30
Transmissometers are among the earliest optical instruments available. Equipped
with a monochromatic light source that beams to a photoresistor across an enclosed
pipe with black walls, the instruments allow single band estimates of k d . Photodiodeequipped instruments are available today.
Turbidity meters provide semi-quantitative characterization of light scattering.
These instruments irradiate a small volume of water with IR and record the incident
light resulting from scattering with an orthogonal or adjacent IR-sensitive photodiode. Proxy calibration is achieved with aqueous suspensions of formazin, an organic
polymer, and readings are reported as nephelometric units (NTU). Both instruments
are useful for estimating visibility and light penetration.
2.3 Electrochemical Sensors for Coastal Ocean Observing
2.3.1 Seawater Salinity
History of Practical Salinometry
Simply stated, the concept of salinity refers to the content of dissolved matter in
seawater. Nevertheless, a robust operational definition has been difficult to achieve.
Early experiments of boiling the water to dryness were unsatisfactory, attributed
subsequently to the chemical complexity of the seawater matrix. It was found for
example that carbonate and bicarbonate ions could boil off as carbon dioxide, that
organic matter in solution could undergo oxidation as well releasing carbon dioxide,
and that absolute dryness was difficult to achieve and maintain due to the hygroscopic nature of many of the resulting salt crystals. For example, after sodium chloride (halite), the most abundant salt resulting from evaporation, is in fact magnesium
sulfate, the well-known Epson salt, which crystalizes out at hepta-hydration resulting in the chemical formula MgSO 4 .7H 2 O with significant water retention.
In 1901, recognition of the preponderance of the chloride ion in the composition
of seawater (55% of total ions in solution by mass), and of the largely conservative
proportionality of elements dissolved in seawater (a property known as Marcet’s
principle), brought about a solution through wet chemical titration of the chloride
ion precipitating out as the insoluble silver salt resulting in a derived property
termed chlorinity. This solution was achieved by means of an international effort
advanced by the International Council for the Exploration of the Sea (ICES).
The Practical Salinity Scale and the International Equation of State
of Seawater
The Chlorinity approach was robust but laborious and was abandoned upon the
advent of reliable means of electronic measurement of electrical conductivity resulting then in the Practical Salinity Scale 1978 under the auspices of the
2 Electronic Sensors and Instruments for Coastal Ocean Observing
Transmissometers are among the earliest optical instruments available. Equipped
with a monochromatic light source that beams to a photoresistor across an enclosed
pipe with black walls, the instruments allow single band estimates of k d . Photodiodeequipped instruments are available today.
Turbidity meters provide semi-quantitative characterization of light scattering.
These instruments irradiate a small volume of water with IR and record the incident
light resulting from scattering with an orthogonal or adjacent IR-sensitive photodiode. Proxy calibration is achieved with aqueous suspensions of formazin, an organic
polymer, and readings are reported as nephelometric units (NTU). Both instruments
are useful for estimating visibility and light penetration.
2.3 Electrochemical Sensors for Coastal Ocean Observing
2.3.1 Seawater Salinity
History of Practical Salinometry
Simply stated, the concept of salinity refers to the content of dissolved matter in
seawater. Nevertheless, a robust operational definition has been difficult to achieve.
Early experiments of boiling the water to dryness were unsatisfactory, attributed
subsequently to the chemical complexity of the seawater matrix. It was found for
example that carbonate and bicarbonate ions could boil off as carbon dioxide, that
organic matter in solution could undergo oxidation as well releasing carbon dioxide,
and that absolute dryness was difficult to achieve and maintain due to the hygroscopic nature of many of the resulting salt crystals. For example, after sodium chloride (halite), the most abundant salt resulting from evaporation, is in fact magnesium
sulfate, the well-known Epson salt, which crystalizes out at hepta-hydration resulting in the chemical formula MgSO 4 .7H 2 O with significant water retention.
In 1901, recognition of the preponderance of the chloride ion in the composition
of seawater (55% of total ions in solution by mass), and of the largely conservative
proportionality of elements dissolved in seawater (a property known as Marcet’s
principle), brought about a solution through wet chemical titration of the chloride
ion precipitating out as the insoluble silver salt resulting in a derived property
termed chlorinity. This solution was achieved by means of an international effort
advanced by the International Council for the Exploration of the Sea (ICES).
The Practical Salinity Scale and the International Equation of State
of Seawater
The Chlorinity approach was robust but laborious and was abandoned upon the
advent of reliable means of electronic measurement of electrical conductivity resulting then in the Practical Salinity Scale 1978 under the auspices of the
2 Electronic Sensors and Instruments for Coastal Ocean Observing