop are required.
These meters are used to measure the velocity of a liquid through ultrasound to analyze volume flow.
These meters measure the disparity between the transit time of ultrasonic pulses which transmits with the direction of liquid flow
The applications of these meters range from process to custody flow
This is one kind of device for volumetric flow measurement for liquids as well as gases.
These are excellent alternatives for both vortex & electromagnetic flowmeters.