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 Ultraflow QSD 6537 and applied in the correction of velocity measurements.
The Ultraflow QSD 6537 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.
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.
Post time: Dec-02-2022