Specific gravity (S.G.) is a measure of the relative density of a substance as compared to the
density of water at a standard temperature. Physicists use 39.2°F (4°C) as the standard, but
engineers ordinarily use 60°F. In the International System of Units (SI Units), the density of
water is 1.00 g/cm3 at the standard temperature. Therefore, the specific gravity (which is
dimensionless) for a liquid has the same numerical value as its density in units of g/cm3. Since
the density of a fluid varies with temperature, specific gravities must be determined and specified
at particular temperatures.
Humidity is the amount of moisture (water vapor) in the air. It can be expressed as absolute
humidity or relative humidity. Absolute humidity is the mass of water vapor divided by a unit
volume of air (grams of water/cm3 of air). Relative humidity is the amount of water vapor
present in the air divided by the maximum amount that the air could contain at that temperature.
Relative humidity is expressed as a percentage. The relative humidity is 100% if the air is
saturated with water vapor and 0% if no water vapor is present in the air at all.
Intensive and Extensive Properties
Thermodynamic properties can be divided into two general classes, intensive and extensive
properties. An intensive property is independent of the amount of mass. The value of an
extensive property varies directly with the mass. Thus, if a quantity of matter in a given state
is divided into two equal parts, each part will have the same value of intensive property as the
original and half the value of the extensive property. Temperature, pressure, specific volume,
and density are examples of intensive properties. Mass and total volume are examples of