4. What is a Psychrometric Chart?
A chart that shows the properties of humid air is called a psychrometric chart. The psychrometric chart can be used to find the
(1) Dry bulb temperature, (2) Wet bulb temperature, (3) Absolute humidity, (4) Relative humidity, (5) Dew point and (6) Enthalpy
(total heat) of a certain air condition. If two of these values are known, the other values can be found with the chart. Now air
conditions will change when it is heated, cooled, humidified or dehumidified can also be seen easily on the chart.
(1) Dry Bulb Temperature t (°F, °C)
Generally referred to as standard temperature, the DB
temperature is obtained by using a dry bulb thermometer
(2) Wet Bulb Temperature t' (°F, °C)
When a dry bulb thermometer is wrapped in a piece of wet
gauze and an ample air flow (3 m/s or more) is applied, the
heat from the air and evaporating water vapor applied to the
wet bulb will balance at an equal state and the wet bulb
temperature is obtained.
(3) Absolute Humidity x (Ib/Ib', kg/kg')
Weight (Ib, kg) of the water vapor that corresponds to the
weight (Ib' , kg') of the dry air in the humid air.
(4) Relative Humidity ϕ (%)
Ratio of the water vapor pressure Pw in the humid air and the
water vapor pressure Pws in the saturated air at the same
temperature. Relative humidity is obtained with the following
ϕR = P
(5) Dew Point t" (°F, °C)
Water content in the air will start to condense when air is
cooled and the dry bulb temperature at that condition is the
(6) Enthalpy i (Btu/Ib, kJ/kg)
Physical matter has a set heat when it is at a cer tain
temperature and state. The retained heat is called the
enthalpy, with dry air at 32 °F (0 °C) being set at 0.
Lossnay Construction and Technology
The dew point t" of the air at point A is the temperature
of the point at the same absolute humidity as point A
on the saturation curve.
Parallel to absolute
temperature scale line
t" °F, °C dew point