M. Le Menn 18/10/15 Development of an optical salinity sensor Salinity measurements: development of a new sensor M. Le Menn ARCTIC CIRCLE ASSEMBLY october 2015 Breakout session ‘Metrology for the Arctic’

M. Le Menn 18/10/15 Development of an optical salinity sensor 2 / 5 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.

M. Le Menn 18/10/15 Development of an optical salinity sensor 3 / 5 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 S a 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 S p, based on conductivity – temperature – depth (CTD) measurements. S p has no unit. Salinity is used to calculate the physical properties of seawater.

M. Le Menn 18/10/15 Development of an optical salinity sensor 4 / 5 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 S p measurements. Since the creation of the TEOS-10 or International Thermodynamic Equation Of Seawater, the absolute salinity S A must be used. It is defined by the relation: S A = S R + δS A (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 S A dens or ‘salinity-density’ defined by the TEOS-10. Hence the interest to develop a refractometer 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

M. Le Menn 18/10/15 Development of an optical salinity sensor 5 / 5 New needs in salinity measurements The 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