Residence Time

Residence time is the time (seconds or minutes) required for the active flame zone to pass a stationary point at the surface of the fuel. Numerically, it is the flame depth divided by the rate of spread. Residence time can be measured easily and cheaply by visual observation, or using video analysis. A more accurate, and more expensive, way to measure residence time is to analyze time-temperature curves obtained from thermocouples.

Residence time is influenced by fuel size. Fuel particle size has a linear relationship to residence time (Anderson 1969). Small fuels with high surface-to-volume ratios burn quickly and have low residence times. As fuel size increases, residence time increases (Wade 1986). A rule of thumb for woody fuels with moisture contents ranging from 4-10 percent is that the residence time in minutes will be approximately 3 times the fuel thickness in centimeters (Chandler et al. 1983). For example, a stick 10 cm in diameter will continue to flame after ignition for about 30 minutes.

Residence time tends to be a better predictor of basal stem damage than either fireline intensity or flame length (Wade 1986). Heat damage to plant tissue depends on both the maximum temperature and the length of exposure (Alexandrov 1964). Since flame temperatures are relatively similar, approximately 1400^o F, and are independent of intensity and fire type, the length of exposure to flames becomes a critical factor in the extent of tissue damage (Wade 1986).