Manual – Starflow Ultrasonic Doppler Instrument
Model 6526
Unidata Manual - 6526 Starflow User Manual Issue 17 01 2020.docx
Page 55
12.2.6
Measuring Flows in Very Small Channels
The Starflow can be installed in channels as narrow as 75mm. It can measure minimum velocities
to 21mm/second in depths as low as 25mm. This flow rate is well below the measuring ability of
most instruments, flumes or weirs. However the Starflow may struggle to operate correctly in
these extreme conditions. The following conditions may be seen.
•
The number of samples will be reduced. As velocities slow it takes the Starflow longer to
measure each Doppler shift. At shallow depths the volume of water from which to acquire signals
is reduced. These factors combine to mean the Starflow will generally not be able to acquire the
full number of samples within the time allowed in a cycle. However, unless the water is extremely
clean it will normally acquire enough signals for a sensible result.
•
There will be an increase in the “signal noise”. Because of the difficult environment there
will be an increase in very high velocity signals. This can bias most Doppler data beyond the ability
of the signal analysis process to extract meaningful velocities. Logged data will jump between the
correct value around 3m/sec.
These can be overcome by the use of a dynamic filter that can be enabled in the Starflow
configuration to minimise the impact of signal noise. The level below which the filter is active can
be set during the preparation of the Starflow for a site. Setting this level near the maximum
Starflow velocity will result in the filter always operating. This should not be done at sites where
legitimate high velocities are experienced as the filter may affect the results.
12.2.7
Effects of Filter on Low Velocity Signal in Shallow Water
Developments are continuing to improve the integrity of low velocity measurements. Software
enhancements can be loaded into existing units. Hardware changes will be incorporated into future
units.
