31
Intergovernmental Oceanographic Commission of UNESCO and the Scientific
Committee for Oceanic Research of the International Union for Science (Lewis
1980). Since electrical conductivity varies widely with temperature, practical salinity (S p ) is dependent on accurate measurements of both temperature and electrical
conductivity.
Rather than relying on an absolute standard, following the ICES example, the
Practical Salinity Scale is based on a conductivity ratio between that of the sample
and that of an empirical standard supplied commercially to users. This standard
known as Standard Seawater is prepared using seawater from the North Atlantic
Ocean adjusted to a practical salinity of 35 (close to the average salinity of the
world ocean) against an absolute KCl standard of known conductivity. This conductivity ratio, paired to a concurrent temperature reading of high accuracy, is used to
compute practical salinity making use of the empirical equations of the Practical
Salinity Scale 1978.
Through careful reevaluation of historical chemical measurement of the individual salts in solution, it is now apparent that the absolute salinity (S A ) of standard
seawater in mg.kg
−1
is about 1% higher than the unitless S P value (Millero 2013). Of
late, a new reference salinity (S R ) has been defined to bridge the gap in the
Thermodynamic Equation of Seawater (TEOS-10, 2010) between the unit-less S P
and recognized salt content:
S
S
R
P
g kg
= (
)
−
35 16504 35
1
.
/
.
(2.4)
Equation (2.4) can be applied directly to north Atlantic surface water to estimate S A
since this water, by definition, exhibits zero salinity anomaly (δS A ). Dissolved silicic
acid is in large part responsible for further discrepancy of global ocean salinity
derived from conductivity since, at the pH of seawater, it remains well protonated
(i.e., ionizes little) and is thus nonconductive. For the different ocean basins, algorithms based on silicic acid content and latitude have been developed for estimating
δS A such that
S
S
S
A
R
A
= −δ
(2.5)
Large values for δS A are mostly restricted to deep ocean basins where silicate content is highest. Absolute salinity provides the most accurate estimate of seawater
salinity to date (McDougall et al. 2012).
Practical Salinometry
Advances in the instrumental measurement of salinity were made possible by the
development of electronic circuitry and, at first at least, by the development of the
electrodeless induction conductivity sensor. In essence, this sensor is a voltage transformer consisting of two insulated copper windings and an external conductive core.
Alternating current (AC) supplied to the primary winding induces AC in the
2.3 Electrochemical Sensors for Coastal Ocean Observing
Intergovernmental Oceanographic Commission of UNESCO and the Scientific
Committee for Oceanic Research of the International Union for Science (Lewis
1980). Since electrical conductivity varies widely with temperature, practical salinity (S p ) is dependent on accurate measurements of both temperature and electrical
conductivity.
Rather than relying on an absolute standard, following the ICES example, the
Practical Salinity Scale is based on a conductivity ratio between that of the sample
and that of an empirical standard supplied commercially to users. This standard
known as Standard Seawater is prepared using seawater from the North Atlantic
Ocean adjusted to a practical salinity of 35 (close to the average salinity of the
world ocean) against an absolute KCl standard of known conductivity. This conductivity ratio, paired to a concurrent temperature reading of high accuracy, is used to
compute practical salinity making use of the empirical equations of the Practical
Salinity Scale 1978.
Through careful reevaluation of historical chemical measurement of the individual salts in solution, it is now apparent that the absolute salinity (S A ) of standard
seawater in mg.kg
−1
is about 1% higher than the unitless S P value (Millero 2013). Of
late, a new reference salinity (S R ) has been defined to bridge the gap in the
Thermodynamic Equation of Seawater (TEOS-10, 2010) between the unit-less S P
and recognized salt content:
S
S
R
P
g kg
= (
)
−
35 16504 35
1
.
/
.
(2.4)
Equation (2.4) can be applied directly to north Atlantic surface water to estimate S A
since this water, by definition, exhibits zero salinity anomaly (δS A ). Dissolved silicic
acid is in large part responsible for further discrepancy of global ocean salinity
derived from conductivity since, at the pH of seawater, it remains well protonated
(i.e., ionizes little) and is thus nonconductive. For the different ocean basins, algorithms based on silicic acid content and latitude have been developed for estimating
δS A such that
S
S
S
A
R
A
= −δ
(2.5)
Large values for δS A are mostly restricted to deep ocean basins where silicate content is highest. Absolute salinity provides the most accurate estimate of seawater
salinity to date (McDougall et al. 2012).
Practical Salinometry
Advances in the instrumental measurement of salinity were made possible by the
development of electronic circuitry and, at first at least, by the development of the
electrodeless induction conductivity sensor. In essence, this sensor is a voltage transformer consisting of two insulated copper windings and an external conductive core.
Alternating current (AC) supplied to the primary winding induces AC in the
2.3 Electrochemical Sensors for Coastal Ocean Observing