40 
BULLETIN 1373, U. S. DEPARTMENT OF AGRICULTURE 
In calculating the air velocity, the pitot-tube formula, V= 1096.5,- /— , 
\ w 
was used, where V is air velocity in feet per minute, ~h is the velocity 
head in inches of water pressure, obtained by subtracting the static 
head from the total head, and w is the weight of air mixture in pounds 
per cubic foot. 
The weight of the air mixture was determined by a formula fur- 
nished by the Weather Bureau, of the United States Department of 
Agriculture, w = 1.3245 X - — 4 ^ Q , < — , where w is the weight or 
density of the air mixture in pounds per cubic foot, B is barometric 
pressure in inches of mercury, e is vapor pressure, 4 and t is tempera- 
ture of air in degrees Fahrenheit. 
Curves showing the relation between the air velocity in feet per 
minute and the percentage of the grain lifted from the screen (fig. 31) 
o 
too 
2000 
WOO /500 
Air Velocity -Feet per minute 
Fig. 31.— Percentage of grain removed at various air velocities 
indicate that there is a difference of several hundred feet per minute 
between the velocities at which the light and heavy kernels of the 
different grains are carried by the air currents. The data may there- 
fore be of use in designing grain-cleaning equipment. The big varia- 
tion between the high and low velocities for corn is due to the different 
sizes and shapes of the kernels. Large, flat grains were picked up at 
a lower velocity than the medium-sized kernels. The maximum 
velocity at which corn and wheat were not lifted is very nearly the 
same, but the velocities at which all the kernels of these two grains 
were lifted differ greatly. This difference is due to the slight varia- 
tion in the size and shape of wheat kernels as compared with the 
large variation in corn. 
4 These values are taken from Psychometric Tables for Obtaining the Vapor Pressure, Relatrre Humid- 
ity, and Temperature of the Dew-Point, by C. F. Marvin, U. S. Department of Agriculture, Weather 
Bureau Bulletin 235 (1910). 
