conversion
Salinity and specific gravity converter
Convert between specific gravity, practical salinity and conductivity for brackish water — with both of the temperatures a specific gravity is a ratio between, because without them it is not a measurement.
A low brackish tank is usually 1.002 to 1.015.
Practical salinity 9.64 · specific gravity 1.0050 · 1004.011 kg/m³
Mesohaline. Middle estuary. The band most brackish aquarium species are actually kept in.
The same water reads 1.0065 at 20 °C and 1.0044 at 28 °C on the instrument you selected. That spread is the reason a specific gravity quoted without its temperature is not a measurement anybody else can reproduce.
A conductivity meter in this water at 26 °C should read about 16.787 mS/cm.
The commonest calibration on aquarium glass hydrometers, inherited from the brewing and petroleum trades. If the stem says 60 °F, this is the one.
Working — every step, so you can check it
- Sample temperature
- 26 °C
- Instrument reference temperature
- 15.56 °C
- Density implied by the reading
- 1.005 × 999.016 kg/m³ = 1004.011 kg/m³
- Practical salinity giving that density at the sample temperature
- 9.64
- Density at the sample temperature
- 1004.011 kg/m³
- Specific gravity
- 1004.011 ÷ 999.016 = 1.00500
- Conductivity that salinity implies at this temperature
- 16.787 mS/cm
Formula
- Practical salinity from conductivity uses PSS-78 (Lewis 1980, UNESCO Technical Papers in Marine Science 44): with R = C(S,t,0) / 42.914 mS/cm and rt(t) = 0.6766097 + 2.00564e-2·t + 1.104259e-4·t² − 6.9698e-7·t³ + 1.0031e-9·t⁴, RT = R / rt(t) and S = Σ aᵢ·RT^(i/2) + ((t−15)/(1+0.0162(t−15)))·Σ bᵢ·RT^(i/2).
- a = [0.0080, −0.1692, 25.3851, 14.0941, −7.0261, 2.7081], which sums to exactly 35. b = [0.0005, −0.0056, −0.0066, −0.0375, 0.0636, −0.0144], which sums to exactly 0. Those two identities are why a ratio of 1 at 15 °C is practical salinity 35.
- Density at one atmosphere uses EOS-80 (Millero & Poisson 1981): ρ(S,t) = ρw(t) + S·(0.824493 − 4.0899e-3·t + 7.6438e-5·t² − 8.2467e-7·t³ + 5.3875e-9·t⁴) + S^1.5·(−5.72466e-3 + 1.0227e-4·t − 1.6546e-6·t²) + 4.8314e-4·S², where ρw is the pure-water polynomial 999.842594 + 6.793952e-2·t − 9.09529e-3·t² + 1.001685e-4·t³ − 1.120083e-6·t⁴ + 6.536332e-9·t⁵.
- Specific gravity = ρ(S, sample temperature) ÷ ρw(instrument reference temperature). Both temperatures are inputs because specific gravity is the ratio between them and means nothing without both.
- The reverse directions — salinity from specific gravity, conductivity from salinity — are solved by bisection on the equations above rather than by a fitted inverse, so they cannot disagree with the forward calculation shown on the page.
Assumptions
An estimate whose assumptions are invisible is indistinguishable from a guess.
- One atmosphere. Pressure terms are omitted because an aquarium is at the surface, which is the condition both equations' one-atmosphere forms are stated for.
- The water is of standard seawater composition, diluted. Practical salinity is defined by conductivity against a potassium chloride standard, so water whose ion ratios differ from seawater — a brackish tank made up with a freshwater-hardness salt mix, for instance — will read differently from a genuine estuarine sample of the same total dissolved solids.
- Your instrument is reading correctly. A converter cannot detect a refractometer that has drifted out of calibration, and a swing-arm hydrometer is the least repeatable instrument in the hobby.
Limitations
Where this calculation stops being reliable. Read these before acting on a result.
- PSS-78 is defined between practical salinity 2 and 42. Below 2 this tool gives density and specific gravity — which come from an equation defined from 0 — and gives NO conductivity figure, because converting one there requires a different algorithm (Hill et al. 1986) and running PSS-78 outside its range would return a number indistinguishable from a valid one. That floor is lower than it sounds: practical salinity 2 is about 0.9993 specific gravity at 26 °C, below what any hobby hydrometer resolves, so it is the lightly salted tank measured on a conductivity meter that meets it rather than the brackish one.
- A specific gravity below 1.000 is an ordinary result rather than a fault. Warm water against a cold calibration temperature is less dense than the pure water the scale was referenced to, and the ratio goes under one without anything being wrong.
- Practical salinity is not parts per thousand. It is a conductivity ratio, it carries no unit, and although the two are numerically close in seawater they are different quantities and this tool does not print a ‰ figure.
- A specific gravity quoted without both temperatures is not convertible, and that is a fact about the figure rather than a limitation of this page. The same water reads about 0.0025 lower at 28 °C than at 15.5 °C on a 60/60 °F instrument.
- A refractometer sold for aquarium use generally has a scale computed for seawater at 20 °C and an automatic temperature compensation designed for that scale. Reading brackish water on it is outside what it was built for, and the discrepancy is largest at low salinity.
- This is arithmetic about water. It says nothing about which salinity any animal should be kept at, and this catalogue holds no marine species.
References
- /water/salinity
- /learn/kinds-of-water-fish-come-from