first if the cool down chamber is not pumped during cool down the amounts of argon and oxygen initially pulsed into the cool down chamber determine the argon and oxygen partial pressures in the cool down chamber throughout cool down one embodiment is illustrated in the schematic diagram of fig8 of a gas supply system to the cool down chamber 80 argon is supplied from an argon line 132 and its flow is metered by a manual needle valve 134 and gated by an electro pneumatic valve 136 similarly oxygen is supplied from an oxygen line 138 and its flow is metered by a manual needle valve 140 and gated by an electro pneumatic valve 142 the outputs of the electro pneumatic valves 136 142 are supplied into the cool down chamber 80 the electro pneumatic valves 136 142 each include two stages of valves a first valve typically actuated by an electrically driven solenoid gates the supply of clean dry air cda supplied from a clean dry air line 144 through a gate valve 146 a second valve actuated by the gated clean dry air opens and closes the flow of the argon or oxygen through the electro pneumatic valve the electro pneumatic valves 136 142 themselves perform no effective metering a controller 148 issues electrical control signals to open the supply of clean dry air through the cda gate valve 146 and to open and close the two electro pneumatic valves 136 142 at known argon and oxygen pressures the.