Table 8 Water vapor capacity of air

Mechanical and Electro
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SM0486

the term "partial pressure"

can

interchanged

with

relative

humidity.

The

meanings are the same.

(5) Now that we have defined the common terms associated with humidity,

let's see how temperature affects humidity. A cold room in a house usually feels

damp. However, if the same room is heated the dampness seems to disappear and the

room becomes dry.

The amount of moisture in the room has not been reduced by

heating the room.

However, the capacity of the air in the room for moisture

increases when the temperature is raised.

EXAMPLE: Suppose the air in a room at 32F. contains 2 grains of water vapor per

cubic foot. As shown in Table 8, the capacity of air at 32F. is 2.118 gr/ft3 or 94

When the temperature is raised to 68F., the

percent.

Such air will feel damp.

amount of moisture is unchanged, but the capacity of the air is now 7.56 gr/ft3

divided by 7.56 gr/ft3, or 26 percent. This air feels dry. We see that increasing

the temperature decreases the relative humidity.

On the other hand, if the

temperature is lowered, the relative humidity will increase.

TABLE 8

Water vapor capacity of air

Instruments that are used to

determine relative humidity are called either psychrometers or hygrometers.

Generally, the act and dry bulb instrument is called a psychrometer and direct

indicators of relative humidity are called hygrometers. Hygrographs determine and

record relative humidity.