Using a Velocimeter 31
© 2018 Nortek AS
In a situation where tilt (and therefore heading) values are incorrect due to the instrument and probe
not being aligned the XYZ and Beam coordinates are still valid. XYZ and Beam velocity data are
gathered with respect to the probe head only.
Figure: Orientation of Vector Cable Probe
Special consideration must be taken when using a Vector with a cabled transducer array (opposed
to the firmly attached stem type). For the cabled type, it is critical that during deployment the
instrument’s array relative to its body (cylinder) is the same as it is with the Vector with the stem.
This is because the internal tilt sensor determines up or down, and ultimately ensures that the
correct coordinate system is used for the directional estimates.
2.1.8 Measuring Waves
For readers interested in information about wave measurements in general it is recommended to take
a look in the Comprehensive Manual - ADCP, as this manual covers this subject in detail. In this
manual it is focused on how wave estimates can be retrieved by using a Vector.
Waves on the ocean surface are clear to us all, however less obvious are the subsurface dynamics
generated by these waves. Beneath the surface, waves generate an orbital motion as they pass by a
point (indicated in the figure below). The motion associated with the waves penetrates down into the
water column. Additionally there is also dynamic pressure that is associated with the combined
effect of the vertical acceleration and the change with depth of the water column. The ability to
measure these dynamics below the surface makes it possible to interpret the waves on the surface
and therefore provides the means to estimate many of the wave parameters that are commonly used
to describe a sea state, such as height, period, and direction.
The name "PUV" itself is a description of the method as it is an abbreviation of the three quantities
measured: Pressure and the two horizontal components of the wave’s orbital velocity, U and V.
These measurements are made at the instrument’s deployment depth and because they are co-
located at the same point, this is referred to as a “triplet” measurement.
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