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CIRCULAR 6 35, U. S. DEPARTMENT OF AGRICULTURE 



To obtain the best results, the pyrethrum powder should be milled 

 to such a fineness that most of it will pass through a 200-mesh sieve, 

 and it should contain a minimum of about 0.80 percent of total 

 pyrethrins. 



The powder should be applied at the rate of 3 ounces for each 1,000 

 cubic feet of space not occupied by tobacco. For example, the pound- 

 age of pyrethrum powder required for one application in an open 

 storage section having a total volume of 237,312 cubic feet and a storage 

 capacity of 1,728 hogsheads would be calculated as follows : Allowing 

 54 cubic feet as the volume of an average hogshead of flue-cured 

 tobacco, the space occupied by tobacco would be 54 times 1,728. or 



Figure 15. — Power duster for applying pyrethrum powder, with operating crew and 

 equipment for dusting open warehouses. 



93,312 cubic feet. The space unoccupied by tobacco then equals 237,312 

 less 93,312, or 144,000 cubic feet. Multiplying 144 by 3, the weight 

 required is 432 ounces, or 27 pounds. 



A power duster suitable for applying pyrethrum powder in tobacco 

 warehouses (figs. 15 and 16) is equipped with a fan 20 inches in 

 diameter turned at a speed of 3.450 or 3,600 revolutions per minute 

 by a 3-horsepower motor operated from a suitable electric connection 

 in the warehouse. The diameter of the delivery hose at its outlet is 

 3!/2 inches, and the air velocity at this point is approximately 15.000 

 feet per minute. The short delivery hose with the proper diameter 

 at the outlet is more efficient than a longer one. The hopper has a 

 capacity for about 40 pounds of pyrethrum powder (fig. 15). 



The recommendations of the Southeastern Underwriters' Associa- 

 tion for a duster which meets satisfactorily the fire hazards in tobacco 

 warehouses are the following: 



