Floating glass hydrometer for marine aquarium water specific gravity (salinity) measurement.

The scale in the narrow upper portion shows the specific gravity at the water surface level, while the temperature scale at the lower section shows the water temperature. Floating Glass hydrometers are calibrated with distilled water which equals 1.000 reading at the temperature of 60F (15,55°C). As water volume expands according to temperature, specific gravity value, which is the density of water, changes along. As the water temperature in tropical marine aquariums is significantly higher, a water sample should be let to cool down to 60F (15,55°C) in order to get exact specific gravity reading. Alternatively, actual Specific Gravity value can be seen from a conversion table according to the water temperature.

Measuring is preferably carried out by taking water to a tall drinking glass or similar container, as it is much easier to read the value when hydrometer is floating in stagnant water. Hydrometer is inexpensive and reliable as it will not loose its accuracy over time, but glass frame can break easily if dropped. Hydrometer is a bit slower to use that other types of specific gravity meters but it is an excellent backup meter and can be used to confirm the accuracy of other gravity and salinity meters.

The normal specific gravity of sea water is 1.023-1.025, but it varies somewhat depending on the location.

NOTE! Hydrometer measurement value must be adjusted according to the water temperature with the conversion table. It is very important to specify whether the discussed value is TEMPERATURE ADJUSTED VALUE or HYDROMETER READ VALUE at a specific temperature, as the values are confusingly close to each other.