How to use a refractometer
A seawater refractometer uses the refraction of light in liquid to measure specific gravity. Place a few drops of water under the refractometer lid, close the lid, and point it at a light source. Look through the eyepiece for a change in color created by refracted light. The line will run through a chart inside the eyepiece that indicates the specific gravity.
A digital refractometer is even more precise than a mechanical refractometer.
Another option is a digital water tester, a handheld meter that combines a thermometer and salinity tester to make water changes a breeze.
Refractometers
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Hanna Instruments
Hanna Instruments Salinity Tester (HI98319)$69.00
Marine Depot
Marine Depot Aquarium Refractometer$39.99
Milwaukee Instruments Inc.
Milwaukee Seawater Digital Refractometer$124.99
$57.99
Milwaukee Instruments Inc.
Milwaukee Instruments Protective Hardshell Case for Refractometers & Photometers$23.99
Red Sea
Red Sea Refractometer$64.99
Milwaukee Instruments Inc.
Milwaukee Seawater Digital Refractometer with Protective Hardshell Case$144.49
Hanna Instruments
Hanna Instruments Digital Seawater Refractometer$180.00