Introduction to the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 15

1.5.3 The measurement of the currents: the ADCP

While the need for direct measurement of ocean currents was evident from the start
of oceanography, the emergence of effective technologies for current measurements
is a comparatively recent development. Early current meters (e.g. the Ekman current
meter, designed by Vagn Walfrid Ekman in 1903) were mechanical devices lowered
from an anchored vessel; speed was measured from the revolutions of a propeller
and direction from the deviation of a vane measured relative to an integral compass.
Such devices were labour intensive and required a dedicated research vessel for the
duration of the observations. Autonomous recording current meters deployed on a
taught wire mooring, such as those shown in Fig. 1.7 , did not appear until the 1960s.
Even then, they were logistically demanding, expensive and vulnerable to damage
by fishing vessels. Current meter moorings were, however, a great improvement on
profiling with direct reading current meters from a research vessel, and were increas-
ingly used in the shelf seas from the 1970s. As well as recording velocity data, recording
current meters are frequently equipped with sensors for conductivity and temperature,
so that salinity and density can be determined. The inclusion of a pressure sensor

Figure 1.7 A mooring ready
for deployment in the eastern
Irish Sea (depth about 35
metres). The instruments are
Aanderaa recording current
meters (RCMs), which use a
rotor with the large vane
aligning the instrument to the
flow. The large spherical buoy
(about 1 metre diameter)
stretches the instrument line
upward from an anchor on the
seabed. Each RCM also had
temperature and conductivity
sensors to provide a coarse
vertical profile of salinity.
(Photo by J. Sharples.)