One of the most important parameters when keeping a marine tank is the salinity ( salt concentrations) of your water. Natural Seawater has a salinity of about 35 ppt (Parts Per Thousand). This unit of measure (ppt) is the amount of salt (g) in 1 kg of water. Measuring Salt Concentrations in water can be measured a variety of way—Specific Gravity (use of Hydrometer), Refractive Index (use of Refractometer), and Conductivity (use of electronic meters). An important feature when choosing a method of monitoring depends on how accurately the instrument can be calibrated. The C-Scope Refractometer is designed for testing the concentrations of saltwater by providing a direct reading of the specific gravity and concentration (Parts Per Thousand) of salt in water. The C-Scope provides a reliable refractive index and salinity readings of total dissolved solids of aqueous solutions. The refractive index is the ratio of the speed of light in a vacuum to the speed of light in the medium (saltwater) being measured. Light is slowed by passing through saltwater, so the index of refraction is an indication of this speed change. Marine aquarists use this refractive index to determine how much salt is present in the aquarium water. Saltwater becomes less dense as the temperature rises. The C-Scope has the Automatic Temperature Compensation feature. This allows you to get accurate readings of your saltwater every time.