Hydrometer

A hydrometer is a sealed glass or plastic float with a graduated stem, used to measure the specific gravity of a liquid compared to plain water. You drop it into a sample of wort or beer inside a tall tube, and the depth it floats to gives you a gravity reading, which is the standard way homebrewers track sugar content before, during, and after fermentation.

For it to be reliable, the sample needs to be at or near the hydrometer's calibration temperature, usually printed on the stem, since warmer or cooler liquid changes density and shifts the reading. You also need enough sample volume in a straight-sided tube for the float to spin freely without touching the sides, and you read the gravity at the flat surface of the liquid, not at the curved edge where it climbs the glass.

Common buying considerations are the gravity range covered, since triple-scale models often show specific gravity, potential alcohol, and Brix on one stem, and how sturdy the glass feels for regular brew day use. A frequent mistake is reading from the top of the meniscus instead of the liquid's true surface, and another is reusing a sample straight from a fermenter without giving it time to settle, which can trap bubbles that throw off the float.