Manual – Starflow QSD Model 6527B and 6537A
Unidata Manual - 6527B and 6537A Starflow QSD User Manual 17 01 2020.docx
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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.
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 fresh water 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 sea
water 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, 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 a best fit for water temperatures between 0°C to
30°C.
The following table shows how the speed of sound varies with temperature and between fresh and
sea water.
Temperature (ºC) "Fresh Water" "Sea Water"
0
1402
1449
5
1426
1471
10
1447
1490
15
1466
1507
20
1482
1521
25
1497
1534
30
1509
1545
35
1520
1555
Velocity of Sound in Water (m/s) at atmospheric pressure
Bubbles in the water are desirable as scatterers, but too many can affect the speed of sound. In air
the speed of sound is about 350 m/s.
