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:
Abbreviations:
“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
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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.
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.
