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I learned that Apollo missions used a simple system providing 100% oxygen atmosphere in the cabin at lower air pressure from the question, "Decision factors for using 100% O2 cabin atmosphere in early US Space Program".

Atmospheric systems have evolved much since then. See: Air temperature and humidity inside the ISS.

Does anyone know what the approximate Apollo cabin temperature and humidity was kept at? I am curious if the unusual 100% O2 atmosphere affected other aspects of the cabin air.

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    $\begingroup$ I think an excellent follow-up question would explore how the humidity was maintained; did the humans reliably put out so much humidity during respiration that only a dehumidifier was necessary to maintain this range? $\endgroup$ – uhoh Feb 26 '18 at 10:14
  • $\begingroup$ A man at rest exhales about 10 litres of breathing gas per minute with 100 % humidity at about 37 °C, That is pretty much water from 3 astronauts in several days. $\endgroup$ – Uwe Feb 26 '18 at 11:30
  • $\begingroup$ I always thought that later Apollo missions switched completely away from 100% O2 because of fire hazards, but indeed you are correct that after launch they switched to 100% O2. $\endgroup$ – jpa Feb 26 '18 at 12:43
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    $\begingroup$ @uhoh: I have posted a follow-up question as I was curious about the environmental mechanisms used as well: space.stackexchange.com/questions/25742/… $\endgroup$ – Dan Sorensen Feb 28 '18 at 7:09
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From this NASA report:

The design range for temperature and humidity control in the Apollo Command Module was 294° to 300°K (70° to 80°F) [i.e., 21 to 27 °C] with a relative humidity of 40 to 70 percent. Similarly, the design range for the Lunar Module was 291° to 300°K (65° to 80°F) [i.e., 18 to 27 °C] with a relative humidity of 40 to 70 percent.

See also 'Table 1 Command Module Cabin Temperatures in °K (°F)' of this document with values for launch, average, range and reentry of all missions.

Crew comments indicated that the Command Module was uncomfortably cool during several missions, especially during sleep periods. These occurrences were not serious problems and crewmen compensated by increasing their clothing insulation.

During the Apollo 13 mission, the LM environmental control system provided a habitable environment for approximately 83 hours (57:45 to 141:05 ground elapsed time). Cabin temperature remained low due to low electrical power levels. This caused crew discomfort during much of this time, with cabin temperatures ranging between 283° and 286°K (49° and 55°F) [i.e., 10 to 13°C].

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    $\begingroup$ @JCRM: Kelvin aren't degrees. I know that, but somebody at NASA did not. I think temperatures were measured in Fahrenheit and not in Kelvin, therefore it is better to convert from Fahrenheit to Celsius, not from Fahrenheit over Kelvin to Celsius. $\endgroup$ – Uwe Feb 26 '18 at 10:26
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    $\begingroup$ The temperatures appear to have been reported with a precision of 5 °F, or 1 K. Converting from F to C introduces precision that doesn't exist, by converting from K to C one maintains the level of precision in the original. $\endgroup$ – JCRM Feb 26 '18 at 10:55
  • $\begingroup$ It may well be spurious precision for K in the original, but we've not introduced it. $\endgroup$ – JCRM Feb 26 '18 at 11:01
  • $\begingroup$ The temperatures do not appear to have been reported with a precision of 5 °F, see the values for reentry in Table 1, there are values of 57, 58, 59, 61, 62 and 67 °F too, not only values like 55 and 60 °F. $\endgroup$ – Uwe Feb 26 '18 at 11:30
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    $\begingroup$ @Uwe - You are correct that the modern conventions are to use "kelvin" (lowercase as a temperature unit; uppercase referring to the Kelvin scale) or "300 K" instead of writing "degrees Kelvin" or "300 °K". But the style before the 13th General Conference on Weights and Measures (1967-68) was to write °K. As the Apollo command module was designed and first launched (Feb 1966) before this conference, the quoted specifications used the old conventions. $\endgroup$ – dr jimbob Feb 26 '18 at 15:42

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