Manual – Starflow QSD Model 6527B and 6537A
Unidata Manual - 6527B and 6537A Starflow QSD User Manual 08 05 2023.docx
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When comparing Starflow QSD velocity and mechanical current meter readings, the display
should be observed long enough to estimate the mean of the readings. The Starflow QSD
will do most of this processing internally but, if an external logger is being used to record the
readings, the averaging could also be done here. This will help attenuate short frequency
variations.
8.3. Conversion of Logged to Mean Velocity
The measured velocity data may have to be adjusted during post-processing to reflect a
mean velocity for the channel. The factors used will be site-specific and must be determined
by the operator. This is done by obtaining a mean channel velocity by conventional
techniques and comparing it with the average logged velocity. If necessary, this process
should be repeated at various discharges. Where the relationship is complex or unstable, the
accuracy of this method is compromised.
In laminar flow conditions, the channel mean velocity could be expected to be between 90%
and 110% of the logged velocity.
In small channels (say a 500mm diameter pipe), the factor may be close to 100% as a
representative area of flow will have been “seen” by Starflow QSD and contributed to the
logged velocity.
In larger channels, only the area adjacent to Starflow QSD will be “seen”, and the
relationship will depend on how this portion relates to the vertical and horizontal velocity
distribution in the channel. An instrument located in the centre of the stream would normally
be in a higher velocity area. However, in a deep channel, Starflow QSD may only see the
slower portion of the velocity profile.
8.4. The Speed of Sound in Water
Velocity measurements are directly related to the speed of sound in water. The factor used
to scale the velocity measurement is based on the speed of sound in freshwater at 20°C
(see table below). This velocity of sound gives a calibration factor of 0.550mm/sec per Hz of
Doppler shift.
This calibration factor may be adjusted for other conditions; for example, the calibration
factor for seawater is 0.5618mm/sec/Hz.
The speed of sound varies significantly with water density. Water density is dependent on
pressure, water temperature, salinity, and sediment content. Of these, the temperature has
the most significant effect, and it is measured by the Starflow QSD and applied in the
correction of velocity measurements.
The Starflow QSD corrects for the variation of the speed of sound in water due to
temperature using a factor of 0.00138mm/s/Hz/°C. This correction is the best fit for water
temperatures between 0°C to 30°C.
