The original break spark test apparatus for intrinsically safe circuits was modified to allow the measurements of minimum ignition currents (MICs) at different initial pressures between 20 and 120 kPa. The MICs of different propylene/air mixtures at ambient temperature and at both atmospheric and sub-atmospheric pressures were measured. The corresponding minimum ignition energies (MIEs) using break sparks were calculated and compared with those derived from MIE/quenching distance correlations using high voltage sparks between flanged electrodes. (C) 2003 Elsevier Ltd. All rights reserved.