52 MISC. PUBLICATION 540, tT. S. DEPT. OF AGRICULTURE 



Humidity Charts 



Several different forms of humidity charts are in use. Some of them 

 are based primarily on relative humidities; figure 22 is an example. 4 

 Figure 23 is based on absolute humidities. Both are calculated for a 

 barometric pressure of 29.92 inches. 



Example 1. What is the relative humidity of air with a dry-bulb 

 temperature of 160° F. and a wet-bulb temperature of 110° F. ? From 

 figure 22, percent relative humidity = 21 percent. 



RELATIVE HUMIDITY (PERCENT) 

 10 20 30 4 50 60 70 80 90 10 



10 2.0 30 40 50 60 70 8 90 

 RELATIVE HUMIDITY ( PERCENT) 



Figuee 22. — Humidity chart — percent relative humidity. 



Example 2. What is the absolute humidity under those same condi- 

 tions ? From figure 23, absolute humidity = 0.0463, and percent abso- 

 lute humidity = 15.5 percent. 



Example 3. What is the dew point of the air under those conditions ? 

 The dew point is the temperature at which air of the same absolute 

 humidity would be saturated. From figure 23, the dew point = 

 102.3° F. 



* The fundamental measurements on which humidity charts are based are scanty and 

 to some extent conflicting, except within the usual range of outdoor temperatures and 

 humidities. Discrepancies between different charts are serious at the higher temperatures 

 and humidities characteristic of commercial dehydrators. The Committee on Psychrometry 

 of the American Society of Heating and Ventilating Engineers is now engaged in redeter- 

 mining the fundamental properties of mixtures of air and water vapor in an effort to arrive 

 at a fully trustworthy humidity chart. The charts given in this manual are based on 

 data: by Keenan and Keyes (24), and at temperatures below 125° P., on data of the 

 American Society of Heating and Ventilating Engineers (2). 



