The Milwaukee MA887 digital refractometer is a precise optical instrument designed to measure salinity in saltwater aquariums. It operates on the principle of light refraction through liquid, providing highly accurate readings for PSU (Practical Salinity Units), PPT (Parts Per Thousand), and Specific Gravity (S.G.). The device features an automatic calibration system and offers a temperature compensation range from 0 to 80 degrees Celsius or 32 to 176 degrees Fahrenheit. It includes specifications such as a resolution of 0.001 for specific gravity, accuracy within ±0.002, and can be used across various salinity levels essential for maintaining optimal conditions in marine aquariums. This tool is best suited for experienced reef aquarists who require precise measurements to ensure the health and stability of their saltwater environments.