How to Convert a Hydrometer to Salinity

While most reef aquarists choose to monitor their water's ion levels with a simple specific gravity measurement, advanced aquarists and hobbyists keeping difficult species may choose to monitor the actual salt level, or salinity. This calculation more precisely measures the ion levels and allows aquarists to maintain the extraordinarily stable water parameters necessary for difficult species. Specific gravity is simply a ratio of the density of any given liquid (seawater in this case) to the density of pure water. This ratio changes with water temperature fluctuations. As a result, water temperature must be considered when converting from specific gravity to salinity.

Things You'll Need

  • Thermometer Hydrometer Calculator
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Instructions

    • 1

      Measure the temperature of the aquarium water in Fahrenheit. If the thermometer only reads Celsius, convert to Fahrenheit by multiplying by 1.8, and then adding 32 to the new value.

    • 2

      Measure the specific gravity of the aquarium water using the hydrometer. Take several readings to ensure the hydrometer is working; salt buildup often skews the initial reading on older hydrometers, and several dips in the aquarium will loosen and dissolve this buildup.

    • 3

      Determine the salinity using a two-step process. First, calculate the value of "z" when z = 0.2x + 28, where "x" is the observed value of departure from 74 degrees. For example, if the aquarium is 79 degrees, "x" is 5 (79-74 =5).

    • 4

      Use the calculated "z" value in a second formula, n = 1300w + z. Here, "w" is the difference between observed specific gravity and 1.020. The result of the second formula is the salinity of the aquarium, measured in parts per trillion. A hypothetical tank with a temperature of 79 degrees and a specific gravity of 1.025 would have a salinity of 35.5.