Figure 3. --Instrumented transverse and longitudinal fuel members as they 

 appear in the fuel crib array. Fire spread is from the left. 



least 0.6 cm. before coming through the back or bottom of the fuel member. Thus, the 

 junction and the immediately adjacent leads lie in an isothermal surface minimizing 

 heat transfer to or from the junction along the leads. The temperature field is 

 captured by simultaneous measurements at the moment of ignition. (The scan time is 

 extremely short compared to the time of the event.) Since only a limited number of 

 thermocouples could be placed in the transverse fuel member, the highest density was 

 concentrated in the region of the steepest temperature gradient, the upper corner 

 nearest the advancing fire (fig. 4) . Therefore, isotherms generated from the temper- 

 ature field outside the upper corner are less reliable. 



Applying the same instrumentation procedure to the longitudinal fuel member 

 would require an extensive array of thermocouples parallel to those located in the 

 transverse members. The alternate method adopted was based on the average rate of 

 spread of the thermal wave that travels along the longitudinal member. From the 

 average rate of spread it is possible to convert temperature variations with time 

 to temperature variations with distance. Thus, it is only necessary to determine the 

 temperature variation with time at various prescribed vertical depths in the fuel 

 member. The temperature distributions at each depth can be interrelated through the 

 ignition time. Small holes were drilled to varying depths from the bottom (fig. 2) . 

 Chromel-alumel thermocouples were inserted the full depth of each hole and plugged 

 with wooden dowels. There is no appreciable temperature gradient normal to the 

 vertical plane determined by the thermocouples. The velocity of the thermal wave was 

 obtained by superimposing the temperature histories of the two top thermocouples and 

 noting their shift in time and separation. 



9 



