Using a Velocimeter 53
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period of a few seconds. Induced flows of up to 2-3 cm/s are possible in stationary water with the
power level set on HIGH. Reducing the power level will help to reduce the amount of transducer
induced flow at the expense of a reduced signal to noise ratio in the return signal.
A range of options exist for accounting for streaming when it is unavoidable, including measurement
of streaming magnitudes with the Vectrino itself and analytical or numerical modeling. For the
interested reader; a research article covering acoustic streaming, named “Acoustic Doppler velocity-
induced acoustic streaming and its implication for measurement” is available in our web site.
2.4.9 Incorrect Speed of Sound
As already elucidated, the speed of sound in water influences the calculation of the velocity data.
The time lag between the transmitted and received pulse determines how far the pulse travelled
before it was reflected.
The instruments compute the speed of sound based on the measured temperature (accuracy of 0.1°
C). A nominal salinity is assumed, an assumption that works relatively well because the sound
speed is more sensitive to temperature variation than it is to salinity variation.
Sound speed errors are typically small, but if you must correct velocity data for errors, use the
following equation:
Here, V is the velocity and C is the speed of sound.
2.5 Applications
This section contains typical applications for Velocimeters, and tries to summarize what needs to be
taken into account before starting the sampling of data. Also take a look at the Configuration section
of the chapter covering your specific instrument for some more details.
2.5.1 Mean Currents
Choose a measurement interval that captures the variations that is of interest. For example, a
typical tidal cycle is well resolved with a measurements every 5, 10, 20 or even 30 minutes.
The cell size has a default setting that corresponds to the instruments transmit frequency. This
may certainly be changed, but be aware that smaller cells increases the measurement
uncertainty, while large cells reduce the measurement uncertainty at the expence of power
consumption.
Vector: Typical moorings allow the instrument to tilt and rotate freely. The Vector measure tilt and
heading and uses this compass information to convert the data to true earth coordinates. How
often the instrument reads the tilt and heading depends on the deployment method used. Buoy
mounted instruments should have the compass set to update relatively fast (every second) so that
buoy motion is accounted for in the velocity estimates. For bottom-mounted instruments, it is
enough with one compass update per burst, while instruments mounted on a mooring line should
update the compass every other second. IMU should of course be considered if mounting the
Vector on a moving structure.
Vector: If magnetic fields are present, consider collecting data in XYZ coordinates rather than
relying on the magnetometer to rotate the results into ENU coordinates. At least, remember to run
a compass calibration.
Salinity should be adjusted in the advanced menu options if it is significantly different from the
default value.
From signal theory we know that the more pings there are within an averaging period, the better
the estimate of the true value we are measuring.
Make sure you are using the most recent software and firmware for your instrument. Visit the
Nortek Web regularly for updates.
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