Heating, Cooling and HVAC

Wet-bulb temperature is measured using a standard mercury-in-glass thermometer, with the thermometer bulb wrapped in muslin, which is kept wet. The evaporation of water from the thermometer has a cooling effect, so the temperature indicated by the wet bulb thermometer is less than the temperature indicated by a dry-bulb (normal, unmodified) thermometer. The rate of evaporation from the wet-bulb thermometer depends on the humidity of the air - evaporation is slower when the air is already full of water vapour. For this reason, the difference in the temperatures indicated by the two thermometers gives a measure of atmospheric humidity.

Dry-bulb temperature: It is the temperature of air measured by the thermometer freely exposed to the air but shielded from radiation and moisture. In construction, it is an important consideration when designing a building for a certain climate Nall (as cited in “References”) called it one of “the most important climate variables for human comfort and building energy efficiency”.

Relative Humidity:
Relative humidity may be defined as the ratio of the water vapor density (mass per unit volume) to the saturation water vapor density, usually expressed in percent:

Relative Humidity (RH) =

(Actual Vapor Density) ——————– (Saturation Vapor Density)

X 100%

Relative humidity is also approximately the ratio of the actual to the saturation vapor pressure.

RH =

(Actual Vapor Pressure) ————————– (Saturation Vapor Pressure)

X 100%

Actual vapor pressure is a measurement of the amount of water vapor in a volume of air and increases as the amount of water vapor increases. Air that attains its saturation vapor pressure has established an equilibrium with a flat surface of water. That means, an equal number of water molecules are evaporating from the surface of the water into the air as are condensing from the air back into the water.

Saturation vapor pressure is a unique function of temperature. Each temperature may be interpreted as a dew point temperature because as the ground cools, dew will begin to form at the temperature corresponding to the vapor pressure .