w f (.22-M f ) - 1.96 exp 



0.0307 U(py -pvjjt" 



(7) 



where Mf = fuel moisture content, ratio of moisture to dry weight of fuel 



Wf = fuel loading dry weight, tons/acre 

 G = initial retardant concentration, gal. /1 00 sq.ft. 



U = windspeed at 1-foot level, m.p.h. 

 P Vq = vapor pressure at surface of retardant, in Hg 



p v = partial pressure of water vapor in air at ambient dry bulb temperature 

 1 and humidity, Py = (relative humidity) X (saturated vapor pressure), 

 in Hg 



t = effective time of retardant, minutes 

 Equation (7) may be rearranged to solve directly for effective holding time of the retardant. 



w f ( .22-M f ) 



G o l0 S t 



1.96 G 



o 



t = 



•0.0307 U(p v -p v ) 

 o 1 



(7.1) 



where log e indicates natural logarithm or logarithm to the base e. Equation (7.1) can now be 

 solved directly for length of holding time if the environmental conditions and fuel conditions are 

 known or can be approximated. As an example, assume: 



M f = 0.05 lb. /lb. 



wf = 14.5 tons/acre, dry weight of fuel 



G„ = 



U = 



3 gal./lOO sq.ft. , case 1 

 2 gal./lOO sq.ft. , case 2 



5 m.p.h. at 1-foot level 



T amb = 90 ° F - 



RH = 50, 20, 10, and 5 percent 



Substitution in equation (7.1) gives effective retardant durations, which are plotted in figure 10, 



Limitations of Short-Term Retardant Effective Time Equation 



Equation (7) is an empirical equation that was developed for fine dead fuels arranged in a 

 random fuel bed. No consideration has been given for the increased drying rate which would 



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