Comprehensive Manual28
© 2018 Nortek AS
For a given velocity range, the internal ping rate is constant and independent of the "sampling rate"
used in the software setup and which tells how often the Vector should output data.
Internal Sampling Rate (all three beams)
Example: If the Vector is configured with a 1 m/s velocity range, it has an internal sampling rate
of 125 Hz. Each Vector measurement interval uses all of the available samples. If the Sampling
rate is set to 64 Hz, then most of the output records will be averages of 2 samples, but a few will
have only one sample (125/64 = 1.95). At 4 Hz, most of the records will hold an average over 31
samples, but about 1/4 will average 32 samples.
Velocity Range
The velocity range is an important parameter to set properly. The range is a nominal value and the
exact maximum velocity is different along the vertical axis (i.e., toward the transmitter) and in the
horizontal plane.
Example: If the velocity range is set to +/-30 cm/s, the maximum velocity Vz is 30 cm/s toward
the transducer and 81 cm/s in the plane perpendicular to the transmit axis (Vx and Vy). The ratio
between the software adjustable (nominal) velocity range and the maximum value for Vx, Vy, and
Vz does not remain constant but is different for each setting.
As a general rule, the velocity range should always be set as small as possible. If for example, the
maximum expected velocity is 8 cm/s, the velocity range should be set to +/- 10 cm/s. The reason
for this is that the noise in the data increases with increasing velocity range. If we set the velocity
range to +/-70 cm/s and the actual velocity is only 2 cm/s the data will appear excessively noisy.
If the conditions are not known in advance, the velocity range must be set high enough to cover the
whole deployment period. Be aware that the velocity range must cover the full range of wave motion
on top of the mean current. If, for example, the mean currents are expected to be around 10 cm/s
and the wave orbital velocity may reach 40 cm/s, the velocity range should be set to 1 m/s.
Speed of Sound
The instruments use the speed of sound to compute velocity from the measured Doppler shift, so
this information is of vital importance. The speed of sound in water is primarily a function of
temperature and salinity. The temperature is measured by a thermistor embedded in the probe and
reported in degrees Celsius, and this information is used when selecting Measured speed of sound.
If the measurements are to be done in an area with changing salinity (for example, an estuary), it
can be an idea to use a fixed speed of sound and correnct the measurements for speed of sound
errors as a post processing step.
2.1.5 Data Collection
The Vector system allows for both stand-alone (self-recording) and real-time data collection
(monitoring applications). Both applications share the same procedure for the deployment planning
(i.e. instrument configuration). A typical sequence includes:
1. Test the Vector as described in Initial Preparation.
2. Erase recorder. Click Deployment > Erase Recorder.
3. Set the real time clock in your PC (the correct time of day).
4. Set the pressure offset: On-line > Set Pressure Offset.
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