I don't know why I've been in a spacesuit mood lately, but the following crossed my mind recently:

  • the flexible portions of NASA'S EMU suits are inflated by the interior pressure
  • the suits in the National Buoyancy Lab (NBL) are underwater
  • water pressure exceeds the nominal flight pressure of 4.3 psi at a relatively shallow depth, yet the NBL suits stay inflated

Is the atmosphere in a suit in the NBL different than it would be in space?

The closest post I found while looking here was Are the EVA suits used in the ISS and in the NBL same or different?

  • $\begingroup$ I would not be surprised if anyone else has better information or can write a better answer to this; please feel free to do so. $\endgroup$
    – Erin Anne
    Jan 31 at 7:58

2 Answers 2


I managed to stumble across the answer while trying to decide how to ask this question, and I thought it was interesting, so

Oxygen exposures at NASA's Neutral Buoyancy Lab: a 20-year experience, Walker et al, 2018 states that the NBL suits run "a 46% nitrox mixture" at "pressures exceeding 2 atmospheres absolute (ATA)". The entire abstract is fairly interesting, considering a long-running argument I had with some other engineers regarding oxygen toxicity

Astronauts training for extravehicular activity (EVA) operations can spend many hours submerged underwater in a pressurized suit, called an extravehicular mobility unit (EMU), exposed to pressures exceeding 2 atmospheres absolute (ATA). To minimize the risk of decompression sickness (DCS) a 46% nitrox mixture is used. This limits the nitrogen partial pressure, decreasing the risk of DCS. The trade-off with using a 46% nitrox mixture is the increased potential for oxygen toxicity, which can lead to severe neurologic symptoms including seizures. Suited runs, which typically expose astronauts of 0.9-1.1 ATA for longer than six hours, routinely exceed the recommendation for central nervous system oxygen toxicity limits (CNSOTL) published by the National Oceanic and Atmospheric Administration (NOAA). Fortunately, in over 50,000 hours of suited training dives spanning 20 years of EVA training operations at NASA's Neutral Buoyancy Laboratory (NBL) there has never been an occurrence of oxygen toxicity. This lends support to anecdotal sentiment among certain members of the hyperbaric community that the NOAA CNSOTL recommendations might be overly conservative, at least for the oxygen pressure and time regime in which NBL operates. The NOAA CNSOTL recommendations are the result of expert consensus with a focus on safety and do not necessarily reflect rigorous experimental evidence. The data from the NBL suited dive operations provide a foundation of evidence that can help inform the expert discussion on dive-related neurologic oxygen toxicity performance and overnight recovery in young, healthy males.

  • $\begingroup$ As discussed in this question the Apollo astronauts breathed pure oxygen at just over one atmosphere from suitup until a few minutes after launch when the cabin (and suit) pressure began decreasing, a total of about two hours. Apollo 17 had a long delay and the crew spent about five hours at 100% oxygen at 1 atmosphere, approaching the length of an NBL dive. There didn't seem to be an answer though to the question of whether there was an Apollo mission rule putting a limit on the number of hours in that environment. $\endgroup$ Jan 31 at 14:16
  • $\begingroup$ This makes me curious why they don't use heliox, or another technical gas mixture that scuba divers would use in this situation. $\endgroup$ Jan 31 at 17:21
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    $\begingroup$ Nitrox is a diving mixture, but normally in cases where a scuba diver was risking nitrogen narcosis or oxygen toxicity with nitrox, they would switch to another gas mixture. For example, you can add helium, creating trimix (or heliox if you remove the nitrogen); this allows you to keep the partial pressures of both nitrogen and oxygen down to safer levels. (Note: I am not a scuba diver. This is just my understanding of why scuba divers don't normally have this problem.) $\endgroup$ Jan 31 at 20:38
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    $\begingroup$ Over the last 40 years the accepted limits for scuba divers have changed fairly dramatically. One reason is the old Navy data was pretty much all on young, fit men, while scuba divers are much more diverse in age, fitness, and gender. Similarly, I would expect that astronauts skew pretty far from whatever averages NOAA bases their numbers on. $\endgroup$
    – Jon Custer
    Jan 31 at 22:31
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    $\begingroup$ The tank is only 40 ft deep. Their torso depth will be about 35 ft. Suit pressure of 4.3psi adds equivalent of 10 ft water depth. So (worst case) if they are breathing air, they will need a short decompression stop at the end of the workday but have zero worry of oxygen toxicity. Mixed gas diving adds unneeded complexity. After a 6 hour dive a safety stop would be prudent even if they are breathing Nitrox. Make it a few minutes longer and you can call it a decompression stop instead. $\endgroup$
    – Woody
    Feb 1 at 3:46

Question: Is the atmosphere in a suit in the NBL different than it would be in space?

Short Answer: The gauge pressure in the suits is the same. The breathing gas mix is different: Nitrox rather than 100% O2. Nitrox reduces (but does not eliminate) the possibility of the bends in the NBL. There is no concern for oxygen induced seizures or lung toxicity in the NBL.

Long Answer:


  • “A”=absolute: pressure difference with a hard vacuum e.g.: psiA

  • “G”=gauge: pressure difference with the ambient pressure e.g.: psiG

  • psi = pounds per square inch -one atmosphere is 14.7psi

  • bar = 100kPa -one atmosphere is 0.99 bar

  • fsw = feet of salt water -one atmosphere is 33.0fsw

  • ffw = feet of fresh water -one atmosphere is 33.9ffw

  • O2= oxygen

  • N2 = nitrogen

  • Nitrox = Mixture of N2 and O2 gasses

Since a standard atmosphere happens to be 1.013 bar, a “bar” is a useful approximation of an atmosphere.

For example, a SCUBA diver at 33.9 feet is exposed to 1barG of water pressure but is breathing gas at 2barA. Physiologic processes respond to the absolute pressure fraction for each gas. This is known as “partial pressure”. In this example, if the diver at 33.9 feet of fresh water is breathing air (21% O2) the partial pressure of O2 is 21% x 2 barA = .43 barA. If the diver is breathing a Nitox mix of 46% O2, the partial pressure of O2 is 46% x 2 barA = .92 barA

1) Suit Pressure

A central problem with working in an EMU suit is physical work against the volume changes which accompany joint movement: pressure X volume = work. The large joints are constant volume, or as close as possible within design limitations. But it is very difficult to design constant volume joints small enough for gloves. This is why EVA work using pressurized gloves is physically demanding. The physical exertion results in multiple minor injuries such as abrasions and fingernail delamination

[enter image description here] 1https://ntrs.nasa.gov/api/citations/20220004017/downloads/2021%20EVA%20Injury%20Evidence%20Report%20Final.docx.pdf

as well as reduced performance during fatiguing tasks https://www.semanticscholar.org/paper/Monitoring-human-neuromusculoskeletal-system-during-Madden-Djurdjanovi%C4%87/0e2bcb86e77af3f620cb2c7b3bbf2a7310f38bb4

To duplicate the physical exertion of an EVA suit pressurized to 4.3 psiA in space, the NBL suits need to be pressurized to 4.3 psiG above surrounding ambient pressure. ( 4.3 psiG is equivalent to an extra 10 feet of fresh water pressure).

The National Buoyancy Lab tank is 40 feet deep https://en.wikipedia.org/wiki/Neutral_Buoyancy_Laboratory#:~:text=Simulation%20control%20area-,Facility%20features,23%20million%20litres)%20of%20water Presumably astronauts spent their time in the water column, not laying on the bottom of the pool. So a reasonable average working depth for their torso would be 34 feet. With the added suit pressure to give realistic suit movement resistance, that makes the equivalent of an unsuited dive at 45 feet. That is 19.5 psi above surface pressure, or 34.2 psiA.

enter image description here

2) Breathing Mixture and the Bends

Breathing inert gas (nitrogen, helium, ) under pressure eventually results in ambient partial pressure of these gasses in the bloodstream and tissues. If the diver returns to normal pressure too fast, these gasses can come out of solution and form bubbles in the tissues or bloodstream, much like popping a champagne cork. If the bubbles are larger than capillaries (the smallest blood vessels) bubbles can block blood flow to those tissues, resulting in decompression sickness, colloquially known as “the bends”. Effects vary from temporarily unpleasant to fatal.

Short dives (less than an hour) at shallow depths (30 feet) are very low risk for the bends. The NBL dives are potentially deep enough (45ft) and long enough (up to 6 hours) that divers could be at risk, depending on the exact depth profile of a given dive.

Since the gas causing bends is the nitrogen, lowering the nitrogen partial pressure reduces risk of bends. Nitrogen partial pressure can be lowered by substitution with an inert gas such as helium, or by increasing the proportion of oxygen (Nitrox). The NBL has chosen the Nitrox option. Nitrox-45 (45% O2) is used. https://www.nasa.gov/wp-content/uploads/2021/06/739540main_ap_st_chem_divingdowndeep.pdf

As an example, diving to 50ft breathing air (21%O2) requires a decompression stop on dives longer than 80 minutes. A dive to the same depth breathing Nitrox-36 (36% O2) permits 220 minutes without decompression.

The above example uses standard dive tables which assume the “worst case” where a diver descends to, and stays at, the maximum depth for the duration of the dive. Since NBL divers are constantly monitored and their air is surface supplied, nitrogen management can be adjusted “on the fly”. For instance, if the tasks at the end of the dive are scheduled at a depth under 18 feet, the surface supply could be switched to 100% O2 which would rapidly flush nitrogen and could negate the need for a decompression stop.

At this point you are likely asking why have any nitrogen in the mix at all? Why not breathe pure oxygen the whole dive? It turns out that oxygen, which we all know and love, has a dark side.

3) Oxygen Toxicity

High partial pressures of oxygen (above 1.2 barA) can cause seizures.

The maximum partial pressure of oxygen in a pressurized NBL suit, breathing Nitrox-45, at the bottom of the NBL pool is 1.07barA

The maximum operating depth at which it is safe to breathe 100% O2 is 20 feet.

According to https://en.wikipedia.org/wiki/Maximum_operating_depth, the maximum operating depth for Nitrox-50 (50% O2) is 91 feet, so oxygen induced seizures are not an issue in the NBL pool.

Oxygen can also cause lung toxicity, but only at partial pressures greater than what can be attained in the NBL. https://en.wikipedia.org/wiki/Oxygen_toxicity

Fun Fact: Astronauts on the ISS are at risk for developing the bends at the start of an EVA. This is because the atmosphere in the ISS is 79%N2+21%O2 @ 14.7psiA. Their blood is saturated with N2 at a partial pressure of 11.5 psiA. The EVA suit is 100% O2 @ 4.7 psiA. To prevent bends, astronauts pre-breath pure O2 for several hours before an EVA. This flushes out the N2.

Note: You may find that numbers “don’t add up” in this answer. Different agencies have different recommendations, and these change over time. For instance, US Navy dive tables are considered insufficiently conservative by some agencies since they were formulated with an “acceptable” risk of bends even when followed to the letter. Tables for Maximum Operating Depth (O2 seizures) vary considerably.

  • $\begingroup$ wow, I have complicated feelings on that AP Chemistry NASA+Texas Instruments collab. I think the EVA Injury paper also indicates that the shoulder injury is from 'gravity causes the astronaut to “fall into” the head of the spacesuit, pressing the shoulders into the HUT of the suit' but I can see why you didn't crop Figure 4. Interesting. $\endgroup$
    – Erin Anne
    Feb 4 at 1:19
  • $\begingroup$ "To duplicate the physical exertion of an EVA suit pressurized to 4.3 psiA in space, the NBL suits need to be pressurized to 4.3 psiG " Do you have a source for this? The astronauts already have to deal with the resistance of the water, so having the suit pressurized to +4.3 psiG would put the exertion at a higher level than is experienced in space. $\endgroup$
    – Hobbes
    Feb 5 at 9:51
  • $\begingroup$ @Hobbes ... I was referring to joint movement resistance due to volume changes seperate from water resistance. Water resistance contributes little to resistance from the pressurized gloves. $\endgroup$
    – Woody
    Feb 6 at 22:43

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