@LordClownFish You're getting some great replies from some very, very experienced reefers, including at least one professional chemist that I know of right here. So please take my advise with a grain of saltwater ;-)
My quick, non-expert, synopsis of all this is that:
A quality glass hydrometer with something like a graduated cylinder (don't try to use a glass hydrometer in your tank!) is an excellent and accurate solution, but it's clumsy, awkward, and not the easiest to use on a home aquarium, in my opinion. Best suited, I think, for those doing laboratory work, or to double check your other test results.
There are plastic hydrometers with swing arms that some say work well, but to me they are problematic - a single air bubble on the swing arm may throw you off. Others may feel differently, but I wouldn't recommend these.
Then you have refractometers, which I'm just guessing about 75% of reefers here use. These are popular, easy to use, and affordable. But you do get what you pay for, they do need to be calibrated, and you need to make sure you get one that's specifically for the reef tank hobby, and not one that's intended for the hobby of home brewing beer and wine (same thing, but will give you different numbers that don't relate to reef tank keeping)
I think your original post was about the Hanna Salinity Checker. I really like the Hanna Checkers for other parameters, but I personally have never owned a Hanna Salinity Checker.
What I do have and like and recommend, and I don't think anyone else has mention it yet, is I have the Milwaukee Digital salinity meter:
https://www.saltwateraquarium.com/d...vEo_mLKUnqndxM8o3h_O7X-m4pxD5cvcaAlNLEALw_wcB
I love mine, and I'm pretty sure it's a well received product here at R2R. It can be calibrated, and comes with the solution to do so. In use, all you have to do is drop a small drop of water on the sensor, press a button, and in about a second it will report your salinity. It could not be easier.
This is my go-to for measuring salinity, not that I'm an expert, but I hope that helps! Good luck with whatever you choose!