The results of the ignition tests are summarized in Figure 2. From the figure it is obvious that the
mixtures of Table 1 will always ignite above 500 °C. At pressures less than 5 MPa they will most
probably never do so at temperatures below 400 °C, irrespective of their oxygen concentration. In the
range 400 °C ≤ T ≤ 475 °C the ignitability depends both on pressure and on oxygen concentration, the
ignition probability being highest at high O2 concentration and high pressure. There are conditions, which
during one test may lead to a genuine ignition while in another test there is an elevated wall temperature
for a prolonged period of time but no ignition. Hence, from the point of view of operational safety only events marked by blue symbols should be considered as non-ignitions. Doing so, oxygen concentrations
below 3 vol% are considered to be safe.