The Captive Purity Refractometer is an incredibly accurate and easy to use device that eliminates the confusion associated with inexpensive hydrometers. Just place a couple of drops of water on the prism and hold to the light. Reads in percent and specific gravity. An absolute must for the serious marine hobbyist. The Captive Purity Refractometer is an optical device (no batteries) dual scaled 0-100% specific gravity: 1. 0 to 1. 070. Calibrate with distilled water. Adjustable eyepiece, carrying case, pipette and calibration screw driver included. This model is ATC (automatic temperature compensating).