coupled in series with relay contacts 68 the coil of valve 36 and the power source similarly relay 48 includes a coil that is actuated by a signal commanding the flow of the o 2 gas the coil of relay 48 controls a normally closed set of relay contacts 58 and a normally open set of contacts 62 relay contacts 58 are coupled in series with the relay timer 54 and the power source contacts 62 are coupled in series with relay timer contacts 66 the power source and the actuating coil of metering valve 34 contacts 66 are controlled by the relay timer 54 while contacts 68 are is controlled by the relay timer 56 in operation a control signal delivered to relay 46 commanding the commencement of flow of the n 2 h 2 gas mixture energizes the coil of relay 46 causing the normally closed relay 60 to open and the normally closed relay 64 contacts to close at this point relay 48 is de energized consequently normally closed contacts 58 remain closed and relay timer 54 is coupled with the power source causing contact 68 to close thus coupling the electromagnetic valve 36 with the power source with the coil of valve 36 energized this valve is opened thereby.