Q&A for How to Read a Hydrometer

The most common scale, specific gravity, does not have units. It compares the density of the liquid to the density of water, so the result is in the form of a ratio or percentage. For example, a result of 1.050 means the liquid is 1.050 times as dense as water.

A hydrometer with the Plato scale gives you the result directly: one degree Plato = 1% of sucrose by weight. To convert (roughly) from specific gravity to Plato, subtract 1, then divide by 0.004.

Yes. You aren't testing the salt content per se, you're testing the density of the water compared to ideal, pure water. However, for all intents and purposes, if you're testing seawater, then you're testing the salinity by testing the density. Note that the density of water also changes with temperature (and to a lesser and irrelevant for your purposes degree, pressure). There are formulas to adjust your measurement based on temperature.

It depends on what are you measure. If you are measuring a homebrew product, such as wort or mead, then there must be something wrong with your brew. The FG will always be lower than OG because the yeast eats up the sugar to produce ethanol+CO2.

Approximately $17-$23 dollars give or take. But this won't include the shipping costs unless you have a brewery nearby.