This model also has a SG (specific gravity) scale (1.000 to 1.030) and is ideal for measuring the salinity of marine aquariums. A refractometer provides a more acccurate measurement of salt content than a hydrometer. The direct reading scales in ppt (parts per thousand) of salt and SG make it easy to check that the salinity in your tank is correct for healthy fish. Scale divisions on the ppt scale are 1 ppt and on the SG scale 0.001. 

Designed for direct reading of salt density in sea water. 

Features

• Rugged, durable construction - made from corrosion resistant brass.
• High contrast demarcation line for easy, accurate readings.
• Easily adjusted zero line by means of readily accessible screw.