There are a few different ways to measure salinity in a reef tank, from hydrometers to refractometers. Most of us are familiar with the traditional refractometer that you hold up to your eye up and view the reading through a prism. However Milwaukee has a digital refractometer that is very reliable, consistent and easy to operate.
The Milwaukee Digital Refractometer required either distilled or RODI water for calibration and the refractometer is calibrated to zero. After calibration you can take a sample of your aquariums water and fill the sample bowl and then hit the large read button. You should see a light turn on below your water sample and then a reading will be displayed. You can choose from multiple conversions from PPT to S.G.
After you have your reading just discard the sample and wipe the bowl with a soft dry towel. We have found that calibrations are only required about once every six months or so.