32
secondary winding through the pulsing magnetic field generated by the source. In
this case, seawater constitutes the transformer core and its ion content modulates the
induced voltage in the secondary winding providing a measure of the conductivity
ratio to that of standard seawater. This measure, concurrent with a temperature reading, allows calculation of S P . In early years, laboratory bench salinometers were
equipped with such electrodes conformed as two stacked toroidal (doughnut- shaped)
windings embedded in a nonconductive matrix. Today, some CTDs are also equipped
with these toroidal electrodeless induction salinity sensors.
Since the magnetic field of the toroidal cell is external to the sensor, field effects
caused by external currents or the proximity of conductive material (such as CTD
cages) are significant. Also, biological fouling of the necessarily externally mounted
cell causes changes in cell geometry, essential for deriving accurate measurements
of conductivity. These drawbacks motivated continuing research to overcome the
obstacles to operating immersion electrodes in seawater including electrode drift,
external field perturbation, and biological fouling. Design improvements have
achieved the necessary resistance stability of the electrodes as well as that of the cell
constant describing the geometry of the assembly. External field effects have been
overcome by minimizing current leak beyond the physical boundaries of the cell.
Alternating current is used to prevent changes in seawater chemistry. For long-term
deployments, biological fouling is overcome using biocide barriers that prevent the
settling and growth of organisms within the small volume cells.
High precision laboratory salinometers allow calibration of field instrument data
and analysis of samples where direct immersion of the electrode is impossible (such
as sediment porewaters). A widely used bench instrument features four helical platinum electrodes housed in a four-well flow-through glass assembly, all immersed in a
constant temperature bath supplemented with a platinum resistance thermometer.
Field modules in various configurations are also available. One convenient design
incorporates three annular platinum electrodes within a flow-through cylindrical glass
cell. Connection of the two outer electrodes conforming a two-terminal cell isolates
the current field to the interior of the cell and thus prevents current leakage outside the
cell boundaries. Submersible instruments incorporate a thermistor thermometer in
addition to the conductivity cell for accurate salinity computation (Fig. 2.13).
Remote sensing of surface seawater salinity (SSS) from satellites was only
recently achieved through passive radiometric sensing of sea surface microwave
emissions in the radar L band at ca. 1.4 GHz. Thermal emission of seawater in this
band is modulated by salinity with a change of about 0.5 K per unit change in salinity at 293 °K (20 °C). Negligible absorption of radiation by atmospheric gases in
this band creates a satellite remote sensing window. Radiometers aboard the
European Space Agency Soil Moisture and Ocean Salinity (SMOS) satellite (2009–
2010) and the US Aquarius instrument aboard the Argentinian spacecraft SAC-D
(2011–2015) provided proof of concept for eventual operational sensing of the
global SSS field (Lagerloef 2012). Nevertheless, spatial resolution of these instruments as deployed in experimental missions is in the range of 100 km
2
largely
negating effective coastal ocean observing applications where resolutions of at least
1 km may be required.
2 Electronic Sensors and Instruments for Coastal Ocean Observing
secondary winding through the pulsing magnetic field generated by the source. In
this case, seawater constitutes the transformer core and its ion content modulates the
induced voltage in the secondary winding providing a measure of the conductivity
ratio to that of standard seawater. This measure, concurrent with a temperature reading, allows calculation of S P . In early years, laboratory bench salinometers were
equipped with such electrodes conformed as two stacked toroidal (doughnut- shaped)
windings embedded in a nonconductive matrix. Today, some CTDs are also equipped
with these toroidal electrodeless induction salinity sensors.
Since the magnetic field of the toroidal cell is external to the sensor, field effects
caused by external currents or the proximity of conductive material (such as CTD
cages) are significant. Also, biological fouling of the necessarily externally mounted
cell causes changes in cell geometry, essential for deriving accurate measurements
of conductivity. These drawbacks motivated continuing research to overcome the
obstacles to operating immersion electrodes in seawater including electrode drift,
external field perturbation, and biological fouling. Design improvements have
achieved the necessary resistance stability of the electrodes as well as that of the cell
constant describing the geometry of the assembly. External field effects have been
overcome by minimizing current leak beyond the physical boundaries of the cell.
Alternating current is used to prevent changes in seawater chemistry. For long-term
deployments, biological fouling is overcome using biocide barriers that prevent the
settling and growth of organisms within the small volume cells.
High precision laboratory salinometers allow calibration of field instrument data
and analysis of samples where direct immersion of the electrode is impossible (such
as sediment porewaters). A widely used bench instrument features four helical platinum electrodes housed in a four-well flow-through glass assembly, all immersed in a
constant temperature bath supplemented with a platinum resistance thermometer.
Field modules in various configurations are also available. One convenient design
incorporates three annular platinum electrodes within a flow-through cylindrical glass
cell. Connection of the two outer electrodes conforming a two-terminal cell isolates
the current field to the interior of the cell and thus prevents current leakage outside the
cell boundaries. Submersible instruments incorporate a thermistor thermometer in
addition to the conductivity cell for accurate salinity computation (Fig. 2.13).
Remote sensing of surface seawater salinity (SSS) from satellites was only
recently achieved through passive radiometric sensing of sea surface microwave
emissions in the radar L band at ca. 1.4 GHz. Thermal emission of seawater in this
band is modulated by salinity with a change of about 0.5 K per unit change in salinity at 293 °K (20 °C). Negligible absorption of radiation by atmospheric gases in
this band creates a satellite remote sensing window. Radiometers aboard the
European Space Agency Soil Moisture and Ocean Salinity (SMOS) satellite (2009–
2010) and the US Aquarius instrument aboard the Argentinian spacecraft SAC-D
(2011–2015) provided proof of concept for eventual operational sensing of the
global SSS field (Lagerloef 2012). Nevertheless, spatial resolution of these instruments as deployed in experimental missions is in the range of 100 km
2
largely
negating effective coastal ocean observing applications where resolutions of at least
1 km may be required.
2 Electronic Sensors and Instruments for Coastal Ocean Observing