A hydrometer does not know what it is floating in. The instrument sold for wine and the one sold for beer measure the same thing (how dense the liquid is) and the reading means the same thing in both: how much dissolved sugar is still there. What changes is the range you need, the numbers you will see and, importantly, the method that turns two readings into a strength. A must that starts at 1.090 and finishes at 0.995 is a long way outside anything a beer does, and some of the shortcuts written for beer stop behaving out there.
Why a wine hydrometer has a longer scale
Beer wort rarely starts above 1.070 and almost never finishes below 1.006. A wine or mead must routinely starts at 1.090 or higher and, because the yeast eats essentially all of the sugar, finishes below 1.000, dry wine is less dense than water, because alcohol is. A hydrometer with a range that stops at 1.000 cannot read the end of a wine ferment at all. The trade-off is that a longer scale on the same length of stem is a coarser scale, so a wine hydrometer in a 1.048 wort is harder to read to the degree than a brewing one.
Potential alcohol, and why it is a different scale
Most wine hydrometers carry a second scale marked potential alcohol, which reads the strength you would get if every gram of sugar in the must fermented out. That assumption is very nearly true of a dry wine and quite untrue of beer, where a large fraction of the extract is unfermentable by design. This is why the potential-alcohol scale is printed on wine instruments and not on brewing ones, and why reading a wort against it will always flatter the beer.
The strength arithmetic is not the same either
For beer, HMRC's guide gives a factor that changes with strength and is applied to the degrees the beer attenuated. That table is written for beer, and its top line stops at 100.7 degrees of attenuation. A wine that dropped from 1.090 to 0.995 attenuated 95 degrees, which is inside the table but at the far end of it, where the factor is 0.135 rather than the 0.130 an ordinary beer uses. The instrument is shared; the constants are not, and it is worth knowing which one you are borrowing.
Reading a must that still has fruit in it
Pulp, fruit and suspended solids all lift a hydrometer, and a must racked off its fruit will read lower than the same must with the fruit still in. Take the reading from a strained sample, and take the first one after you have finished adding water and sugar rather than partway through, or the original gravity is a reading of something you never actually fermented.
Questions people ask about hydrometer for wine
Can I use my beer hydrometer for wine?
For the starting reading, usually, 1.090 is within most brewing ranges. For the finishing reading, only if the scale goes below 1.000, and many brewing hydrometers stop at 0.990 precisely so that they can. Check the bottom of the stem before you need it.
Why did my wine finish below 1.000?
Because it is mostly water and alcohol, and alcohol is lighter than water. A dry wine at 0.992 is normal and is not a sign that anything went wrong; it means the yeast took essentially all of the sugar.
Does the temperature correction work the same way?
Yes, identically. The instrument is calibrated at a temperature and the liquid expands as it warms, whatever is dissolved in it. Correct both readings against your own instrument's card, or read both samples at the calibration temperature.
Is the potential alcohol scale accurate for mead?
It has the same assumption behind it as for wine (that all of the sugar ferments) and mead usually obliges. Where it misleads is a mead you intend to back-sweeten or one with a yeast that quits early, because then the difference between two readings is the honest answer and the potential scale is a prediction that did not happen.