Ocean salinity

Salinity is a derived variable, meaning that it is not directly measured. In modern oceanography, salinity is determined in part from conductivity measurements done on water samples using laboratory or on-board salinometers, or in situ using an oceanographic instrument called a CTD. The CTD is named for the three variables it measures: conductivity, temperature, and depth or pressure. Learn more about RBR CTDs.

Conductivity sensors all rely on the same general principle: an electrical current is induced in a small volume of seawater to measure its electrical resistivity, i.e. its ability to oppose an electric current. The resistivity of the seawater, expressed in ohms (Ω), is the reciprocal quantity to its electrical conductance, in Siemens (S), and is a measure of the seawater's capacity to pass the electric current. Conductance can then be converted to seawater conductivity, in S/m, by scaling the measured conductance by the conductivity cell constant. Seawater conductivity is also commonly expressed in mS/cm.

Theoretically, the cell constant is defined as the ratio of the conductivity cell's length to its cross-sectional area. However, this idealized definition relies on assumptions that break down when considering the practicalities of realizable conductivity cells. In practice, the cell constant of a conductivity cell is determined empirically in the laboratory, by calculating the ratio of the measured conductance to a known reference conductivity. The cell constant of the conductivity cell must be scaled carefully depending on the objective, as it directly impacts both the range and the resolution of the final conductivity measurements. In the case of seawater salinity, the conductivity cell is typically designed to have a cell constant that allows the sensors to capture a conductivity range of 0mS/cm and 85mS/cm, at a resolution of approximatively 0.001mS/cm.

Where conductivity cell technology differs across CTD manufacturers is in the technical approach taken to generate the electrical current in the sampled volume of seawater. The two most common approaches are inductive conductivity cell sensors and electrode-based sensors. 

The core components of the RBR inductive conductivity cell.

Inductive conductivity sensors, like the RBR conductivity cell, rely on Faraday’s law of induction. The conductivity cell is composed of a wire coiled around an annulus made of ferrite. A current is run through the wire, generating a magnetic field within the ferrite. The circular magnetic field in turn generates an electric field through the centre of the annulus, directly into the seawater. The conductivity sensor also includes a second “receiving” coil that reverses the process: the electric current that has travelled through the seawater is picked up by the receiving coil. The difference in the emitted and the received electrical fields gives a direct measurement of the seawater conductivity the electric field has travelled through. Laboratory calibration of the sensor provides information on the relationship between the measured current and the seawater conductivity, based on the cell's constant, therefore yielding a precise measurement of seawater conductivity. 

Advantages:

• Rugged sensor construction withstands rough handling and can be used in freezing conditions
• Naturally flushed, pump-free design uses relatively little power offering longer autonomous deployments on a variety of platforms
• Noise-free design will not affect passive acoustic monitoring
• Sensors are unaffected by trace amount of contaminants and able to measure within 20cm of the air-sea interface

Drawbacks:

• Sensors need to be calibrated in fully-integrated state in order to avoid proximity effects and obtain stated accuracy

The core components of an electrode conductivity cell.

Electrode-based conductivity sensors rely on measuring seawater resistivity in a small volume of water contained in an insulating material, often borosilicate glass, in which three electrodes are located. The seawater’s conductivity is deduced from the measured resistivity between the electrodes.

Advantages:

• Fully-enclosed electromagnetic field is unaffected by nearby objects (such as antennas, sensor guards, or other instruments)
• Temperature and conductivity are measured on the same small water parcel

Drawbacks:

• Complex assembly is difficult to clean in the field and is not suitable for use in freezing conditions
• Pump requires power to operate which impacts the endurance of the instruments and/or the size of the required power supply
• Pump causes vibrations and noise which may affect sensitive acoustic or microstructure measurements
• Electrode surfaces can be easily fouled by surface contaminants, so pump must be stopped metres before reaching surface

Once conductivity is measured, it can be linked to the seawater salinity. The challenge is that conductivity does not only depend on seawater salinity, it is also a function of temperature and pressure. One must first remove the impact of temperature and pressure on the measured conductivity. This is achieved using a non-linear equation that is considered the international standard for computing salinity from conductivity. This is referred to as the Practical Salinity Scale 1978, or more commonly as PSS-78. 

The PSS-78 scale defines a reference seawater composition with a conductivity of 42.9mS/cm at a temperature of 15°C and a pressure of 1atm. The salinity of a seawater sample is then calculated relative to this reference composition using a set of empirical relationships and correction factors to account for the impact of temperature and pressure. The resulting value is expressed in practical salinity units, which are defined as the ratio of the measured conductivity of the sample to the conductivity of the reference seawater composition, and is therefore dimensionless. 

For reference, a change in conductivity of 1uS/cm corresponds to a change in salinity of approximately 1mPSU. Similarly, a change in temperature of 1m°C corresponds to a change in salinity of approximately 1mPSU.

The PSS-78 scale is widely used in oceanography and other fields as a standard way of expressing the salinity of seawater. While it is considered to be accurate and reliable, it is based on empirical relationships and may not be applicable to seawater compositions that differ significantly from the reference seawater composition.

In 2010, the Intergovernmental Oceanographic Commission (IOC), International Association for the Physical Sciences of the Oceans (IAPSO), and the Scientific Committee on Oceanic Research (SCOR) adopted a new set of standard equations to compute thermodynamic properties of seawater (TEOS-10). In the process, the concept of “Absolute Salinity” was introduced: a true mass fraction, expressed in g/kg, that captures the different chemical composition of real seawater throughout the ocean. This concept is now widely used as it more accurately reflects spatial variations.