As most of us who are serious about our Salt Water use refractometer's ($50 - $150) instead of hydrometers, this is just a reminder of something I forgot to do. Lubricate the calibration nob (if applicable) and adjustment screw. Some refractometer's have a large knob with a screw in the center, others just a screw or dial to fine tune your device. I forgot to lubricate mine 6 months ago as I usually do and when I went to adjust the inner screw it and the knob were "jammed". I fastidiously rinse and clean mine with fresh water after each use, even so, it jammed. A small dab of Krown T-40 (use what you like) and a minute later it was loose. I use Krown as it's "creeping oil" that will find its way into every inch of thread. I now have a reminder set to re-occur every 6 months on my phone. You may wonder why I don't check the calibration more often, it's simple, my calibration knob locks the screw in place and after using it for over two years as well as 1/2 a bottle of ultrapure water I found I did not have too. Set your maintenance to what you are comfortable with of course. I use a Sybon Opticon series FG100SA, $68 from Big Al's store, online may be less.
Life is tough, it's even tougher if your stupid.
If your not angry, your not paying attention...
150G FWLR (Morays) 75G Fresh (Assrtd) 24G Cube (Reef/Goby) 10G Fresh (Beta)