This article discusses flammable limit test methods with reference to the past 200 years of research. Critical examination is made of the various pressure rise criteria currently used to determine whether flame propagation has occurred in closed test vessels. Of these, the most appropriate, with respect to vessels of 5-20 liters volume, is judged to be the “net 7%” pressure rise criterion described in ASTM E-2079. Since the tests are typically carried out in spherical vessels with central ignition, only a small fraction of the limit mixture burns and consequently a small pressure rise criterion must be used. Also, it is impossible to determine the product distribution for near-limit flames. A small pressure rise criterion may cause the flammable range to be overestimated, particularly at the upper limit. For non-routine and standardization purposes at least, use of a tall, large diameter, cylindrical vessel with bottom ignition is suggested.

European test methods are creating a database of flammable limit values outside the range of compositions at which flames can self-propagate. The objective of flammable limit measurement should be to obtain close agreement with large-scale observations and, ultimately, with theoretical models for self-propagating flames. It is recommended that “flammable limits” measured using DIN 51649 and European Standard prEN1839 (or closed vessel methods striving to obtain values consistent with these) not be mixed with the existing database because they correspond to fuel concentrations that do not support flame propagation in any orientation.