Oilers/Plant Tenders (HHC) Civil Service Practice Exam

The instrument used to measure the specific gravity of lubricating oils is the hydrometer. This device operates on the principle of buoyancy, where it floats in the liquid and the level to which it sinks correlates with the density of the liquid being tested. Specific gravity is defined as the ratio of the density of the oil to the density of water, meaning the hydrometer is ideally suited for this measurement as it provides direct indications of density.

In the context of lubricating oils, knowing the specific gravity is crucial because it can indicate the oil's quality and performance characteristics. These properties can impact how the oil performs under various temperature and pressure conditions, essential for ensuring efficient operation of machinery and engines.

Other instruments listed do not serve this specific purpose. A thermometer measures temperature, which is unrelated to the measurement of density or specific gravity. A viscometer is designed to measure the viscosity of fluids, which is a different property regarding flow behavior instead of density. A manometer is used for measuring pressure, again unrelated to specific gravity measurements. Each instrument has its dedicated function, making the hydrometer the appropriate choice for assessing the specific gravity of lubricating oils.