Salinity, Specific Gravity and Conductivity: Units, Temperature and Conversions

How ppt, practical salinity, specific gravity and conductivity relate, why every SG needs reference temperatures, how conductivity is compensated to 25 °C, and the brackish ranges.

What salinity measures

Salinity is the amount of dissolved salt in water. It is usually given in grams per litre (g/L) or grams per kilogram (g/kg). Grams per kilogram is a dimensionless mass fraction and equals parts per thousand (ppt, ‰). By convention, "dissolved" means material that passes a very fine filter, historically 0.45 µm and now usually 0.2 µm. Seawater typically has a salinity of about 35 g/kg. Rivers and lakes range from under 0.01 g/kg to a few g/kg, and the Dead Sea exceeds 200 g/kg. The USGS expresses the same quantity in parts per million (ppm). Ocean water contains about 35,000 ppm of salt, which equals 35 ppt because 1 ppt is 1,000 ppm.

ppt, practical salinity and absolute salinity

Oceanographers have used three related scales. Before the 1980s, salinity was found by titrating halides with silver nitrate to get the chlorinity, which was then multiplied by a factor. The resulting Knudsen salinity is expressed in ppt. The Practical Salinity Scale 1978 (PSS-78) instead derives salinity from electrical conductivity and has no unit. Writing "PSU" (practical salinity unit) after the number is common but is described as formally incorrect and strongly discouraged. The TEOS-10 standard of 2010 introduced absolute salinity, a mass fraction in g/kg.

For typical seawater with a chlorinity of 19.37 ppt, the Knudsen salinity is 35.00 ppt, the practical salinity is about 35.0 and the absolute salinity is about 35.2 g/kg. Its electrical conductivity at 15 °C is 42.9 mS/cm. At aquarium precision the three scales therefore differ only by a few tenths. Limnologists usually report salinity of fresh waters as mass per volume (mg/L or g/L). This implies a reference temperature, because the volume of a solution changes with temperature.

Specific gravity and its reference temperatures

Specific gravity (SG), now preferably called relative density, is the density of a sample divided by the density of a reference, usually pure water. It has no unit, but it is meaningful only when both temperatures are stated, written as SG (Ts/Tr) for the sample temperature and the reference temperature. Water at 4 °C, its densest point, is the default reference, but 15 °C, 20 °C and 60 °F (15.56 °C) are also used. Glass hydrometers for salinity are mostly calibrated on a 60 °F/60 °F basis, and the basis is printed on the scale inside the instrument. A California State Water Resources Control Board procedure warns that reading a 60 °F/60 °F hydrometer with tables made for 15 °C/4 °C instruments gives a value 0.001 too high. That error makes the calculated salinity about 1.3 ppt too high on average.

The reference temperature changes the number noticeably. Seawater with a salinity of 35 g/kg has a density of 1023.6 kg/m3 at 25 °C. Pure water is 0.9970479 g/cm3 at 25 °C and 0.9999720 g/cm3 at 4 °C. Dividing these values (a calculation made for this article, using densities from Wikipedia's seawater and water data pages) gives SG (25 °C/25 °C) of about 1.0266 but SG (25 °C/4 °C) of about 1.0236 for the same water. A difference of 0.003 is three times the table-mismatch error above, so an SG target is only useful together with its reference temperatures. Surface seawater density ranges from about 1020 to 1029 kg/m3, depending on temperature and salinity.

A hydrometer works by buoyancy, sinking deeper in less dense liquid. Because warm water is less dense, the reading must be corrected with the table that matches the instrument's calibration basis. A refractometer measures refractive index instead, and in marine aquarium keeping it is used to read salinity and SG. Refractive index also depends on temperature and on the wavelength of light, which is why refractometers use automatic temperature compensation or temperature control.

Conductivity as a salinity proxy

Electrical conductivity measures how well a solution carries current through its dissolved ions. The SI unit is siemens per metre, but water quality work normally uses microsiemens per centimetre (µS/cm). Conductivity rises by 0.5 to 3% per °C, depending on which ions are present. For that reason, USGS reports specific conductance, which is conductivity corrected to 25 °C, using κ25 = κ / (1 + α(t − 25)). Here t is the water temperature in °C and α is a compensation factor, typically 0.019 to 0.020 per °C in linear meters. This compensation is accurate to about ±5% between 5 and 35 °C at pH 4 to 11, but below pH 4 the errors can reach 50%. As an example, lake water with about 70 mg/L of salts typically reads 80 to 130 µS/cm at 25 °C but only about 50 to 80 µS/cm at 5 °C.

Converting conductivity to dissolved solids requires an assumed ratio. USGS expects total dissolved solids in mg/L divided by specific conductance in µS/cm to lie between 0.5 and 0.9 in natural waters. Wikipedia gives 0.54 to 0.96, or about 0.64 mg of salt per kg of water for each µS/cm if the salt is assumed to be sodium chloride. For reference, laboratory-purified water reads about 0.05 to 1 µS/cm, typical drinking water 200 to 800 µS/cm and seawater about 50 mS/cm (50,000 µS/cm). Meters are calibrated with potassium chloride standards. For example, 0.01 mol/kg KCl reads 1,409.3 µS/cm at 25 °C, close to the 1,413 µS/cm standard in common use. The conductivity and TDS probe guide covers the hardware.

Brackish ranges

Brackish water is defined as water containing 0.5 to 30 g of salt per litre (0.5 to 30 ‰). Wikipedia gives this as SG 1.0004 to 1.0226 but does not state the reference temperatures. Water above 30 ‰ counts as saline. The Venice System (1959) divides the brackish range into three zones:

The USGS uses a groundwater-oriented scheme in ppm. Fresh water is below 1,000 ppm. Slightly saline water is 1,000 to 3,000 ppm, moderately saline water 3,000 to 10,000 ppm, and highly saline water 10,000 to 35,000 ppm. Natural brackish waters such as estuaries, the largest brackish habitats, often vary considerably in salinity over space and time, so a brackish condition is a band rather than a single value. Some invertebrates kept in freshwater aquaria, mainly certain snails and shrimp, need brackish water to breed even though the adults live in fresh water.