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A hydrometer is a device used to measure the specific gravity (sg) of water.

It is commonly used to measure the amount of salt in a body of water when setting up brackish or saltwater tanks. You'll want a hydrometer with a specific gravity scale of at least 1.005 to 1.030, salinity of 5 to 40.

Cheaper passive hydrometers are designed to be used at their calibrated temperature. Usually 25°C (77°F) . If you measure at different temperatures you'll have minor errors.


Hydrometer Temperature correction
°F/°C Adjust
4.4°C (40°F) -0.002
10°C (50°F) -0.001
15.6°C (60°F) -0.000
21.1°C (70°F) +0.001
26.7°C (80°F) +0.002
32.2°C (90°F) +0.004
37.8°C (100°F) +0.005