Introduction to the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 10

Figure 1.4 A CTD being
deployed over the side of the
RRS James Clark Ross. The
CTD rests within the lower
part of the frame below the
grey sample bottles. This allows
the instrument to sample
relatively undisturbed water as
the package is lowered
through the water column.
(Photo by J. Sharples.)

1.5.1 The measurement of temperature, salinity and pressure (the CTD)

The density of seawater is a physical parameter which, as we shall see in Chapter 3 ,
plays an important part in the dynamics of the flow in shelf seas. Until the 1960s,
density determination in the ocean was largely based on a combination of measurements
using specialised mercury-in-glass thermometers to measure temperature and the
collection of water samples to allow the determination of the salinity (salt content)
of the water by titration or laboratory measurements of conductivity. The methods
were reasonably accurate, giving temperatures to 0.01 C and salinity to

∼

one part

∼

in 3000, but they were also rather complicated and labour intensive.
In modern practice these early methods have been almost entirely replaced by
a profiling instrument package measuring conductivity, temperature and depth
andreferredtosimplyasaCTD,shownin Fig. 1.4 . As the package is lowered on
a cable through the water column, information from electrical sensors is transmitted
to the surface via conductors in the cable. In most modern CTDs, temperature
is measured by a high-quality platinum resistance thermometer while conductivity
is sensed by a conductivity cell which may be directly coupled to the seawater via
electrodes or indirectly through an inductively coupled system. Pressure, which is
usually measured by a strain gauge sensor, is used to determine the depth of the CTD.