Gas supply

Two, 9 cf, 2100 psi steel gas cylinders were used for O2 and inert gas. These were lightweight models FAA certified for use in aircraft. Initially we used chrome plating to protect them, later we went to teflon coating. Beckman liked the more military look and there was some concern over possible hydrogen embrittlement from the chroming process.

Standard, old style, "K" valves were used as cylinder valves. On the inert side a SCUBA regulator-type yoke was used to mount a high pressure 1/8" NPT needle valve operated by rotary action of a T-shaped handle. Inert gas was valved in manually directly from the tank as needed using this valve. In use it had a very smooth precise feel. Inert gas was valved into the plenum at the bottom of the absorbent canister so that some mixing would take place before it got to the sensors.

On the O2 side a piston type first stage of a U.S. Divers single hose regulator was used to reduce tank pressure to about 60 psi. This is somewhat lower than such first stages normally deliver and was achieved by using a weaker piston spring. The normal hose to the second stage was used to connect the O2 supply to the solenoid valve. The octopus port of the first stage was used to attach an O2 bypass valve. This was a spring action, lever activated low pressure valve and it was protected by an enclosure which required opening a spring closed cover to get at the valve. The manual bypass valved O2 directly into the sensor compartment so the result was immediately readable.

I will digress briefly on O2. In addition to the physiological risks recently discussed in some detail on the list there is also the danger of fire and explosion. Valves, regulators, fittings and any other equipment used for O2 have to be thoroughly degreased of any petroleum based lubricants. If lubrication is required, as for example with the o-ring seal of a regulator piston, non-combustible silicone based lubricants must be used. Be aware that even a fingerprint oily with suntan lotion can start an explosive fire with O2. Once an O2 fire starts all sorts of things you might not ordinarily think of as combustible burn ferociously. I have heard stories of chamber fires in which everything inside, including the occupants, was reduced to ash.

My partner Kanwisher was on one of the advisory panels to NASA in connection with the Apollo program. Although he recommended using a mixed gas atmosphere in the Apollo capsule he was over-ridden by the engineers who felt that monitoring the PPO2 was too difficult. John knew better as he had been doing it for several years in conjunction with his work on respiration but the engineers prevailed. The result was the fire which killed three astronauts.

The solenoid valve we used was a miniature 12 volt one made for pneumatic control. We equipped it with a miniature screw adjusted needle valve outlet. When the setpoint is reached and the solenoid is triggered it takes perhaps three or four seconds for the sensors to respond and rise enough to cut it off again. The solenoid needle valve was adjusted so that the O2 injected raised the PPO2 to a peak pulse of about 0.75 Atm and would usually trigger a couple of beeps from the audible alarm. Within a couple of breaths mixing brought the level back to perhaps 0.65 after which it dropped more slowly as it was consumed by metabolism until the setpoint was reached again after about a minute or so. That would be for moderate activity such as easy swimming. At complete rest it would of course take longer to drop back to the set point and less time if you were actively swimming.

If the needle valve was adjusted to a lower flow rate solenoid activation would be more frequent and of longer duration placing an unnecessary drain on the solenoid batteries. If much higher flow was adjusted for the O2 spikes would be too high and the alarm would be sounding much of the time. I think there are now smaller, more power efficient solenoid valves available.

The solenoid and manual bypass valves were of the downstream type so that if high pressure leakage from the regulator occured it would release when it reached the level where it overcame the spring tension which normally closed the valve. This is important to prevent either valve lockup or blowing out the supply hose in the event of a high pressure leak. In the event of O2 leakage from either valve the cylinder valve could be used to cut it off.

The gas cylinders were mounted on either side of the central larger cylinder containing the absorbent canister and electronics section. This assembly was worn as a back pack with the valves at the bottom at hip level. Inert gas had to be added several times on descent and at other times if you lost any from nasal exhalation. Manual O2 was normally only used in decompression. The inert gas valve was therefore on the divers left side leaving the right hand free for more complex tasks. Swapping sides for southpaws would have been easy but I don't recall anyone ever raising the question. It was no big thing either way.