Hydrometer

 A Hydrometer is a device for measuring some characteristics of a liquid, such as its density (weight per unit volume) or specific gravity (weight per unit volume compared with water). 

A hydrometer usually consists of a sealed hollow glass tube with a wider bottom portion for buoyancy, a ballast such as lead or mercury for stability, and a narrow stem with graduations for measuring. The liquid to test is poured into a tall container, often a graduated cylinder, and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer correlates to relative density. Hydrometers can contain any number of scales along the stem corresponding to properties correlating to the density.

A simple hydrometer consists of a glass float inside a glass tube, as shown above.

The hydrometer float is weighted at one end and sealed at both ends. A scale calibrated in specific gravity is positioned lengthwise along the body of the float. The float is placed inside the glass tube, and the fluid to be tested is drawn into the tube.

As the fluid is drawn into the tube, the hydrometer float will sink to a certain level in the fluid. The extent to which the hydrometer float protrudes above the level of the fluid depends on the specific gravity of the fluid. The reading on the float scale at the surface of the fluid is the specific gravity of the fluid.

The point at which the surface of the liquid touches the stem of the hydrometer correlates to relative density.

Hydrometers are calibrated for different uses, such as a lactometer for measuring the density (creaminess) of milk, a saccharometer for measuring the density of sugar in a liquid, or an alcoholometer for measuring higher levels of alcohol in spirits. In our application, we measure the Specific Gravity of Battery Acid.

Reference:
[1] https://en.wikipedia.org/wiki/Hydrometer
[2] https://www.britannica.com/technology/hydrometer
[3] https://instrumentationtools.com/how-to-measure-specific-gravity-of-battery/
[4] https://www.batteriesinaflash.com/how-to-measure-specific-gravity