For the most precise measurement of salinity, the experts use a Refractometer. This high-tech instrument is used to measure salinity when perfect conditions are needed for breeding, or keeping very delicate corals. This easy-to-use and read device will allow you to keep track of your tank"s salinity fluctuations. This instrument also features ATC, automatic temperature compensation that will allow the Refractometer to gauge an accurate measurement in a range of temperatures. Simply look through the eyepiece to measure. Complete instructions included. Size: Unit is 8 in. long x 1 in. diameter.