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Wikipedia claims that space suits that use pure oxygen are pressurized to 4.7 psi, instead of 3.0 psi, to account for carbon dioxide and water vapor pressure as per the alveolar gas equation (although noting that the calculation has slight overcorrection).

NASA EMU is pressurized to 4.3 psi according to many sources. The rationale given for this is the same: to give the astronaut the same amount of oxygen as for normal air per the alveolar gas equation.

As for others, A7L Skylab space suits were pressurized to 3.7 psi. Russian Orlan space suits are pressurized to 5.8 psi.

However, none of these values seem to fit when actually viewing the alveolar gas equation. If breathing pure oxygen, the alveolar gas equation becomes simply:

$$\require{mhchem}p_A\ce{O2} = P_{atm} - p\ce{H2O} - p_a\ce{CO2}$$

where $p\ce{H2O}$ is 6.28 kPa at 37 °C and $p_a\ce{CO2}$ is normally 40 mmHg. Given that 104 mmHg is considered normal for $p_A\ce{O2}$, this is achieved with a pressure of 3.7 psi for pure oxygen.

So why 4.3 psi specifically? Are there benefits to having a slightly hyperoxic environment? Is this to allow for slow leaks or partial depressurization without the astronaut becoming hypoxic?

I will obviously be very happy to receive corrections if I have made a mistake in my calculations or if there's some physiological phenomena here that I don't understand. I would love to get a proper answer from someone who actually knows the real reason, but forgoing that I would be thankful for any well-founded arguments as to why this might be so.

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This is what Kenneth S. Thomas and Harold J.McMann have to say about it in U.S. Spacesuits:

Operating pressures:

The Shuttle extravehicular mobility unit (EMU) has an operating pressure of 4.3 psi (30 kPa) and the Shuttle crew escape/launch/entry suits operates at a maximum of 3.5 psi (24 kPA). All Russian spacesuites, in comparison, operate at 5.8 psi (40kPa) to minimize or avoid decompression sickness or other risks.

Decompression:

U.S. Studies have indicated that people could rapidly decompress from 14.7 psi (1 atm) to 8pis (55 kPa) with minimal risk. Similar Russian studies viewed decompression sickness risk slightly differently. From 14.7 psi, a russian suit pressure of 5.8 psi (40 kPa) is judged to be sufficient to avoid decompression sickness after half and hour of breathing pure oxygen, which is approximately how long it takes to perform a suit checkout before going out to do an extravehicular activity. As a result, all Russian spacesuits feature a 5.8 psi operating pressure.

So as well as the specific job that the suit is designed for, it seems that studies about decompression have largely guided these operating pressures.

Some of the other variables will be down to maneuverability, comfort, fatigue and cost. As long as the variables are within safe limits, then there is room to move with other factors in mind.

Earlier spacesuits had lower pressures as described below:

Humans require environmental parameters to be within prescribed limits for comfort and to effectively perform work. One significant parameter is oxygen concentration. In a sea level atmosphere of 14.7 psia, the oxygen partial pressure is 3.08 psia. This results in an oxygen partial pressure with the alveoli of the lungs of 2.0 psia. NASA selected this as the lower limit of alveolar pressure for nominal human space operations. To maximize spacesuit joint mobility and to minimize leakage and loads of the pressure suit, spacesuits are designed to operate at the lowest pressure consistent with other requirements. Hence all spacesuits systems provide a breathing atmosphere of 100% oxygen (discounting small amounts of carbon dioxide and water vapor). However, because breathing efficiency decreases as pressure decreases, the normal operating pressure for U.S. spacesuits in the 1960s and early 1970s was established at 3.7psi.

There was a competitive review between systems and in February 1990 the decision was made to use 4.3 psi. The main reasons were to be able to support both exrtavehicular activity (EVA) and provide more effective crew escape and survival in intravehicular emergencies.

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  • $\begingroup$ Thank you! Excellent resource! Found by Google: "The solution was instituting a reduction of cabin pressure from the normal 14.7 psia (1 atm) to 10.2 psia (70.4 kPa) 24 hours before an EVA and an increase in the EMU operating pressure to 4.3 psi (30 kPa). The EMU pressure had to be raised by this small amount to assure an acceptably low risk of DCS. The cabin pressure couldn't be reduced any further than 10.2 psia (70.4 kPa) due to flammability concerns arising from the reduction of nitrogen, assuring that no less than about 3.08 psi (21.2 kPa) of oxygen was available for breathing. $\endgroup$ – Nakedible Jan 5 '16 at 18:48
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    $\begingroup$ The above paragraph seems to directly give the answer why exactly 4.3 psi (flammability risk in cabin + DCS risk in suit), which is exactly what I was looking for. The answer also validates my math, so I am very happy. $\endgroup$ – Nakedible Jan 5 '16 at 18:57
  • $\begingroup$ Thanks for finding the right paragraph. I thought it was in there but couldn't find it and didn't want to put something in the answer without being sure. Glad it helped. $\endgroup$ – Phil_12d3 Jan 5 '16 at 20:19
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Apparently, it's because putting a person in the suit prevents the atmosphere from being pure O2. The Space Shuttle Flight Rules (Rule A13-53) state:

ONE-GAS ENVIRONMENT (EMU) - IF THE PRESSURE IN THE SUIT FALLS BELOW 3.15 (3.3) PSI, THE EVA CREWMEMBER WILL ABORT THE EVA. ®[050400-7197A]

At 92 percent O2, 3.15 (3.3) psia corresponds to a pressure altitude of 8000 feet and the rationale of paragraph A.1 applies. Exhaled nitrogen from the crewmember dilutes the oxygen atmosphere in the suit to 92 percent.

Breaking the code here, it says that the real limit is 3.15, with instrumentation error it's 3.3, and the canned crewmember exhales enough N2 to dilute the suit atmosphere down by 8%.

It doesn't say specifically, but if 3.3 is the working limit, 4.3 sounds like a nice 1 psi margin over that.

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  • $\begingroup$ I assume that 92% $O_{2}$ would be toxic if the pressure inside the suit were at 1 atm (~15 psi), right? $\endgroup$ – honeste_vivere Jan 4 '16 at 15:55
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    $\begingroup$ I think so. There is a paragraph in the linked document, a few pages ahead of the block of text I quoted, that talks about O2 toxicity at higher pressures, but since it was in reference to the shuttle cabin atmosphere and not the EMU, I didn't really read it. Take a look if you like in the Flight Rules I linked to. $\endgroup$ – Organic Marble Jan 4 '16 at 17:01
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    $\begingroup$ Actually it was in the next rule, sorry, A13-54: Breathing 100 percent oxygen at sea level pressure will result in physiologic changes due to oxygen toxicity after 6 hours minimum. Since oxygen toxicity is proportional to the oxygen partial pressure and not the percent gas, longer exposures of 100 percent oxygen can be tolerated at lower pressures. For 100 percent oxygen at sea level, physiologic changes begin as mild (2 to 5 percent) asymptomatic decreases in vital capacity (the largest volume of air which can be exhaled voluntarily) beginning with fully inflated lungs. $\endgroup$ – Organic Marble Jan 4 '16 at 17:04
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    $\begingroup$ This, like many flight rules, may be extremely conservative since most EVAs are preceded by a long "prebreathe" on pure O2 designed to expel N2 from the crewmembers' bodies. In this nominal case I wouldn't expect much N2 to be exhaled, but the rule is presumably written to cover the case of an emergency EVA where a long prebreathe wasn't possible. $\endgroup$ – Organic Marble Jan 4 '16 at 20:32
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    $\begingroup$ Actually yes, I just verified the same from ISS handbooks. There is a long camp-out at 10.7 psi, lots of breathing pure oxygen and finally 15 minutes long EMU nitrogen purge sequence where suit is vented and fed with pure oxygen. Considering the human body contains about 1 liter of dissolved oxygen, after all this is done, there can't be much nitrogen left in the blood stream to change the suit atmosphere - certainly not 8% in the normal case. $\endgroup$ – Nakedible Jan 4 '16 at 21:24

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