Layers of paper of different thicknesses (d) were used as vent covers to seal the vent; for each paper, the time of paper rupture was determined from the interruption of an electrical circuit by breakage of a thin metal strip fixed on it. The vessel was evacuated by a vacuum pump and then filled with a methane-air mixture with an equivalence ratio (f) of 1.4. Later, the ignition unit, high-speed cameras and oscilloscope were triggered simultaneously using a transistor-transistor logic (TTL) signal outputted by a signal synchronizer to ignite the methane-air mixture and to record the experimental data. For the experiment with no vent cover, the paper used to seal the vent was removed just before ignition. In all experiments, the initial pressure and temperature of the methane-air mixture were 1 atm and 300 K, respectively.