Fire Safety Science Digital Archive

The effect of varying the moisture content on bath the pilot and spontaneous ignition times of different woods of areas 7.6 cm square and 15 cm square has been measured over a wide range of intensities of radiation. Moisture increases the energy required for ignition; it also increases the minimum intensity for ignition though with pilot ignition its effect is only marked for moisture contents above 40 per cent. Results have been correlated on the assumption that the material is inert and ignites at a fixed temperature. Simple heat transfer theory has been used to calculate this temperature with values for the thermal properties appropriate to the given moisture content and with allowance made within the term for thermal capacity for removing the water; the effects of moisture migration have been neglected, following the results of Williams (7). For pilot ignition the correlating temperature is found to be 380C, corresponding to a critical intensity of 0.31 cal cm^-2s^-1, except for fibre insulating board which appears to ignite at a somewhat lower temperature of 330°C. Earlier experiments, with smaller specimens, gave a similar result, of 360C, with the results for fibre insulating board included. The present correlation extends to much longer times (up to 59 min). The results show that the choice of 0.3 cal cm^-2s^-1 as the maximum acceptable level of radiation for building regulation purposes, gives a larger margin of safety than was originally thought. For spontaneous ignition the correlating temperature is found to bo 545°C, the same as found previously for smaller areas, corresponding to a critical intensity of 0.74 cal cm^-2s^-1. The present correlation extends to much longer times (up to 16 min) and the results suggest that the empirical correction necessary for the area effects is linked with density.