PENSON & CO. Salinity Refractometer for Aquarium, Fish Tank Seawater ATC Hydrometer Dual Scale (1.0 to 1.070 S.G.) Measure salt water or salinity of water, ideal for aquariums and marine monitoring. Features automatic temperature compensation (ATC) ranging from 5 to 30 degree C. 20/20 calibration at room temperature for more consistency and accuracy in everyday use. Measures on 2 scales, Specific Gravity (D 20/20) and parts per thousand (). Comes with hard case, dropper, screwdriver and cleaning cloth.