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I'm curious about the pressurization of a full pressure flight suit (such as). For example, the linked suit has an operating pressure of 3.5 psi, but my assumption would be that during normal flight operation the suit and cabin would be at the same pressure.

Is this accurate, and if so, what happens if the suit is exposed to a vacuum? Or perhaps more to the point, what is the maximum pressure differential that the suit may be subjected to, assuming the cabin is at 1 atm and suddenly depressurizes?

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For the ACES suit that you show in the question, there is an integral pressure controller that determines what level the suit is pressurized to.

...pressurization begins when the cabin pressure altitude rises to around 35,000 feet and is “hard pressed” (3.67 psia) above 60,000 feet.

Note that your assumption that when the cabin altitude is sea level, there is no pressurization, is correct.

Source: SCOM, page 2.10-4

Rough equivalent pressure for the cabin altitudes given are 3.5 psi (35,000 ft) and 1 psi (60,000 ft).

Source: Air pressure at altitude calculator

The maximum delta pressure between the suit and ambient pressure is controlled by a positive pressure relief valve (PPRV) that is part of the suit controller. This valve cracks at 5.5 psid and closes at 3.5 psid. This valve keeps the suit from becoming too rigid if, for example, the suit is closed up with a 14.7 environment inside and the cabin pressure subsequently falls. The PPRV vents gas to maintain the proper delta pressure and prevent the suit from swelling up.

I don't have a great reference for this (personal notes) but it's at least referred to in this paper, page 20.

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  • $\begingroup$ The orbiter cabin operated at atmospheric conditions (14.7 psia N2/O2 mix) while the ACES suit appears to be set up for pure O2, providing a survivable level of oxygen at only 3.5 psia. What is the procedure for transitioning from one to the other? Most of Columbia's crew weren't wearing their gloves and/or helmets. Are these donned only if depressurization is known to be eminent? $\endgroup$
    – Kengineer
    Commented Aug 24, 2017 at 22:11
  • $\begingroup$ They were supposed to be wearing them. The suit was useless if they weren't. The idea is that if the cabin pressure drops, you close and lock your visor. When the cabin pressure drops to < ~3.5 the suit starts flowing O2 to keep the pressure at ~3.5. The volume in the helmet isn't very big. $\endgroup$
    – Organic Marble
    Commented Aug 24, 2017 at 22:19
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    $\begingroup$ side note of interesting item: On page 2-11-9 top of second column; Bends treatment adapter (BTA): "The BTA converts the EMU into a hyperbaric treatment chamber, pressurizing the EMU to 8.0 psid over ambient cabin pressure." I think this means the suit is designed to sustain a pressure difference of at least 8 psi in unusual circumstances. $\endgroup$
    – uhoh
    Commented Aug 25, 2017 at 0:01
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    $\begingroup$ The BTA is installed over the positive pressure relief valve on the EMU and essentially changes its setpoint to 8 psid. Note that the EMU and the ACES are different suits! Some info on pages EVA-17 and EVA-18 of this presentation- jsc.nasa.gov/news/columbia/frr/sts-104/10_eva.pdf $\endgroup$
    – Organic Marble
    Commented Aug 25, 2017 at 12:16
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    $\begingroup$ The Asteroid Redirect Crewed Mission Space Suit and EVA System Architecture Trade Study paper is really interesting! It's quite densely written and there is a lot to be learned there. Also the edits are really helpful, and the existence/function of the PPRV seems to be the fast-acting overpressure protection function that the question might be asking about. $\endgroup$
    – uhoh
    Commented Aug 25, 2017 at 21:48

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