Transit time difference process
Doppler flow meters operate similarly to the radar speed traps used on the road.
An emitter sends ultrasonic waves at frequency f1 (approx. 1 - 5 MHz) at angle a into the
flowing product. The ultrasonic waves strike particles moving through the sound field at
velocity Vp. The wavelength of the emitted wave at frequency f1 amounts to:
Due to its rate of motion Vp, the particle moving away from the emitter ‘sees’ the
wavelength: λp= (C-Vp COS α ) / f1
In turn, the receiver now ‘sees’ the reflected frequency out of line because the reflecting
particle is moving further away all the time, and the wavelength changes as follows:
λp= (C- 2Vp COS α ) / f1
For Vp « c we obtain:
This difference in frequency is therefore a linear measure of the rate of motion o
The fluid velocity can be expressed as
v = c ( f2 - f 1 ) / 2 f1 cosΦ (1)
f2 = received frequency
f1 = transmission frequency
v = fluid flow velocity
Φ = the relative angle between the transmitted ultrasonic beam and the fluid flow
c = the velocity of sound in the fluid
Doppler meters may be used where other meters
don't work. This might be liquid slurries, aerated
liquids or liquids with some small or large amount of
suspended solids. The advantages can be summarized
Obstruct less flow
Can be installed outside the pipes(clamp on version)
Low flow cut off
Relative low power consumption
•Needs a sufficient number of reflecting particles in the medium
on a continuous basis.
•The particles must be large enough to provide sufficiently good reflections
•The sound velocity of the particulate material must be distinctly different from that of
• The sound velocity of the medium is directly included in the measurement result.
• The particle velocity often differs noticeably from the velocity of the liquid.
•Usually, the ultrasonic field extends only into the peripheral flow. That is why indication
heavily dependent on the flow profile.
• The velocity needs to be far higher than the critical velocity at which particles settle.
•Very long unimpeded inlet runs (20 x D) are needed to allow conclusions to be drawn
from the flow rate.
1. Medical applications (measurement of the blood flow)
Absolute accuracy is not a requirement. Only good dynamic performance is required which
reproduces blood pulsation in the veins and arteries in great detail for diagnosis (similar to
an ECG). The sensor pad is simply applied to the skin (with coupling gel) avoiding bleeding.
2. Measurement of the flow of slurries (e.g. iron ore)
By its very nature, particle concentration is high. The sound velocity of the particles also
differs sufficiently from that of the carrier medium. The flowmeter is often used to provide
a signal when the velocity drops below the critical level. The penetration depth of the
beam, which depends on the concentration, and also the flow profile tend to cause
considerable errors of measurement.
To diagonally canoe across a river with
the current flow takes less time than
against the current flow.
The stronger the current,then faster
the crossing than against it.
This difference between the travel
times with and against the current is
directly proportional to the flow velocity
of the river.
This effect is exploited by ultrasonic
flowmeters to determine flow velocity and
flow rate. Electro-Acoustic
converters(piezo’s) emit and receive short
electronic pulses through the product
flowing in the tube
Use reflect mode whenever possible,
however for short pipe run and plastic
pipes use direct method
For reflect method, the spacing
between transducers is to be adjusted
according to pipe wall thickness and
hence procedure becomes complex.
** Z-method also called direct method
•Obstruction less flow
•Pressure drop equal to an equivalent length of
•Unaffected by changes in temperature, density or
•Bi-directional flow capability
•Accuracy about 1% of flow rate
•Relative low power consumption
The pipe wal should be cleaned and be rust free.
Pipe is full.
Fluid media conduct sonic energy.
Avoid installing transducer anywhere near a
downstream of throttling valve , mixing tank, etc.
Clamp On method
The more the no of measuring paths= Higher the accuracy at
varying “Reynolds number”
CLAMP ON TYPE
The operating principle for the ultrasonic
flowmeter requires reliability high frequency sound
transmitted across the pipe.
Liquid slurries with excess solids or with
entrained gases may block the ultrasonic pulses.
Ultrasonic flowmeters are not recommended for
primary sludge, mixed liquor, aerobically digested
sludge, dissolved air flotation thickened sludge and
its liquid phase, septic sludge and activated carbon
Liquids with entrained gases cannot be measured