Manual – Starflow QSD Model 6527B and 6537A
Unidata Manual - 6527B and 6537A Starflow QSD User Manual 17 01 2020.docx
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APENDIX A - FACTORS AFFECTING FLOW MEASUREMENT ACCURACY
Starflow QSD measures velocity to an accuracy of ±2% and depth to ±1% of range. This is logged to
a resolution of 1 mm/sec and 1mm respectively.
The purpose of the Starflow QSD system is to produce velocity data. There are many opportunities for
errors to accrue in the process and degrade the result. These can be reduced or eliminated by using
the instrument properly. Some of the more significant potential error sources follow.
Alignment with Flow and Depth
For the calibration to be valid, the transducer needs to be horizontally and vertically aligned with the
flow. While Starflow QSD instruments are calibrated pointing into the flow, they can be pointed
downstream with little loss of calibration accuracy. You may want to do this when fouling of the
sensor face is a problem. Any angled flow in the horizontal plane will reduce the recorded velocity.
The Starflow QSD instrument must be mounted in the water parallel to the surface for depth readings
to measure accurately (~ +/- 10 deg), if not the depth may read inaccurately and therefore the
recorded depth can be recording incorrectly.
Instantaneous Versus “Averaged” Velocity
When you observe Starflow QSD velocities, they will be seen to vary by 10% or more from scan to
scan at some sites. Because Starflow QSD is very sensitive to variations in velocities, you are able to
see the natural velocity changes in the channel.
Although the discharge in a channel may be reasonably constant for a period of time, the velocity
distribution is always changing. Different velocity streams wander from side to side and bed to surface
as they progress down the channel. Turbulent swirls and eddies are carried downstream for long
distances while they slowly decay. Hydrographers will be used to having this action partly removed by
the mechanical inertia of a current meter and the period over which a typical measurement is timed.
However all will have noticed that the rate of revolutions of the current meter varies during the timing
period.
Continual velocity logging at one location with a Starflow QSD will show these cyclic velocity
pulsations. The characteristics will be different for different sites and will vary with discharge. Cycles
will typically include short period fluctuations (a few seconds) overlaid on longer cyclic fluctuations (up
to many minutes). Longer term pulsations may also be seen particularly in larger streams when in
flood.
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 averaging could
also be done here this will help attenuate short frequency variations.
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 have to be determined by the
operator. This is done by obtaining a mean channel velocity by conventional techniques and
