20 BULLETIN 1335, U. S. DEPARTMENT OF AGRICULTURE 
The practical application of the charts in solving various problems 
can best be illustrated by means of examples. The charts given 
here serve merely to indicate the methods of computation. In 
reaching exact results, the writers used larger charts than could be 
reproduced here, showing more subdivisions. 
RELATIVE HUMIDITY FROM KNOWN WET AND DRY BULB READINGS 
Example: Dry bulb =150° F.; wet bulb =100° F. 
Find in figure 7 the intersection point of the lines corresponding 
to a dry bulb temperature of 150° F. and a wet bulb temperature 
of 100° F. The line representing the relative humidity at this point 
reads 18 per cent. | 
POUNDS OF DRY AIR AND OF WATER VAPOR PER CUBIC FOOT UNDER KNOWN 
CONDITIONS OF TEMPERATURE AND RELATIVE HUMIDITY 
Example: Dry bulb=150° F.; relative humidity =18 per cent. 
According to Figure 8, air at 150° F. and 18 per cent relative 
humidity contains 0.062 pound of dry air and 0.002 pound of 
water vapor per cubic foot. The weight of the mixture would then be 
the sum of these weights, or 0.064 pound per cubic foot. 
CHANGE 1N RELATIVE HUMIDITY PRODUCED BY A CHANGE IN TEMPERATURE 
Example: Lowering the temperature from 150° F. and 18 per 
cent relative humidity to 100° F. 
On Figure 9 start at the point corresponding to 150° F. and 18 
per cent-relative humidity and follow parallel to the nearest curve 
until the 100° F. dry bulb temperature line is reached. The relative 
humidity at this point reads 71 per cent. 
Example: To find the temperature at which the air under the 
foregoing conditions would reach 100 per cent saturation. 
Follow parallel to the nearest curve as before until the 100 per 
cent relative humidity line is reached. The temperature at Ghis 
point reads 89° F. 
Example: Raising the temperature from 70° F. and 50 per cent 
relative humidity to 150° F. 
Find on Figure 9 the point corresponding to 70° F. and 50 per 
cent relative humidity and follow parallel to the nearest curve to 
the 150° F. dry bulb temperature line. The relative humidity at 
this point reads 5 per cent. 
CHANGE IN VOLUME PRODUCED BY CHANGE IN TEMPERATURE 
Example: One cubic foot of air at 150° F., 18 per cent relative 
humidity, would have at 100° F. a relative humidity of 71 per 
cent, according to Figure 9. 
According to Figure 8, air at 150° F. and 18 per cent relative 
humidity contains 0.002 pound of water vapor and 0.062 pound of 
dry air, or 0.064 pound of the mixture per cubic foot, while at 100° F. 
and 71 per cent relative humidity the mixture contains 0.002 pound 
of water vapor and 0.068 pound of dry air and weighs 0.070 pound 
per cubic foot. Therefore the volume at 100° F. and 71 per cent rela- 
06s or 0.914 cubic foot. 
tive humidity would be 0.070" 
