Hydrometer

Intermediate

Measures liquid density using flotation depth.

A weighted glass float whose sinking depth indicates liquid density. It sinks deeper in less dense liquids.

Worked example

Checking battery acid or alcohol content.

Common mistake

Calibrated at a particular temperature.

In more detail

A hydrometer floats in a liquid and sinks to a depth that depends on the liquid's density. The scale reading gives the relative density directly.

Brewers measure alcohol content using hydrometers.
Car battery acid density is checked with a hydrometer.
Battery acid, antifreeze and beer are tested with a hydrometer, which floats higher in denser liquids.

Why does it sink deeper in lighter liquid?

Because a lighter liquid needs more volume displaced to support the same weight.

Do hydrometers need calibration?

Yes, they are calibrated to specific temperatures for accurate readings.

How does a hydrometer work?

It sinks until the weight of the liquid it displaces equals its own weight, so the depth reading indicates the liquid's density.