9.2 Field and Laboratory Measurement Techniques
307
temperature resistance and very small size. In typical oceanographical applications, thermistors are fixed along a mooring cable forming a thermistor string
used for stationary measurements or used in expendable bathythermographs
(XBT). A thermistor is placed in the nose of the probe as the temperature
sensing element. The accuracy of an XBT is ±0.1°C (Emery and Thomson,
1997). Resistance thermometers are also widely used in CTD (conductivitytemperature-depth) profilers.
9.2.2 Salinity
As we showed in Chap. 1, salinity is one of the major agents determining
the density of ocean water. A first measure of salinity is related to another
measured parameter 'chlorinity' by Knudsen's equation:
S(ppm) = l.805 CI(ppm) + 0.03.
(9.1 )
At present, water conductivity is used as a measure of salinity rather than the
titration of chlorinity. Conductivity of ocean water depends on the ion content
of the water and therefore is directly proportional to the salt content. Modern
conductivity-temperature-depth (CTD) profiling systems record conductivity
directly as the water flows through the instrument. Then, an empirical relationship between observed conductivity and salinity is used to compute salinity.
This new salinity definition, so called 'practical salinity scale', or PSS78 is now
recommended as the scale in which salinity data should be reported (Lewis and
Perkin, 1981).
9.2.3 Sea Level Measurement
The periodic rise and fall of the sea surface has been observed for thousands of
years. Pytheas in the fourth century B.C. probably recorded the tides during
his journey from Massilia in the Mediterranean northward into the English
Channel and southern part of the North Sea (von Arx, 1977). Since then,
tidal variations of sea level have been observed by a variety of methods. The
simplest is the float stilling well gauge, in which the float rises and falls with
the water level. Modern recording systems use sensitive and accurate pressure
or acoustic gauges.
However, conventional sea level measurement systems require a fixed installation platforms and therefore are only possible at or near the coast. Over the
vast ocean, away from any islands, sea level is measured using satellite-borne
radar altimetry. We will describe this technique in Sect. 9.3.
9.2.4 Ocean Current Measurements
There are a variety of techniques and instruments used for measuring current
velocity and direction. They can be measured at fixed locations (Eulerian
307
temperature resistance and very small size. In typical oceanographical applications, thermistors are fixed along a mooring cable forming a thermistor string
used for stationary measurements or used in expendable bathythermographs
(XBT). A thermistor is placed in the nose of the probe as the temperature
sensing element. The accuracy of an XBT is ±0.1°C (Emery and Thomson,
1997). Resistance thermometers are also widely used in CTD (conductivitytemperature-depth) profilers.
9.2.2 Salinity
As we showed in Chap. 1, salinity is one of the major agents determining
the density of ocean water. A first measure of salinity is related to another
measured parameter 'chlorinity' by Knudsen's equation:
S(ppm) = l.805 CI(ppm) + 0.03.
(9.1 )
At present, water conductivity is used as a measure of salinity rather than the
titration of chlorinity. Conductivity of ocean water depends on the ion content
of the water and therefore is directly proportional to the salt content. Modern
conductivity-temperature-depth (CTD) profiling systems record conductivity
directly as the water flows through the instrument. Then, an empirical relationship between observed conductivity and salinity is used to compute salinity.
This new salinity definition, so called 'practical salinity scale', or PSS78 is now
recommended as the scale in which salinity data should be reported (Lewis and
Perkin, 1981).
9.2.3 Sea Level Measurement
The periodic rise and fall of the sea surface has been observed for thousands of
years. Pytheas in the fourth century B.C. probably recorded the tides during
his journey from Massilia in the Mediterranean northward into the English
Channel and southern part of the North Sea (von Arx, 1977). Since then,
tidal variations of sea level have been observed by a variety of methods. The
simplest is the float stilling well gauge, in which the float rises and falls with
the water level. Modern recording systems use sensitive and accurate pressure
or acoustic gauges.
However, conventional sea level measurement systems require a fixed installation platforms and therefore are only possible at or near the coast. Over the
vast ocean, away from any islands, sea level is measured using satellite-borne
radar altimetry. We will describe this technique in Sect. 9.3.
9.2.4 Ocean Current Measurements
There are a variety of techniques and instruments used for measuring current
velocity and direction. They can be measured at fixed locations (Eulerian