Salinity sensors

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See also: Instruments and sensors to measure environmental parameters

Salinity Definition

This definition is impractical and the procedure difficult to carry out with precision. Alternatively, other parameters have been used in recent history as a proxy to calculate salinity.

Salinity measurements and definitions throughout history

Although attempts have been made throughout history (as far as Ancient Greece times) to address the "saltiness" of seawater, the low sensitivity of the analytic methods meant that measurements were not sufficiently accurate to be considered. During the Modern History new and more precise methodologies were developed: weighing after evaporation (Boyle,1693; see Birch, 1965), solvent extraction (Lavoisier, 1772) and precipitation (Bergman, 1784). In 1865, Forchhammer introduced the term salinity and dedicated himself to measure individual components of seasalt rather than the total salinity. He found that the ratio of major salts in samples of seawater from various locations was constant. This constant ratio is known as Forchhammer's Principle, or the Principle of Constant Proportions, from which the salinity could be calculated from the chlorine content. Towards the end of the nineteenth century, William Dittmar, following the work of Forchhammer, tested several methods to analyse the salinity and chemical composition of seawater. The Dittmar methods for chemical analysis of the seawater were extremely precise. Dittmar analysed the chlorine content in seawater using silver nitrate precipitation of the chloride, and compared it with synthetically prepared seawater samples to vouch for the method's accuracy. Dittmar later analysed 77 samples taken during the Challenger expedition and noticed the same constancy of composition observed by Forchhammer: "although the concentration of the waters is very different, the percentage composition of the dissolved material is almost the same in all cases".

Constant composition of seawater (Dittmar, 1884)

Still, a clear definition of salinity and international standard were yet to be developed.

In 1889, Martin Knudsen was named by ICES to preside a commission to address the salinity issues, chlorine was established as a proxy to calculate salinity. Knudsen and his colleagues proposed the following equation to calculate salinity based on chlorine percentage, chlorinity. S = 0.03 + 1.805Cl

and in 1902 the following definition was published:

"Salinity is the total amount of solid materials, in grams, dissolved in one kilogram of sea water when all the carbonate has been converted to oxide, the bromine and iodine replaced by chlorine and all organic matter completely oxidized .Although this is a correct definition, the methodology is impractical and difficult to carry out with precision.

Sensors

Salinity is a ratio and not a physical parameter that can be measured. Thus, “Salinity sensors” do not exist. There are several different ways to calculate this ratio with pros and cons for each method.

Salinity Scales

Conductivity meter

Calibration

See also

Historical overview of salinity measurements and definitions