Manual – Starflow Ultrasonic Doppler Instrument
Model 6526
Unidata Manual - 6526 Starflow User Manual Issue 17 01 2020.docx
Page 4
2.2
Speed of Sound in Water
Velocity measurements are directly related to the speed of sound in water. The factor used to
scale Starflow 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 and applied in the correction of velocity
measurements.
The Starflow 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.
