Manual – Starflow Ultrasonic Doppler Instrument
Model 6526
Unidata Manual - 6526 Starflow User Manual Issue 17 01 2020.docx
Page 49
12.1
Inherent Factors in Doppler Histograms
The width and shape of the peak displayed is caused by a
range of factors all of which are measured by the Doppler
as a frequency shift.
Some of these factors are inherent in the method.
In flowing channels there are always a range of different
velocities present but even in “perfect” conditions, with all
water flowing at the same velocity the histogram displayed
will still have a characteristic “skinny bell” shape.
This is a composite of a variety of effects related to using
Dopplers to measure water velocity in streams.
12.1.1
Varying Angle as Particles Pass through the Main Beam
The acoustic beam has a
width, or spread, of about
10°. The centreline of the
beam is aligned at 30° above
horizontal but as a particle
passes horizontally through,
the actual angle within the
beam varies from 25° to 35°.
The Doppler shift measured is the velocity component along the
line between the particle and the transmitter, so this changes 5%
as the particle traverses the beam.
12.1.2
Measuring Errors Due To “Signal Noise”
“Signal Noise” is a convenient term used to classify any Doppler shifts that cannot be explained as
legitimate measurements of water velocity. The Starflow is measuring tiny signals echoed back
from small particles along a path scattered with many other particles, all in motion. The returned
signals are sometimes echoes of echoes, vague and ambiguous and can be misinterpreted by the
measuring circuits resulting in an incorrect Doppler shift.
