Step-by-step explanation:
A hydrometer is an instrument that is mainly used to measure the specific gravity. It works on the Archimedes principle
It measures the relative density liquid that is ratio of density of liquid to that of water.
To perform the test, the car mechanic would take the oil in some graduated cylinder and then hydrometer is lowered into the oil till it floats freely. The point at which the oil touches the stem of the hydrometer is noted. Now oils specific gravity would be more incase of more dirt and this is the benchmark of oil replacement
It measures the relative density of that