The Tests
The tests, performed at a racetrack, involved seven two-third-ton lumber fuel cribs. Optimum crib size was determined by fire scaling tests in 1994. Sixty eight-foot-square, seven-layer fuel cribs were built of 2 ¥ 4 spruce-pine-fir lumber, according to the Underwriters Standard 711 pattern. Fire tests began in September 1995, after crib moisture content was determined by testing to have dropped to six percent.
Each crib was ignited by 74 pounds of heptane in an eight-foot-square ignition pan under the crib. The heptane burned out in about two minutes. The crib was attacked four minutes after ignition. The nozzle was eight feet above and 16 feet in front of the crib. To reduce variation in the method of application, the crib top was attacked with a constant flux rate of the suppressant under test. The total nozzle sweep time was 22 seconds.
Twenty seconds after the first attack ended, the rekindled fire was attacked again in the same manner as above. (Rekindle time is the time from the end of the nozzle sweep until flame reappears at the top of the crib.) If knockdown was not achieved, the flow rate was below the critical flow rate (CFR)–the flow rate below which fire cannot be knocked down. The rekindled fire was attacked for a third time 20 seconds later. The third rekindle was attacked with various foams to estimate their CFR.
