M. Le Menn 18/10/15Development of an optical salinity sensor

Salinity measurements: development of a new sensor

M. Le Menn

ARCTIC CIRCLE ASSEMBLY 16-18 october 2015Breakout session ‘Metrology for the Arctic’

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Introduction

Marc Le Menn: head of the metrology and oceanographic chemistry department of the SHOM

The SHOM is the hydrographic and oceanographic service of the french navy.Its missions are to acquire knowledge and to describe the ocean physical environment in relations with atmosphere, seabed, coastal areas, to foresee the evolutions and to ensure the diffusion of corresponding information's.It sustains the needs of the national defense and of the french public policies of the sea and coastal areas.The SHOM is the heir of the first official hydrographic service in the world (1720).

The SHOM, by its metrology department, is a non-funded partner of Meteomet 2 – WP2.We collaborate to the development of a facility to study the pressure dependence of deep-sea thermometers and of a new generation of absolute salinometers.

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Reminders about the salinity

In polar regions, changes in salinity affect ocean density more than changes in temperature : it has been proved that exchange of salt between sea ice and the ocean, influences ocean circulation across hundreds of kilometers.

Artic is a place of salt exchanges when sea ice is forming and when it is melting, so high concentrations and very low concentrations of salts can be met in Arctic.

But, what is salinity exactly and how can it be measured ?Salinity is defined as the total amount of dissolved matter per kilogram of water.

It is the definition of the so-called absolute salinity Sa expressed in g/kg.

Problem: for ever, the value of this mass is difficult to determine in practice.

Since the 70’th salinity is assessed by a practical salinity Sp, based on conductivity – temperature – depth (CTD) measurements. Sp has no unit. Salinity is used to calculate the physical properties of seawater.

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Evolutions in the salinity definitions and measurements

Until 2009, these properties were calculated with the Equations Of State of Seawater of 1980 or EOS-80, based on Sp measurements.

Since the creation of the TEOS-10 or International Thermodynamic Equation Of Seawater, the absolute salinity SA must be used. It is defined by the relation:

SA = SR + δSA(p, λ, ) where

By the Lorentz-Lorenz relation, the refractive index n is in direct relation with the density , and for seawater, with its absolute salinity SA

dens or ‘salinity-density’ defined by the TEOS-10.

PR SS

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Hence the interest to develop arefractometer usable in situ, and the origin of the NOSS (NKE Optical Salinity Sensor) project.

Three different prototypes of NOSS have been calibrated in the SHOM laboratory and tested at sea during oceanographic campaigns.

Source: NKE InstrumentationPhoto © SHOM

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New needs in salinity measurementsThe third prototype was tested at sea in last April on two profiling floats.The following graph shows a typical result among all the profiles acquired.

At low pressure, the salinity (in red) calculated from NOSS refractive index measurements agree with the reference salinity calculated from CTD measurements and Millard - Seaver relations.

Discrepancies appear at high pressure. They can be explained by the inaccuracy of Millard – Seaver relations as the pressure increases, and perhaps by salinity anomalies.

The problem to solve before using routinely this kind of sensor, is to rebuilt and improve Millard and Seaver relations to obtain more accurate results when refractive index measurements are made under pressure.

For that, we need funds to set up an experiment with the help of a metrology laboratory, to equate, with a low uncertainty, refractive index measurements with temperature, salinity and pressure variations.

Source: NKE Instrumentation