Knowing the current at one depth tells you much less than people assume. Currents are
layered: a surface flow driven by wind can run in a different direction from the tidal stream
beneath it, and both can differ from what is happening near the seabed. For a riser, a mooring, a
diving operation or a dynamic positioning system, the profile through the water column is the
useful quantity, not a single number.
An acoustic Doppler current profiler produces that profile from one instrument. It transmits
a pulse of sound and listens to the echo returned by particles suspended in the water. Those
particles drift with the water, so the echo comes back frequency-shifted by the Doppler effect
in proportion to how fast the water is moving toward or away from the instrument. Because sound
returning from further away arrives later, the return can be sliced by arrival time into depth
cells, each giving a velocity at a known distance. Several angled beams resolve those velocities
into current speed and direction at every cell.
The practical consequences are worth stating. There are no moving parts in the water, so
there is nothing to foul or seize - which is the failure mode of mechanical rotor current
meters. The instrument does not have to be at the depth being measured; it profiles away from
itself, so it can sit on the seabed looking up or under a structure looking down. And one
deployment yields the whole column rather than requiring a string of instruments at different
depths.
The trade-off that governs selection is between range and resolution, and it is set by
acoustic frequency. A high frequency gives fine depth cells over a short range; a low frequency
reaches much further with coarser cells. That single decision determines which instrument suits
a site, which is why an ADCP is specified against water depth and the profile detail actually
required rather than bought as a general-purpose device. Nortek span this with the Aquadopp
series, which they position for users who are less experienced with the technology, and the
high-end Signature series, which offers a wider range of configurations.
Setup is done in Nortek's deployment software, which lets the measurement scheme be
configured and the power requirement assessed before the instrument goes in the water. That
second function is not a nicety - on an autonomous deployment, cell size, ping rate and
averaging interval together determine whether the battery lasts the intended period, and there
is no correcting it once the instrument is on the seabed.
We should flag something rather than quietly work around it: "S4SH" is not a model
designation we could find on Nortek's own site or in Nortek distributor catalogues. The "S4"
naming pattern is used by InterOcean Systems for their S4 current meter family, which measures
current electromagnetically rather than acoustically - a different instrument entirely. This
looks like a brand or model mismatch carried over from the source data behind this catalogue
line, and we would rather tell you that than print a specification for a product we cannot
verify exists. Tell us the actual instrument - Nortek or otherwise - and we will write this
page against it properly; until then, treat the figures above as background on ADCP technology
in general rather than a specification for this exact line.
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