Manual – Starflow QSD Model 6527B and 6537A
Unidata Manual - 6527B and 6537A Starflow QSD User Manual 08 05 2023.docx
Page 107
In practice, a series of histograms in such a situation will move back and forth between the
two extreme conditions, logging an average that is not the correct velocity. At some sites,
ripples will be a characteristic at certain levels. Positioning and orienting the Starflow QSD
so that it does not point directly into a “mirror” ripple face is the best prevention.
17.12.
Measuring Flows in Very Small Channels
The Starflow QSD can be installed in channels as narrow as 75mm. It can measure
minimum velocities to 21mm/second in depths as low as 40mm. This flow rate is well below
the measuring ability of most instruments, flumes, or weirs. However, the Starflow QSD may
struggle to operate correctly in these extreme conditions. The following conditions may be
seen.
The RSSI will be reduced. As velocities slow, it takes the Starflow QSD 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 QSD will generally not be
able to acquire good RSSI 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.
17.13.
Velocities Too Slow to Measure
When velocities fall below the minimum level the Doppler can measure (21mm/sec), signals
are not processed. However, under certain conditions, the signal noise is processed, a
messy signal is interpreted and a higher velocity results. Usually, the processed spread is
too large. Using the filter will prevent most of these signals.
