Okay, this one has me interested.
Changed the coolant (eyeballed 50/50 ethylene glycol); bought a hydrometer to test it.
How does one test–warm or cool–and how much difference does it make? I understand the principle of thermal expansion, making the coolant less dense at temp. Theoretically, this should make warm coolant look more “watery.”
However…the “5 floating balls” are also being warmed (and given their small size, one would expect them to warm quickly.) If the manufacturer did his homework, he could design the balls with the exact expansion coefficient as the coolant, as to make the measurement insensitive to temperature.
So, how ought one test, and what effect does changing the temp have on the final result? (Also, how long to wait…warm coolant floats 3 at first, eventually floating all 5 after a minute or so.)
Note that I said “warm” coolant, as opposed to “boiling hot coolant spilling all over my hands!”
Can’t answer the temp question but I believe it is not as relevant as you think it may be. What I mean is unless you live ina area where hard freezing happens, anyhere from a 40/100 to a 60/100 range will be good enough for cooling,lubrication of the pump and rust inhibation,notice “good enough”. I myself want a accurate 50/50 mix thats why I bought a much more accurate device to judge coolant mix with. (its a lens you look through with a prizim at the end,all the rage in Europe but the tool trucks have them here in the States. Perhaps others can tell where to buy here.
I have never trusted the floating ball type but they are good enough. I do think you can discard the temp concern on the coolant because your gague is much more innacurate than the effect of the temp would be. Almost like worrying about losing heat out a window when the roof is missing.
This could make a interesting scientific type question almost as obscure as worrying about your connecting rods streching at high rpm 
Actually, I’d argue that of the deigner designed balls that made the device insensitive to temperature changes it would be because he did NOT do his homework!
But it isn’t only the size of the balls that counts…it’s their specific gravity, their total weight, as opposed to the total weight of the fluid they’re displacing.
The balls operate on a known principle of physics that’s constant. Much as mechanical measuring tools are calbrating by measuring them against known principles of physics, the balls are reliable just as the cabration “standards” are. Just as you can test a bubble level by turning it 180 degrees to see if the reading stays the same when turned around, so your floating ball device will stay constant.
It’s reliable, but I would not rely on it as a substitute for draining and refreshing your coolant every few years. It’s only reliable IMHO for measuring the mix proportions. The risk is too huge.
How long has the coolant been in your car? Generally it is a good idea to replace it every 3 - 5 years.
I just changed it, that’s why I wanted to know if the hydrometer is to be used hot or cold. The balls absolutely float more easily on the 2nd, etc, tests, presumably because they’ve warmed up.
The only thing that might concern me is if it’s >70% glycol, but that seems really unlikely, even “eyeballing” a 50% mix.