48 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE 



vapor if it were cooled without change in proportion of vapor and 

 air. Then if the mixture is already saturated, its dew point coin- 

 cides with its temperature, while if it is less than saturated, its dew 

 point is lower than its temperature. If air is saturated at any given 

 temperature and then heated to a higher temperature without change 

 in proportion of water vapor, its dew point remains unchanged. 



The term "humidity" is used for any of several different ways of 

 expressing the amount of water vapor carried by air. 



The expression "relative humidity" is most commonly used in 

 meteorological work and air conditioning for human com- 

 fort, and is simply the following ratio: Percent relative humid- 



1AAv Partial pressure of water vapor in air rr , 1 



ity=100X* f ± j 7 1 ^i 1 Then sat- 



J Vapor pressure oi water at same temperature 



urated air has a relative humidity of 100 percent, and air contain- 

 ing no water vapor has a relative humidity of percent. At. tem- 

 peratures above the boiling point of water the relative humidity is 

 always less than 100 percent, even for pure water vapor unmixed 

 with air. 



In work with dehydrators there are many advantages in the use 

 of the expression "absolute humidity," which is defined as follows: 

 Absolute humidity = pounds (or grains) of water vapor per pound 

 of dry air (1 pound equals 7,000 grains). 



Whenever absolute humidity is referred to in this manual pounds 

 of water vapor per pound of dry air should be understood. The 

 convenience of working with absolute humidities arises from the 

 fact that they are expressed on a weight basis. A pound of dry 

 air entering a dehydrator, for example, still weighs just a pound 

 at any other point in the dehydrator, no matter what changes in 

 temperature or pressure may have taken place, or how much water 

 vapor may have been added to it. 



The absolute humidity of completely dry air is, of course, pound 

 per pound. The absolute humidity of saturated air rises as the tem- 

 perature rises, and becomes infinite at and above the boiling point of 

 water. A column in table 7 gives its values at a barometric pressure 

 of 29.92 inches and at temperatures below 212° F. The definition for 

 percent absolute humidity is as follows: Percent absolute humidity = 



10n Absolute humidity, pounds vapor per pound dry air 



Absolute humidity saturated air, pounds vapor per pound dry air 

 Note that percent absolute humidity is different numerically from 

 percent relative humidity ; see the following paragraph for a formula 

 for converting one to the other. Percent absolute humidity, like the 

 absolute humidity of saturated air, loses its significance at air tempera- 

 tures above the boiling point of water. 



Any of the various ways of expressing humidity may be converted 

 into any of the others through the use of one or more of the fol- 

 lowing formulas. Some of these formulas are not exact, particu- 

 larly at the higher humidities, but their accuracy is sufficient for 

 practical purposes. 



_0.625ff 



a s = 



B-p 

 0.625P S 



B-P s 



