The Milwaukee MA887-BOX is a digital refractometer (optical instrument) that employs the measurement of the refractive index to determine the salinity of natural and artificial seawater, ocean water or brackish intermediates, and includes a protective hardshell case. The digital refractometer eliminates the uncertainty associated with mechanical refractometers and is easily portable for ship, shore or home use. The MA887 refractometer is an optical device that is simple and quick to use. Samples are measured after a simple user calibration with distilled or deionized water. Within seconds, the refractive index and temperature are measured and converted into one of three popular measurement units: Practical Salinity Units (PSU), salinity in parts per thousand (ppt), or Specific Gravity (S.G. (20/20)). All conversion algorithms are based upon respected scientific publications using the physical properties of seawater (not sodium chloride). The temperature (in °C or °F) is also displayed on the large dual level display along with helpful message codes. Key features include: Waterproof models offers IP65 waterproof protection Automatic Temperature Compensation (ATC) battery operation with low power indicator automatically turns off after 3 minutes of non-use.