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This report describes the explosion of a Salvus breathing apparatus in the Derbyshire County Fire Brigade and an investigation into the possible causes of this and similar explosions. Since all reported explosions appear to have occurred immediately upon opening the main valve of the apparatus, it is suggested that those which were not due to the failure of defective Bourdon tubes in the pressure gauges, are likely to have been due to the heat generated by the sudden compression of oxygen in the high-pressure side of the apparatus. It was found that this was sufficient to melt a heat-sensitive paint of melting point 232C, and it was shown that the rate of compression, and hence the maximum temperature attained, could be reduced by fitting a spigot in the centre of the valve seating, provided that the diameter of the spigot did not differ from that of the hole by more than 0.001 in.; even so the temperature rise was sufficient to melt a lacquer which fused at a temperature of 176Â°C. It is shown that the materials of the washers in the high-pressure side of the apparatus present an explosion hazard and it is recommended that an attempt should be made to replace these washers by ones which are incombustible in high pressure oxygen or alternatively by using metal seatings.