Brand new Salinity Refractometer that is used to measure the salinity levels of saltwater aquariums and the worlds oceans.Calibrate with either distilled water or a 35ppt Salinity standard.Wipe off the saltwater after each use.This unit comes with Automatic Temperature Compensation or "ATC". This feature allows you to get a reading outdoors when the temp is between 50"F-86" (10"C-30"C). If you test outside of temp range your readings will become gradually more and more inaccurate. For best results, test indoors.No batteries necessary as you only need strong light - daylight (sunny or cloudy) or a lamp/flashlight.