Not only are those hydrometers more accurate than swing arms, good quality ones are actually more accurate and repeatable than 99.9% of refractometers on the market. I use a lab grade hydrometer of a similar type that is calibrated accurate on manufacture at +-0.05, and no refractometer on the market is calibrated that close unless you are paying several hundred dollars plus for a calibrated lab grade. Hobbyist ones are good, but not that good. Check out Brennans website for a plethora of calibrated lab grade hydrometers
Where refractometers rate highly is on conveniance, and they are usually pretty close. Where they are a little wobbly is on repeatability as the drops of water applied are so small they evaporate quite quickly, so changing the saliniity, and as the differences in RI (refractive index) are so small they are susceptible to this change. So if you use a refracto. get on with doing it. Don't put a drop of water on, and then do something else and go back and read it - it might have changed.
So, FWIW, if you use this guy properly he will be accurate , repeatable, and probably more so than a refractometer. Try to find out what temp the scale is calibrated at - densities change quite quickly with temp, and need a little correction - I don't have a table handy. At least you know, if you wash it each time with tap water the results will be repeatable.
Strangely, despite the slagging hobbyists give them, cleaned swing arms on a surface measured flat with a spirit level proved accurate and reperatable on the only actual test I've seen done (by Steven Pro last year), using I think 25 swing arms obtained from various sources blind. It would seem the errors are caused by the fact that most hobbyists don't do either thing mentioned (cleaning, levelling) properly.
Oh, and measure at the water level, not the meniscus, and turn off any circulation pumps to let it settle.
