Figure 11. --Effect 

 of total moisture 

 on rate of spread. 



FUEL & RETARDANT MOISTURE AT TIME OF IGNITION 

 (PERCENT OF DRY FUEL) 



The effect of total moisture upon rate of spread is shown in figure 11 . Each point is the 

 average of three or more tests. The spread in data is a result of burning in three environmental 

 conditions. Total moisture is shown as a percent of the dry fuel weight. Rate of spread is 

 shown as a percent of the rate that would occur in an untreated fuel bed at 6-percent moisture 

 content. A line showing the relation between rate of spread and fuel moisture content in un- 

 treated fuel beds is shown for comparison with the data points. Data for this line were obtained 

 from previous study of untreated beds. One -hundred-percent rate of spread was taken at 6- 

 percent fuel moisture, and any other rate along the line is based on the 6-percent value. The 

 comparisons of fire retardants were made on this same basis. Note that the line passes through 

 some of the short-term retardant points and intercepts the fuel moisture coordinate at 22 per- 

 cent, which was the moisture level used to calculate the effective holding time of short-term 

 retardants in equations 6 and 7. 



Refer to figure 11 and note that the short-term retardants are designated by solid symbols 

 and long-term retardants by open symbols. The shape of the symbols designates whether 1,2, 

 or 3 gal. /1 00 sq.ft. were applied initially. Some interesting conclusions can be drawn from 

 studying figure 11. Data points that fall below 20 percent of the untreated rate of spread were 

 characterized by smoldering combustion; they burned deep in the fuel bed where the retardant 

 had not penetrated. Fuel beds treated with short-term retardant required total fuel moistures 

 greater than the 22-percent fuel moisture limit to suppress the flaming surface fire. When 

 total fuel moisture of the short-term retardant -treated fuel beds was less than 22 percent, the 

 rate of spread began to follow the flaming combustion line of untreated fuel beds at the same 



" Rothermel and Anderson, op. cit., p. 3. 



24 



