An hydrometer is an instrument that measures the specific gravity of fluids. 'Specific gravity' means the relative density of the fluid in comparison to the density of water.
The hydrometer is usually made of glass and it consists of a chamber to contain some of the fluid to be measured and a calibrated float.
If the mass of any object is greater than its equivalent volume of fluid, then it will not float in the fluid, but will sink. For instance, a steel ball is heavier than an equivalent-sized ball of water, so it will not float in water. By contrast, if the mass of an object is less than the equivalent volume of fluid, it will float. For instance, a table tennis ball is much lighter than a table tennis ball-sized volume of water, so it will float in water.
The floating force acting on the object is always equal to the weight of the fluid being displaced by the mass of the object. Therefore, if the fluid was a very heavy fluid, it is possible that the steel ball could float, and if the fluid was an extremely light fluid, it is possible that a ping-pong ball would not float, but instead would sink.
In the hydrometer, the float will ride higher or lower in the water depending on the density of the fluid it is floating in. The calibrations on the float at the fluid level indicate the specific gravity of the fluid.
In the automotive industry, hydrometers are used to measure the specific gravity of fluids such as battery electrolyte to assess the acid concentration; radiator coolant to assess the freeze point, and diesel fuel to determine the Cetane level.
