Practical Salinity Unit (PSU): Ocean Conductivity Scale
The Practical Salinity Unit (PSU) expresses seawater salinity based on conductivity measurements. Unlike mass-based units, PSU derives from the ratio of a seawater sample’s conductivity to that of a standard KCl solution at defined temperature and pressure. Introduced with the Practical Salinity Scale of 1978, PSU enables consistent global monitoring across shipboard, moored, and satellite instruments. Typical open-ocean salinity hovers around 35 PSU, while estuaries, polar regions, and hypersaline basins depart from this benchmark. Although “PSU” remains entrenched in operational datasets, the scale is formally dimensionless; reporting values as practical salinity SP alongside any PSU shorthand preserves SI coherence.
This article details the PSU definition, traces its evolution from titration-based measurements, and highlights best practices for calibration, reporting, and application in oceanography and engineering. Cross-references to the decisiemens explainer and thermodynamic quantities such as molality show how PSU harmonises with SI-based concentration systems.
Definition and Core Relationships
Conductivity ratio to PSU conversion
PSU is computed from the conductivity ratio R = C(S, T, p) / C(KCl, Tref, pref), where C denotes electrical conductivity. The Practical Salinity Scale defines salinity SP such that SP = 35 × g(R, T), where g is a polynomial calibrated to replicate chlorinity-based salinity at 35 PSU, 15 °C, and standard atmospheric pressure. Conductivity is measured in siemens per metre, tying PSU to the SI via standards maintained by national metrology institutes. Because SP is dimensionless, document whether reported numbers are labelled with “PSU” or presented explicitly as practical salinity to avoid ambiguity. Always report measurement temperature and pressure because conductivity varies strongly with both.
Relationship to absolute salinity
PSU approximates mass fraction salinity but does not directly express grams of dissolved salts per kilogram. The Thermodynamic Equation of Seawater 2010 introduced Absolute Salinity SA, measured in g·kg⁻¹, accounting for regional variations in seawater composition. Conversion between PSU and SA uses geographic correction terms derived from oceanographic datasets. When modelling seawater density or sound speed, specify whether PSU or SA was used, referencing the density article for consistency.
Historical Development of Salinity Scales
From chlorinity titration to conductivity
Early salinity estimates relied on titrating seawater with silver nitrate to measure chlorinity, a laborious process sensitive to sample handling. As electrical conductivity instrumentation matured in the mid-twentieth century, oceanographers adopted conductivity-based indices for faster, automated measurements. The Practical Salinity Scale standardised this approach in 1978, harmonising data from CTD (conductivity-temperature-depth) profilers and reducing inter-laboratory discrepancies.
Modern refinements and international adoption
Organisations such as UNESCO, the International Association for the Physical Sciences of the Oceans (IAPSO), and the Intergovernmental Oceanographic Commission supported global adoption of PSU. Contemporary efforts focus on refining conductivity sensor calibration, characterising regional deviations in seawater composition, and integrating satellite-derived salinity from missions like SMOS and SMAP. Data centres provide conversion tools between PSU, practical density anomaly, and thermodynamic properties for assimilation into climate models.
Measurement Techniques and Calibration
Instrument considerations
CTDs employ inductive or electrode-based conductivity cells. Regular calibration with IAPSO Standard Seawater, maintained at 15 °C, preserves traceability. Field teams rinse sensors with deionised water, avoid air bubbles, and correct for thermal lag between conductivity and temperature sensors. Autonomous platforms such as Argo floats and gliders require drift monitoring; laboratory comparisons before and after deployment quantify sensor stability and inform quality-control flags.
Data processing and quality control
Salinity processing involves applying cell-specific calibration coefficients, aligning conductivity and temperature time series, and correcting for pressure effects. Quality-control procedures flag spikes, density inversions, and unrealistic gradients. Documentation should include calibration batch numbers, processing software versions, and comparisons to bottle samples analysed in the laboratory. Cross-reference PSU data with the decisiemens article when translating values for agronomic or industrial water use.
Applications and Impacts
Ocean circulation and climate
Salinity gradients drive thermohaline circulation by altering seawater density. PSU data feed global ocean models, informing predictions of meridional overturning strength, stratification, and mixed-layer depth. Combining PSU observations with the Sverdrup article contextualises how transport anomalies redistribute heat and freshwater, affecting climate variability.
Water resource management and engineering
Desalination plants monitor feedwater PSU to optimise energy use and scaling control. Aquaculture operators adjust PSU to match species tolerances, protecting growth and health. Coastal infrastructure designers assess corrosion risk and material selection using PSU-driven chloride load estimates. Tools such as the water hardness converter and the salt dosage calculator help translate PSU into operational setpoints.
Importance of Transparent Reporting
Metadata standards
Provide comprehensive metadata with PSU datasets: instrument type, calibration reference, sampling depth, processing version, and applied corrections. Include uncertainty estimates derived from calibration drift, measurement noise, and algorithmic approximations. Adhering to formats such as Climate and Forecast (CF) conventions facilitates interoperability with climate data portals.
Communication with stakeholders
Translating PSU into practical guidance supports policymakers and industry partners. Compare PSU trends with freshwater inflow, precipitation anomalies, or desalination output. Integrate salinity insights into dashboards alongside temperature and nutrient data, and use decision-support tools—such as the ventilator sizing calculator —to highlight cross-sector implications of marine air and water chemistry.