Manual – Starflow QSD Model 6527B and 6537A
Unidata Manual - 6527B and 6537A Starflow QSD User Manual 08 05 2023.docx
Page 44
8. FACTORS AFFECTING FLOW MEASUREMENT ACCURACY
Starflow QSD measures velocity to an accuracy of ±2% and depth to ±1% of the 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.
8.1. 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
˚
), if not, the depth may read inaccurately, and
therefore the recorded depth can be recorded incorrectly.
8.2. 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 are 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.
