Glass Hydrometer for SG, OG , FG readings.
A hydrometer measures the difference in gravity (density) between pure water and water with sugar dissolved in it by flotation. The hydrometer is used to gauge the fermentation progress by measuring one aspect of it, attenuation. Attenuation is the conversion of sugar to ethanol by the yeast. Water has a specific gravity of 1.000. Beers typically have a final gravity between 1.015 and 1.005.