Could the information contained in NFPA 921, Minimum Ignition Energy (mJ) for specific gases be misleading. Table 23.8.1 give specific values in mJ for the minimum ignition energy for each of the gases listed. A number of investigators have used these values as absolutes numbers when stating what the minimum ignition energy (mJ) would have been required to have caused ignition on a specific incident. If the person reaching that conclusion does not know the specific concentration from which the posted value was obtained and does not know the specific concentration of the gas at the incident at the time of ignition, how then can a conclusion be reached using the values listed in NFPA 921.

What NFPA 921 does not do is to state these values were based on specific test with specific concentrations. ASTM, when discussing the precision of the test method states the following; “Available indications are that reproducibility (and presumably accuracy) of +/- 10 % in minimum ignition energy and +/- 2% in ignition quenching distance arc readily achieved in those instances where results are relatively independent of fuel-oxidizer concentration. For mixture compositions approaching the limits of flammability the minimum ignition energy varies so rapidly with mixture composition that reproducibility of the test mixture becomes the limiting and controlling parameter (it is possible that a 0.1% variation in fuel concentration could change the ignition energies by factor of 100 to 1000).”

Jim Mazerat
Forensic Investigations Group