Hand Held Optical Refractometer
This has got to be one of the neatest toys, err, piece of equipment I have. It provides a very accurate temperature compensated reading of both Salinity and Specific Gravity. Just place two or three drops of tank water on the prism (blue) and aim at a bright light source while looking in the eye piece. You will see a dual scale reading. Just look where the blue line touches the scale for your reading.
The unit is calibrated with deionized water.