I think what people have done is use one in conjuction with a refractometer and figure out where the proper level should be on the hydrometer. But I would imagine that every hydrometer is different. When in doubt pull out the refractometer, if not itll be just a guess
i dont think any are accurate, but they are consistant, meaning if you do measure it against a refracto, and mark the actual levels, you can do pretty well with it from there, just make sure to rinse it after every use, salt build up can throw your reading off over time.
A good lab grade bulb hydrometer is very accurate, I used them for years and like them, I don't know about the 5.00 ones that are around. As far as the plastic hydrometer, I have a few of them and all are in the ball park. I used them for a fast check when making up water. but I have seen some that are really off.
I agree with Andy. I haven't found a particular model that is accurate from one hydrometer to the next, but I've used Deep Six and Aquarium Systems swing arms in the lab. When I checked them against refractometers, one of the Aquarium Systems hydrometers was spot on, but the other one and the Deep Six were off by about 0.01-2. As long as they were rinsed after using they read the same consistently.
I recommend that if you do plan to use a hydrometer, either get the LFS or a local reefer to test it against a refractometer occasionally so you know how far off it is and ensure that it's still reading the same error.
After spending tons of money on bulb and plastic hydrometers and finding out they were inaccurate, I spent the bucks and bought a refractometer. Sure works well and is very easy to use. I don't recommend any hydrometers, just a decent refractometer!
The glass bulb hydrometers are much better than the plastic swing-arm (I consider them (swing-arm style) pretty useless now after having the refractometer) but they break so easily! We broke 2 or 3 and were careful.