Psychrometrics Page 5 - Terms Defined and Explained

Relative Humidity
The amount of moisture in the air measured as a percentage of the air’s capacity to hold moisture.  Warm air can hold more moisture by weight than cold air can.  So, 50% relative humidity means that the air is 50% saturated, but doesn’t tell you much unless you know the temperature of the air.  Condensation occurs when the relative humidity reaches 100%.

Specific Humidity
The amount of water in the air measured by weight.  Specific humidity is generally listed as weight of water in the air per weight of air.  The specific humidity does not change with temperature, unless water is added or removed from the air.

Dry Bulb Temperature
The temperature of air that without moisture considerations.  This is the temperature that thermostats read.

Wet Bulb Temperature
The temperature of a wetted surface if air is blown over it.  Ever notice that you feel colder when you get out of a pool?  This is because air is moving over your wetted surfaces and the temperature of your skin is lowered below the outdoor temperature (dry bulb temperature).  The more humid the air is, the closer the wet bulb temperature is to the dry bulb; dry air has a significantly lower wet bulb temperature associated with it.

Dew Point Temperature
The dew point temperature is the air temperature at which condensation forms, given the humidity in the air.  If a surface is cooled below the air’s dew point temperature, then condensation forms and the surface gets wet.  When moisture is added to air, the dew point temperature rises.

Dew point should be considered when managing the indoor environment and when studying the spaces internal to walls and roofs(the building envelope).

Psychrometric Chart
Graphical representation of the physical properties of moist air.  This chart illustrates how the dry bulb temperature and wet bulb temperature indicate relative humidity. Apollo BBC has simplified a common psychrometric chart for the use by building operators.  Our chart can be downloaded and printed for your reference.  For a copy, refer to Psychrometric Chart.



Psychrometrics
Page 1 - Introduction
Page 2 - Indoor Comfort
Page 3 - HVAC Systems
Page 4 - Building Enclosures
Page 5 - Definition of Terms