Cylindrical Density Jar 250, 500 ml. A hydrometer or barometer is an instrument that measures the specific gravity (relative density) of liquidsthe ratio of the density of the liquid to the density of water. A hydrometer is usually made of plastic, and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. 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 specific gravity. Hydrometers usually contain a scale inside the stem, so that the person using it can read specific gravity.for petrol and gas stations.