Man has sensuously supposed that the electronic circuits do not have enough energy to ignite inflammable gases, when electric sparks are produced because supply voltages of quartz watches are as low as, usually less than 3 V at the dry cell.
Since research work on the intrinsical safety properties of electric circuits in quartz watches has not been reported until now, we report the results of an investigation was carried out to find out whether or not fundamental circuits of quartz watches which have been produced up to present time are sufficiently safe in methane-air mixture.
The summary of results is as follows:
(1) As intrinsical safety properties of dry cells in methane-air mixture are shown in Fig. 1, so it became clear that the common type of dry cell in each quartz watch is safe in methane gas.
(2) Though we could not examine the detail of inner circuit of quartz watches because of manufacturers' secret, the possibility is assumed to be very low that methane-air mixture happens to ignite by breaking of wire or a short circuit.
(3) Explosion tests on crashing of the illuminating lamp installed on liquid crystal panel in various quartz watches were carried out for 200 pieces of 1.55V-13mA lamp, 200 pieces of 2.8V-8.5mA lamp and 70 pieces of 3.0V-6.5mA lamp, but none of them could ignite 7-9% methane-air mixture.