Safe Material:Made of plastic,aquarium clear Automatic Accurate Readable SaltWater Hydrometer. Durable Clear Acrylic:Accurate, durable, and easy to read your marine aquarium's salt level and specific gravity,Small and waterproof.if the specific gravity between 1.000 - 1.019,show that the specific gravity is small;if the specific gravity between 1.020 - 1.023,show that the specific gravity is nomal;if the specific gravity between 1.024 - 1.030,show that the specific gravity is big; Clearly Safe Zone Indicated:Includes solinity and specific grovity scales.The outside rang of scale indicates salinity(PPT).The inside range of scale indicates gravity.And with safe zone clearly indicated.Range of use: 0-40 degrees (1.000-1.030).If the salinity ppt between 32 - 40，show that the salinity hydrometer is high;If the salinity ppt between 27 - 31，show that the salinity hydrometer is normal; If the salinity ppt between 0 - 26，show that the salinity hydrometer is light.