Understanding gravity is essential in meadmaking. Long story short – gravity deals with the density of the fluid – relative to the density of pure water – in your brew. For us brewers, this would amount to the “amount of sugar in your brew”. As there are several hydrometers in sale, each type calibrated for its intended use – make sure that when you want to buy a hydrometer for brewing- get one that is calibrated for brewing and not -let’s say – for milk ( yes – they do exist..)
Specific gravity on a hydrometer for this purpose, shows on a range, usually from below 1.000 and upward to well over 1.500 ( one, point xxx to one point xxx). Also here it is important to note you could find hydrometers with different ranges. Some specialized in the big ranges and lower details, while others have more narrow ranges and higher details.