I first purchased a Swing arm type hydrometer then after joining this forum, I purchased a refreactometer, I tested the hydrometer against the refractometer and found the numbers to be the exact same!
However after several months of use I found my refeactometer would not zero, and some how was stuck. I had to buy another. But I check my salinity with my refractometer weekly, and I check it with the hydrometer from time to time just to get a second opinion if you will.
90 Gallon Drilled Glass Tank w/ Corner Overflow: 492w (2x 150w 14,000K MH, 2x 96w Actinic CF) Coralife Aqualight Pro | Custom 34x15x15 Sump/Refugium | SWC Skimmer | Coralife Turbo-Twist 6x UV Sterilizer | Maxi-Jet Power Head w/Sure Flow Upgrade Kit | 120lbs of Live Rock | 100lbs of Live Sand