Glass hydrometers usually come calibrated to a set temperature, so you’ll have to adjust based on your current temp. The Tropic Marin hydrometer is pretty damn good and reliable. Refractometers on the other hand are usually way off if not of a certain quality and type. Most are made for a brine solution and not actual seawater. There are two known brands that I trust, VeeGee STX-3 and DD H2Ocean. Also, if your calibration solution requires shaking or mixing, it’s garbage and shouldn’t be used. If I don’t make my own, then the only brand I’ve ever trusted was Aqua Craft. RHF has a DIY recipe with table salt to create your own solution.
The thing to remember with both refractometers and hydrometers is, you need to keep them clean and free of saltwater.
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