Manual – Starflow Ultrasonic Doppler Instrument
Model 6526
Unidata Manual - 6526 Starflow User Manual Issue 17 01 2020.docx
Page 5
2.3
Factors Affecting Data Accuracy
Starflow measures velocity to an accuracy of ±2% and depth to ±0.25% of calibrated range. This
is logged to a resolution of 1 mm/sec and 1mm respectively. The transducers have been
calibrated and are expected to be stable for long periods provided they are not physically
damaged, blocked or buried.
The purpose of the Starflow system is to produce flow data. This is the product of the cross section
(derived from measured depth) and velocity data, each of which is modified by user defined factors
before use. 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.
2.3.1 Alignment with Flow
For the calibration to be valid, the transducer needs to be horizontally and vertically aligned with
the flow. While Starflows 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. A 10 degree
angle will reduce the velocity recorded by 1.5%.
More significant errors will result from angled flow in the vertical plane. The sensors are
manufactured to project acoustic signals at 30° above horizontal when the sensor is mounted
horizontally. A 10° vertical flow angle change will cause errors of approxi8.5% (@20°) and
–11.5% (@40°).
2.3.2 Instantaneous Versus “Averaged” Velocity
When you observe Starflow velocities, they will be seen to vary by 10% or more from scan to scan
at some sites. Because Starflow 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 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 velocity and mechanical current meter readings, the display should be
observed long enough to estimate the mean of the readings. This should be adopted as the
logged velocity to compare with any check measurements.
When measuring velocity data Starflow samples for several seconds during each 1 minute scan. It
is recommended that all velocity measurements are averaged over the log interval. This attenuates
short frequency variations.
