Salinity Refractometer, Aquarium & Seawater – Dual Scale (1.0 to 1.070 S.G.) by Agriculture SolutionsThis is one amazing instrument and the only way to inexpensively test for Salinity, worked great and is a way better made then I expected, easy to calibrate just use distilled water ( if you don’t have distilled water just freeze a water bottle, microwave a cup of water to almost a boil, hold frozen water bottle over hot cup of water, when moister from steam forms on the outside of water bottle it will start to drip use this to calibrate as you only need a drop or two) put two drops on the blue glass put clear glass cover over blue glass check you can’t have any bubbles and glass needs to be totally covered with distilled water. Look through and focus as needed. Use the screwdriver provided and adjust to 1.00 Wipe off the distilled water from the glass (with the cleaning cloth provided) now put two drops of your aquarium water on the glass and test it the same way you tested the distilled water (don’t forget to dry before you put it away). Then take a sample to your fish shop to test if your refract meter is accurate. Instructions are hidden inside the case under the molded plastic. You will need to screw in the eye piece every time to get it back into the case. Very easy to read. Don’t have a saltwater aquarium without this.A NoteIf you put your hand a few inches above or over the prism glass to shade direct sunlight scale will clear up turn darker blue and make reading the scale very accurate.Now take the hot cup of water and make coffee for the trip to the fish shop. Save the water bottle for later when you mow the lawn.