?? ATC ?? Automatic Temperature Compensation, 0-10°C; ? It is necessary to keep the surrounding environment with sufficient brightness and a suitable temperature during use. There is no error when the temperature is 10-30?. If it deviates from this temperature, there will be a certain error. ? Because people’s focal lengths are different, reverse rotation may be necessary when focusing. ?? Concentration Measurement Range ?? 0-100‰, Minimum Scale is 1‰. ?? Density Measurement Range ?? 1.0-1.070, Minimum Scale is 0.001. ?? Measures on 2 Scales ?? Specific Gravity (D 20/20) and parts per thousand and replaces your poor quality and difficult to read hydrometer. ?? Salinity Refractometer ?? Measure Salt Water or Salinity of Water,Monitor and Control the Concentration of Salt Water and Brine,Ideal for Aquariums and Marine Applications.