Nortek Vectrino - Using the Doppler Effect; Vectrino Sonar Principles

NORTEK VECTRINO VELOCIMETER
User Guide
17
Rev. C • 10.2004
Using the Doppler Effect
You hear the Doppler effect whenever a train passes by – the change in pitch you
hear tells you how fast the train is moving. The Vectrino uses the Doppler effect
to measure current velocity by transmitting short pairs of sound pulses, listening
to their echoes and, ultimately, measuring the change in pitch or frequency of
the returned sound.
Sound does not refl ect from the water itself, but rather from particles suspended
in the water. These particles are typically zooplankton or suspended sediment.
Long experience tells us that these small parti cles move with the same average
speed as the water – the velocity it measures is consequently the velocity of the
water.
Vectrino Sonar Principles
In contrast to standard Doppler profi lers and current meters, the Vectrino is a
bistatic sonar. This means that it uses separate transmit and receive beams. It
transmits through a central beam and receives through four beams displaced off
to the side.
The fi gure below shows how the beams intersect each other 50 mm from the
transmitter. The measurement volume is defi ned by this intersection and by
range gating in time. The transmit transducer sends a short pulse that covers
3–15 mm vertically (user adjustable), and the receivers listen to an echo that cor-
responds from this volume. The diameter of the volume is 6 mm. The Vectrino
uses four receivers, all focused on the same volume, to obtain the three velocity
components from that very volume.
The beams intersect at
approximately 50 mm from the
transmitter. The exact position
varies from one Vectrino to
another, but all Vectrinos are
individually measured and
calibrated before leaving the
factory. This exact position is
then stored in each Vectrino’s
head confi guration fi le.
The Vectrino uses four
receivers – all focused on the
same volume – to obtain the
three velocity components. See
also the text for more on this.
Approximately 50mm
3–15 mm (user adjustable)
The fi gure below shows that a transmit/receive beam pair is sensi tive to velocity
in the direction of the angular bisector between the beams. The arrow indicates
a positive velocity.