I have used almost every type of specific gravity including expensive refractometers. Besides being expensive, they often need to be calibrated and are difficult to use. Coralife floating arm hydrometers never need to be calibrated. Season it with salt water for the first 24 hours, then rinse with fresh water after each use and you'll be trouble-free for years. I tested them on refractometers, accuracy +-0.001. In any case, stability is known to be much more important than perfect accuracy.
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