Air Conditioning Fundamentals
TC070-05-01S
3 – A/C TERMS AND CONDITIONS
14
Humidity as a Percent
Humidity — also called relative humidity — is usually expressed as a percent. For example,
80 percent humidity means the air is holding 80 percent of the water vapor it could hold at
its current temperature. One hundred percent humidity means that the air can hold no
additional moisture unless its temperature increases.
High Humidity
High humidity makes a hot day feel even hotter because of the way the human body cools
itself. To cool off, the body releases water to the atmosphere. During low humidity, the
atmosphere absorbs the moisture given off by the body, providing a cooling effect. During
high humidity, the atmosphere cannot absorb the moisture the body gives off. The moisture
condenses on the skin as sweat, causing an uncomfortable “sticky” feeling.
During periods of high humidity, an otherwise normal air conditioning system may appear to
need servicing. When evaluating the performance of an A/C system, you must consider the
relative humidity. The following table shows acceptable readings for various combinations of
temperature and humidity.
Typical System Temperatures
Ambient Air
A/C System
Relative
Humidity
Temperature
(°F)
Evaporator Pressure
(psi)
Vent Outlet
Temperature (°F)
30%
70
28.0
37
80
29.0
37
90
30.5
40
100 36.0
45
110 43.0
56
60%
70 28.0 39
80 30.0 42
90 35.0 46
100 43.0
60
90%
70 28.0 41
80 32.0 48
90 39.0 56
100 50.0
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