Salinity sensors

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See also: Instruments and sensors to measure environmental parameters

Salinity measurements and definitions throughout history

Since as far as Ancient Greece times, attempts have been made to address the "saltiness" of seawater. However, the low sensitivity of the analytic methods meant that measurements were not sufficiently accurate to be considered. During the Modern History 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 from around the world, 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. Now, with more accurate techniques to measure salinity, there was a need to have a common definition and methods across the countries. In 1889, Martin Knudsen was named by ICES to preside a commission to address the salinity issues. Knowing that the seawater composition was constant and chlorine could be easily measured by Knudsen's titration, and his colleagues proposed an equation to calculate salinity based on chlorine percentage, chlorinity.

Salinity based on chlorinity can be measured accurately by a simple chemical analysis, salinity was redefined using chlorinity:

     S  = 1.805 Cl  + 0.03                                                                               (3-2)

where chlorinity Cl is defined as “the mass of silver required to precipitate completely the halogens in 0.328 523 4kg of the sea-water sample.’

S = 0.03 + 1.805Cl

Definition of Salinity, ICES, 1902 "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. What is commonly referred to as a salinity sensor is in fact a conductivity sensor.

The Conductivity Sensor measures the ability of a solution to conduct an electric current between two electrodes. In solution, the current flows by ion transport; therefore, an increasing concentration of ions in the solution will result in higher conductivity values. Conductance is defined as the reciprocal of resistance. When resistance is measured in ohms, conductance is measured using the SI unit, siemens (formerly known as a mho). Since the siemens is a very large unit, aqueous samples are commonly measured in microsiemens, or µS. Conductivity, C, is found using the following formula:

C = G• kc

where G is the conductance, and kc is the cell constant. The cell constant is determined for a probe using the following formula: kc = d/A where d is the distance between the two electrodes, and A is the area of the electrode surface. For example, the Conductivity Sensor has a cell constant: kc = d /A = 1.0 cm/0.1 cm2 = 10 cm-1 The conductivity value is found by multiplying conductance and the cell constant. A potential difference is applied to the two probe electrodes in the Salinity Sensor. The resulting current is proportional to the conductivity of the solution. This current is converted into a voltage.

WHAT IS PSU? by Frank J. Millero in Oceanography Magazine, 1993

After receiving the latest issue of Oceanography, I was irritated by the Sea-Bird advertisement on the inside cover. It shows a TS diagram that is labeled with the term PSU. Although I have been unsuccessful in getting the company to discontinue the use of this term, I thought I should write this letter to express my concerns about its use my oceanographers in published articles. The term apparently is used to denote the use of the Practical Salinity Scale and is an abbreviation for Practical Salinity Unit. As a member of the Joint Panel on Oceanographic Tables and Standards that was instrumental in the development of the international equation of the state of seawater and the practical salinity scale, I am amazed that the practice that seems to have been adopted by oceanographers in using PSU. The practical salinity scale was defined as conductivity ratio with no units. A seawater sample with a conductivity ratio of 1.0 at 15ºC with a KCl solution containing a mass of 32.4356 g in a total mass of 1 kg of solution has a salinity of 35.000 (no units or ‰ are needed). The salinity and temperature dependence of this ratio for seawater weight evaporated or diluted with water led to the full definition of the practical salinity scale. This definition was adopted by all the National and International Oceanographic Organizations. It also was published in all the journal publishing oceanographic studies. Somewhere along the line oceanographers started to use the term PSU (practical salinity unit) to indicate that the practical salinity scale was used to determine conductivity salinity. This apparently resulted from the previous use ‰ to represent parts per thousand, which some oceanographers felt was a unit. The bottom line is that salinity has always been a ratio and does not have physical units. The use of the term PSU should not be permitted in the field and certainly not used in published papers. Whenever the practical salinity scale is used to determine salinity this should be stated somewhere in the paper. The use of the term PSS can be used to indicate that the Practical Salinity Scale is used. One certainly does not have to use to term PSU on all figures showing TS data. I should also point out that UNESCO (1985) has published a SUN report that carefully outlines the use of units in the field of oceanography. This report was also adopted by all the International Oceanographic Societies but is not generally used by the oceanographers and the journals publishing oceanographic data. If the field of oceanography is to become a recognized science, it must adopt the units that are basic to the fields of chemistry and physics. It also should not adopt new units for variables that are unitless.

UNESCO (1985) The international system of units (SI) in oceanography. UNESCO Technical Papers No.45, IAPSO Pub. Sci. No. 32, Paris, France.

Salinity Scales PSS

Conductivity meter

Calibration

Future standards

See also

Historical overview of salinity measurements and definitions