content of the needles to lie between 6.0 and 7.0 percent of their ovendry weight. Further con- 

 ditioning of the needles to bring them into equilibrium moisture content with the environment in 

 which they would be burned was not necessary because the entire fuel bed was to be coated with 

 a moisture -laden retardant. 



Pine needles were distributed over each fuel bed to a loading density of 0.5 pound per 

 square foot. Thus, each 18- by 24-inch drying pan contained 1.5 pounds of needles, and each 

 18- by 96-inch burning tray contained 6 pounds. To build reproducible 8-foot-long fuel beds 

 with uniform compactness, we followed Schuette's 4 published instructions. A similar procedure 

 was followed in preparing the 2 -foot-long drying pans and the 3 -foot-long "igniter" fuel beds. 



Environment 



All testing reported here was performed in the laboratory's large wind tunnel. Test ob- 

 jectives specified three variations of environment for both drying and burning tests; to achieve 

 these, we combined two relative humidities and two wind velocities at a temperature of 90° F. 

 (table 2). 



Table 2 . --Environmental conditions 



Wind velocity 



Height above fuel 



1 foot 



20 feet 1 



National spread 

 index 

 equivalent 2 



M.p.h. 



2 

 2 

 5 



±0.25 



6 

 6 

 15 



36 

 40 

 68 



Condition 



: Temperature : 



Relative 

 humidity 





Degrees F. 



Percent 



I 



90 



50 



II 



90 



20 



III 



90 



20 



Tolerance 



±1.0 



±1.0 



A 3-to-l difference in windspeed between the fuel surface and an anemometer at 20 feet is 

 assumed. This value may change drastically according to the boundary layer created by the 

 surface vegetation. 



2 National Fire-Danger Rating System, Fine Fuel Moisture- -Cured Herbaceous Stage, U.S. 

 Dept. Agr. Forest Serv. Form 5100-24 (2/64). 



CHEMICAL APPLICATION 



Essential Features 



Immediately before impact with the fuel, physical characteristics vary greatly among 

 retardant formulations --even within a specific drop of a single retardant. Viscosity may be 

 much less while a retardant is falling through the air than when it is at rest; this change 

 in viscosity influences droplet size, both in average diameter and in range of diameters; in 

 turn, the droplet's velocity is affected. All these factors, along with the total amount applied, 

 affect the retardant's penetration into the fuel bed. The following six essential features were 

 incorporated into the application technique to produce a retardant drop pattern that would most 

 closely simulate actual drop conditions: 



Schuette, op. cit., p. 3. 



4 



