There are two main types of ultrasonic flowmeters – transit-time and
Doppler -- but much of the research and development work on ultrasonic flowmeters
today is directed toward transit time meters, specifically multipath
transit time meters. These meters send multiple signals across a pipe at different locations to achieve higher accuracy.
Ultrasonic flowmeter transducers
apply voltage to piezoelectric crystals to generate an ultrasonic signal. The same
piezoelectric crystals can also be used to detect the presence of an ultrasonic
signal. In this way, an ultrasonic flowmeter transducer can be both a sender and a
receiver of signals.
A transit-time ultrasonic flowmeter has both a sender and a receiver. It sends two ultrasonic signals across a pipe at an angle – one with the flow and one against the flow. The meter then measures the “transit time” of each signal. When the ultrasonic signal travels with the flow, it travels faster than when it travels against the flow. The difference between the two transit times is proportional to
Some ultrasonic meters have a single
path, requiring one pair of transducers, and some have dual paths, requiring two
transducer pairs. An important group of ultrasonic flowmeters has three or more
paths; these meters are called multipath.
Multipath transit time
flowmeters use more than one ultrasonic signal, or “path,” in calculating flowrate. Each path
requires a pair of sending and receiving transducers. By using more than one path, the flowmeter
measures flow at more than one location in the flowstream, leading to greater accuracy.
flowmeters have been particularly important for measuring natural gas flow,
and have become many companies' meter of choice
for custody transfer of natural gas.
Info on Doppler meters:
of improvements in electronic processing technology, transit-time meters
are better able to handle fluids that are not completely clean. This has
enabled transit-time flowmeters to be used for applications that could
previously only be handled by Doppler flowmeters. These improvements have
also increased the accuracy of ultrasonic meters, which has led to broader
use of these meters in a wider variety of conditions.
about New Technology Flowmeters:
In flowmeter terminology, a path is
defined as the route of travel between two ultrasonic transducers. Another term now in common use is the
term “chord.” Mathematically speaking, a chord is a straight line within a circle
whose endpoints lie on the circumference. However, the term “chord” is also used by
some ultrasonic manufacturers to refer to the route of travel between two transducers. In this way, a chord is like a path.
However, a chord is considered to be the
route of travel between a transducer and the pipe wall or reflector when the signal is
bounced off a wall or a reflector. So in this sense, an ultrasonic signal that bounces off
a wall or reflector to a receiving transducer has one path and two chords. One chord is
the path of the signal from Transducer A to the pipe wall or reflector, and the second
chord is the path of the signal from the pipe wall or reflector to transducer B.
In a direct path, the signal goes from
one transducer to another without being reflected. In a reflected or bounced path,
the signal from one transducer is reflected or bounced off a reflector or the pipe wall
on the way to the second transducer. Unlike a reflected path, a direct path is considered
to have only one chord.
of bounced or reflected paths
agree about the definitions of path and chord:
A path requires
two transducers, and it represents the path of an ultrasonic signal traveling back and forth between the two transducers.
represents the path of an ultrasonic signal going between two transducers.
If the path is direct, then the transducers have one
path and one chord going between them.
If the path is reflected,
or bounced, then there is one path but two or more chords between the two transducers, depending on the number of reflections or bounces.
have different views about the value of bounced or reflected paths.
believe that the added length of the chords in bounced or reflected paths provides additional diagnostic information and enhanced
performance -- and also can result in greater measurement accuracy and
Others maintain that
suppliers who emphasize the number of chords over the number of paths are engaged in “marketingspeak” and that
end-users may interpret the number of chords to be the same as the number
For further information, please see
www.FlowUltrasonic.com and www.UltrasonicFlows.com.