4. What is a Psychrometric Chart?
The chart which 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 beforehand, the other values can be found with
this chart. The way that the air will change when it is heated, cooled, humidified or dehumidified can also be seen easily on
(1) Dry bulb temperature t (°C)
Generally referred to as standard temperature this is
measured with a dr y bulb thermometer (conventional
The obtained value is the dr y bulb
(2) Wet bulb temperature t' (°C)
When a dry bulb thermometer's heat sensing section is
wrapped in a piece of wet gauze and an ample air flow
(3 m/s or more) is applied, the heat applied to the wet bulb by
the air and the heat of the water vapor that evaporates from
the wet bulb will balance at an equal state. The temperature
indicated at this time is called the wet bulb temperature.
(3) Absolute humidity x (kg/kg')
The weight (kg) of the water vapor that corresponds to the
weight (kg') of the dry air in the humid air is called the
(4) Relative humidity ϕ (%)
The 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 is called the relative humidity. This is obtained
with the following formula:
ϕR = P
(5) Dew point t" (°C)
The water content in the air will start to condense when air is
The dry bulb temperature at this time is called the dew point.
(6) Enthalpy i (kJ/kg) (kcal/kg')
Physical matter has a set heat when it is at a cer tain
temperature and state. This retained heat is called the
enthalpy, with dry air at 0 °C being set at 0.
CHAPTER 2 G Lossnay Construction and Principle
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" °C dew point