Using a Velocimeter 51
© 2018 Nortek AS
Table: Weak spots listed as the distance from Sample Volume to boundary for the Vector and Vectrino.
Note that the weak spot location is dependent on the speed of sound and the boundary surface, so the
values given are only estimates and weak spots m ay be encountered a centimeter or more away from
these values.
The Velocity range specifies the separation in time for the two pulses. This parameter can be
adjusted in the Configuration dialog. The rule of thumb is to set the velocity range as low as possible
without the potential for water velocities to exceed the horizontal or vertical velocity ranges.
A weak spots is related to the spatial separation between the pulse pairs transmitted by the
Velocimeter. A weak spot occurs when the first pulse hits the bottom and the reflected signal of the
first pulse reaches the sampling volume at the same time as the second pulse goes through the
sampling volume. The two pulses passing through the same volume at the same time creates
interference, reducing data quality and generally leading to unusable data. The figure below shows
how a weak spot may show up in the data.
Figure: X, Y, Z1 and Z2 velocity plots. The upper plot show s X velocity, w hich is subject to interference
due to a weak spot
The vertical extent of the weak spots is a function of the bottom composition. If the bottom is well
defined (e.g. sand) the extent is no bigger than the transmit pulse or about 1 cm. If the bottom is
rough, the vertical extent can be larger. It is also a matter of the relative strength between the water
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