What does a hydrometer measure?

A hydrometer is an instrument designed to measure the specific gravity of liquids. Specific gravity is defined as the ratio of the density of a substance to the density of a reference substance, typically water. When using a hydrometer, it floats in the liquid being measured, and the level to which it sinks indicates the specific gravity of that liquid. This is particularly useful in various applications, including determining the concentration of solutions, the density of fluids, and the quality of refrigerants in refrigeration systems.

The hydrometer operates on the principle of buoyancy, following Archimedes' principle, where the weight of the liquid displaced is equal to the weight of the hydrometer itself. As such, the deeper the hydrometer sinks, the lower the specific gravity of the liquid, and vice versa. This measurement is valuable in many fields, including HVAC, where it can help analyze the refrigerants and ensure optimal performance of systems.

The other options do not pertain to the function of a hydrometer. Temperature is measured by thermometers, pressure is measured by manometers or pressure gauges, and humidity is quantified with hygrometers. Thus, the correct answer focuses squarely on the capability of the hydrometer to measure specific gravity.