A hydrometer is an instrument used to measure the specific gravity of liquid in comparison to pure water. The hydrometer is important because it allows a brewer to determine several things:
- When the mash is no longer contributing sufficient levels of sugar during a sparge
- How much dissolved sugar is in the finished wort
- If your beer has under or over fermented
- Brew house efficiency
- Original gravity
- Final gravity
- Fermentation progress
- Fermentation completion
- Alcohol by volume.
Below are three examples of different hydrometers that are available to brewers, including one used for determining your final gravity.
Here is the final gravity reading from a hydrometer that also registers temperature so that corrections can be made if the beer is too hot or too cold.