This is ISI Mark Density Petroleum Hydrometer Comes with NABL Certificate. A hydrometer or barometer is an instrument that measures the specific gravity (relative density) of liquids—the ratio of the density of the liquid to the density of Petroleum. A hydrometer is usually made of glass, 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 kit, 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. Petroleum usually contain a scale inside the stem, so that the person using it can read specific gravity.