Hydrometer is used to measure specific gravity of the electrolyte of the battery. It consists of a glass tubular body with a rubber bulb at the top and a sampler tube at the bottom. There is a glass float inside the glass body. There is a mark inside the glass (red & green).
This glass float has a vertical density scale. To test the specific gravity of the electrolyte :
1. Immerse the sample tube in the cell containing electrolyte. (Mixture of water and sulphuric acid).
2. Squeeze the rubber bulb and release the same which would cause a sample of the electrolyte to be drawn inside the glass body. Let the float inside rise and then read off the scale drawn at the surface of the sampler tube. The specific gravity of a fully charged battery is 1.280. If the specific gravity is less than 1.200 then it should be charged.
