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🔹 1. Units of Salinity in Seawater
Salinity refers to the concentration of dissolved salts (mainly sodium chloride) in water. It can be expressed in several units:
① PSU (Practical Salinity Unit)
- The most commonly used unit.
- Based on electrical conductivity measurements.
- It is dimensionless, but "PSU" is used conventionally.
- Example: 35 PSU means approximately 35 grams of salt per 1 kg of seawater.
② ‰ (Per mille)
- Expressed in parts per thousand.
- Example: 35‰ means 35 grams of salt in 1,000 grams of solution.
③ ppt (parts per thousand)
- Similar to ‰; indicates the ratio of salt in 1,000 parts of solution.
- Example: 35 ppt = 35 grams of salt per 1,000 grams of water.
🔸 In practice, PSU, ‰, and ppt are often used interchangeably. However:
- PSU is a dimensionless value based on conductivity.
- ‰ and ppt are mass-based measurements from experimental data.
🔹 2. Conversion Between Units
| Unit | Conversion Formula |
| PSU → ‰ | 1 PSU ≈ 1 ‰ |
| PSU → ppt | 1 PSU ≈ 1 ppt |
| ppt → PSU | 1 ppt ≈ 1 PSU |
| ‰ → ppt | 1 ‰ = 1 ppt |
※ These conversions are approximations. In most oceanographic practices, the units are treated as equivalent.
🔹 3. Classification Based on Salinity
| Type | Salinity (in PSU) | Description |
| Freshwater | < 0.5 PSU | Drinking water, rivers, lakes |
| Brackish Water | 0.5 – 30 PSU | Estuaries, where freshwater and seawater mix |
| Seawater | 30 – 38 PSU | Typical ocean water |
| Hypersaline Water | > 38 PSU | Salt lakes, evaporation-dominated environments (e.g., Dead Sea) |
🔹 References
- Millero, F. J. (2006). Chemical Oceanography. CRC Press.
- UNESCO (1981). Background papers and supporting data on the Practical Salinity Scale 1978. UNESCO Technical Papers in Marine Science.
- Talley, L. D., Pickard, G. L., Emery, W. J., & Swift, J. H. (2011). Descriptive Physical Oceanography: An Introduction. Academic Press.
- NOAA – National Oceanic and Atmospheric Administration. https://www.noaa.gov
- USGS – U.S. Geological Survey. https://www.usgs.gov
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