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Apollo spacecraft used a pure oxygen atmosphere while in space and an atmosphere with 60% oxygen at launch time. This chapter describes how the atmosphere composition varied from 60% to 100% oxygen in the firsts hours of flight. This can be seen on this graph. While getting back to Earth, the inside atmospheric composition should haveequalized with what the outside atmosphere would have been when the hatch was opened. I imagine this change must have been progressive.

How many times did it take to change this composition from 100% of oxygen to the Earth atmosphere levels (about 21% of oxygen)?

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The Command Module repressurized during entry when a negative pressure relief valve (on Apollo, called Cabin Pressure Relief Valve) cracked and allowed ambient atmosphere to flow in.

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In the following graph, you can see the CPRV opening at ~125 seconds and then being moved to the Dump position at ~175 seconds. Shortly after Dump positioning, the cabin pressure starts to track the ambient pressure closely. The pressure continues to equalize and attain ambient atmospheric composition until the valve is closed for water impact at 800 feet (~410 seconds).

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Source CSM/LM Operational Data Book

These drawings show the location of the control valve on panel 325.

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Source: Apollo Operations Handbook Controls and Displays (annotation mine)

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    $\begingroup$ Wooo! less than 7 minutes to go from 100% of oxygen at 5psi to 21% of oxygen at 14psi, it seems really fast (and even more when considering time taken for atmosphere changes at launch) $\endgroup$ – Manu H Apr 21 at 14:55
  • $\begingroup$ @ManuH spacecraft typically aren't very strong in the "crush" direction so they have to limit the negative delta-p, shuttle had a similar set-up. $\endgroup$ – Organic Marble Apr 21 at 14:57
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    $\begingroup$ If air just flows in only, 21 % oxygen could not be reached. 5 psi of 100 % oxygen plus 9 psi of air would result in 7.11 psi partial pressure of nitrogen. That is 51 % nitrogen and 49 % oxygen. A lot of air flowing in and out is neccessary to acheive 21 % oxygen. You need to wash out excess oxygen. $\endgroup$ – Uwe Apr 21 at 15:37
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    $\begingroup$ After splashdown, "the cabin is ventilated with ambient air by postlanding ventilation fan and valves" More mixing probably took place then. $\endgroup$ – Organic Marble Apr 21 at 15:56

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