With swingarm & glass hydrometers people often don't realize or forget that these are calibrated at a set temperature and if the water you are measuring is significantly off that temperature, it will affect the reading. Most of the newer refractometers have automatic temperature compensation so provided you wait a few moments for the liquid to reach temperature parity with the refracto, you're good to go.
I just measured my tank water with 4 different components:;
Glass Hydro with thermometer: 1.024+ @ 74 Fahrenheit (there's a mark at 78 F so I assume that's the manufacturer's calibration temperature)
Refractometer: 1.024+ (auto temp compensation)
Instant Ocean swing arm: 1.0234 @ ? Fahrenheit, presumably same as glass hydro
Coralife Deep Six swing arm: 1.0216 @ ? Fahrenheit - same story as IO swing arm
Note the extra decimal place for the cheapo swing arm jobbies. That's because the scale on both of these is fairly large making it easier to read off to that level of accuracy. Both the glass hydrometer and refracto scales are much tighter and for me this makes it almost impossible to read anything beyond that 3rd decimal. So one could argue that the swing arm hydros offer an advantage in this regard, provided they are accurate to begin with and one has the temperature compensation chart handy.
I kind of like the floating glass hydrometer. It's a tried and true technology. Different versions of these have been used for decades to measure specific gravity of battery acid, anti freeze in your radiator, sugar content for potential alcohol of that home made wine.... Same goes for refractos, one for almost every flavour.
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