The standard dust explosibility test is performed in a 20-L vessel with either one or two 5 kJ pyrotechnic igniters. The dust is deemed to be explosible if the ratio of the maximum deflagration pressure to the initial pressure exceeds some threshold value. This type of test is widely accepted and used. However, marginal dusts may be over driven in the 20-L standard test and yield a false positive result (i.e., indicate that the dust is explosible), even when such a dust is not capable of forming a dust cloud through which a flame would actually propagate any significant distance. This can be avoided by testing such dusts in a larger vessel, where the flame must propagate over a more reasonable distance in order to develop a maximum pressure sufficient to classify the dust as explosible. This article reports on urea dust testing where this type of result was obtained, but the approach taken in this work is applicable to other dusts as well. (c) 2013 American Institute of Chemical Engineers Process Saf Prog, 2013