The following terms are used in any discussion of the subject of
humidification. They are defined here so that you will be able to
understand the differences, however slight they may be, between such
terms as relative humidity and percentage humidity.
• Humidity is the water vapor within a given space.
• Absolute humidity is the weight of water vapor per unit volume,
expressed as grains or pounds of moisture per cubic foot of dry air.
• Percentage humidity is the ratio of the weight of water vapor per pound
of dry air to the weight of water vapor per pound of dry air saturated at
the same temperature.
• Relative humidity, according to the ASHRAE Guide, is the ratio of the
mol fraction of water vapor present in the air to the mol fraction of water
vapor present in saturated air at the same temperature and barometric
pressure. Approximately, it equals the ratio of the partial pressure or
density of the water vapor in the air to the saturation pressure or density,
respectively, of water vapor at the same temperature. More simply,
relative humidity is the ratio of the actual moisture content of the air to
the maximum possible amount of moisture that the air could hold at the
same temperature. If the relative humidity is 100%, the air is saturated.
There is a difference between percentage humidity and relative humidity.
However, it is very slight and almost negligible at normal room temperatures.
For the purpose of this text, relative humidity is the amount of water vapor
actually present in the air, expressed as a percentage, compared to the maximum
amount that the air could hold under the same conditions.
The warmer the air, the more moisture it can hold. Air in a home heated to 70°F
can hold about 8 grains of moisture per cubic foot. That is 100% relative
humidity. H there are only 2 grains per cubic foot in the home, that is 1/4 of the
air’s capacity to hold moisture. Therefore, the relative humidity is also 1/4, or
25%. The air could hold four times as much water.
When you step out of a shower in a closed bathroom, it can feel quite warm,
actually muggy. The temperature is probably about 75°F. The relative humidity
is in the 70 to 80% range. The humidity is high because water vapor was added
to the air from the shower. If you go into another part of the house, it feels
noticeably colder, even though the temperature is nearly the same. But the
relative humidity is probably about 10 or 15% in the rest of the house. This dry
air begins evaporating the moisture from your skin. The result is exactly the
effect of an evaporative cooler, and you feel cold.
This same thing goes on day after day, every winter, in millions of homes. People
turn their thermostats up to 75°F or higher to feel warm. Even then it feels drafty
and chilly, because the evaporative cooling process is going on. Proper relative
humidity levels make 70°F feel more like 75°F.
This cold effect is not the only discomfort caused by air that is too dry. Static
electricity is a definite indication of low relative humidity levels, and is a
condition that annoys most people. Proper relative humidity will eliminate or
at least reduce this discomfort.
The addition or reduction of moisture drastically affects the qualities,
dimensions, and weight of hygroscopic materials. Wood, leather, paper, and cloth may feel dry to the touch, but they contain water. It is not a fixed amount of
water, but an amount that will vary greatly with the relative humidity of the
surrounding air. Take, for example, a cubic foot of wood with a bone-dry weight
of 30 lb. At 60% relative humidity, the wood will hold over three pints of water.
Now lower the relative humidity to 10%. The water held by the wood will not
even fill one pint bottle. In effect, lowering the relative humidity from 60% to
10% withdrew 2lf2 pints of water from the wood (1 pint= approximately one lb ).
This action goes on with every single material in the home that is able to absorb
and release moisture. This includes paper, plaster, fibers, leather, glue, hair,
skin-practically everything in the home. These materials shrink as they lose
water and swell as they take on water. If the water loss is rapid, warping and
cracking take place. As the relative humidity changes, the condition and
dimensions of the materials change. It is as constant as the weather. This is why
humidity must be added, and why relative humidity must be controlled. This is
why proper humidity is important.
Excerpt from Technical Institute Manual 2 from the Refrigeration Service Engineers Society. – www.rses.org