The Doppler effect was first observed in 1842 by Christian Doppler, a
19th century physicist and mathematician who studied the behavior of light. He noted that the observed frequency of sound and light waves
depends on how fast the source and observer are moving relative to each other. This principle has come to be known as the Doppler effect.
Doppler did not work with flowmeters, his principle was used over 100 years later to form the basis of Doppler
flowmeters. Doppler flowmeters have the distinction of readily making one of the most difficult
measurements in flow: measurement of fluids containing bubbles, sand, dirt and other impurities.
(With the exception of magnetic flowmeters, most other flowmeter types thrive on clean water, gas or steam, with few
or no impurities.) Doppler ultrasonic flowmeters are effective with dirty liquids or slurries, and used with both liquids and gases
meters slip in
were more widely used in the early days of ultrasonic flowmeters than they are
today for three main reasons:.
Transit time flowmeters are
now also capable of handling fluids that are not completely clean. Since transit time flowmeters typically
have greater accuracy than Doppler flowmeters, end-users select them for
many applications that would previously have required Doppler flowmeters. Alternatively, they might select another technology like magnetic or
even vortex over Doppler meters.
Doppler remains almost exclusively a clamp-on
technology, although there does not appear to be anything in Doppler technology that makes a
Doppler inline meter impossible. Clamp-on flowmeters have advantages and disadvantages.
They are mobile, but the pipe wall can attenuate the signal, lending some uncertainty to the resulting measurement. Build-up on the
pipe wall can also impact the pipe diameter, affecting signal accuracy and
reliability. Clamp-on meters must be installed properly to work correctly.
Flowmeter markets trend toward greater reliability and higher
accuracy, and much of the research and development work on ultrasonic flowmeters today is directed toward multipath transit time meters that send
multiple signals across a pipe at different locations to achieve higher accuracy. Multipath ultrasonic meters are invariably inline
meters; there is no analogue in Doppler meters to inline multipath transit time
Doppler ultrasonic flowmeters send
an ultrasonic signal across the pipe to a receiver. When there is no flow in the pipe, the signal frequency received
is the same as the signal transmitted. When there is flow in the pipe, and the fluid contains particles or bubbles, the signal reflects off these particles or bubbles, back to the sensor or receiver.
These particles are traveling at the same
speed as the flow.
This reflected signal has
an altered frequency that differs from the frequency of the transmitted
signal, and this shift is proportional to the flowrate of the fluid. As the signal passes through the stream, its frequency
shifts in proportion to the mean velocity of the fluid.
A receiver detects the
reflected signal and measures its frequency. The meter calculates flow by comparing the transmitted and reflected signal frequencies.
about New Technology Flowmeters:
information on ultrasonic flowmeters, please see
www.FlowUltrasonic.com and www.UltrasonicFlows.com.