Hydrometers give us the SG or specific gravity reading of our reef aquarium water. SG swing arms hydrometers will vary with water temperature, as most are designed to work normally with 25 degrees C or 77 degrees F. Now if you are testing water at a temperature that has NOT been calibrated for your swing arm then you will get a false reading. Cold test water will give a false positive reading and warm test water will give a false negative reading. Most swing arms claim to be .001 + - in accuracy. As most reefers have used these types of hydrometers and have found them not to very accurate at all do to many factors involved when testing with them. Swing arm hydrometers must be kept clean and free of any partials and air bubbles on the swing arm. The temperature must be at the temp it was calibrated for, which most of us in this hobby really don't have the time or effort to do this. I personally have used 2 brand new SeaTest and 1 used 1 new deep 6 at the same time and got 4 different reading which all were below what I thought it should be. I have taken water in to a LFS and have had it tested with a refractometer and it was the same reading as my refractometer. Now refractometers are known to be the most accurate SG testing devices. Most have ATC (automatic temperature compensating) which makes it a fine tool to use for us lazy reefers. Just apply 4 or 5 drops of tank water on to the lens and view the results. When finished wipe lens off with a damp towel. Most are .001 in accuracy. Disclaimer: These are not the views of 3reef.com and or any other company that there equipment that was mentioned. These are only for discussion purposes with other reef aquarium users as an information tool and for discussion. This post was written by Nautilus aka David A. and are his views while using said equipment while doing his own water testing with his personal reef aquarium.