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OK, still in my Victorian Steampunk Shadowrun setting, we've got some underwater cities (care of a certain Prince Dakkar). They're on average 50m below sea level, with a couple being deeper at 150-200m.

They are "open" to the sea, so are at higher pressure than normal atmosphere, so we are at risk of the Bends.

I want my characters to be able to move back to the surface without waiting up to 24 hours to degas, so are there any methods other than decompression chambers to do this, or at least to help reduce the overall time? I've thought of blood degassing maybe?

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    $\begingroup$ No, because you need to degass the entire body. In fact current decompression times are not 100% successful. Professional divers very often end up with body damage due to microbubbles which cause buildup of damage over multiple dives. $\endgroup$ – Carl Witthoft Dec 20 '19 at 14:09
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Given this is a Shadowrun setting, there's magic of various kinds. Magic ought to be able to do a pretty good, rapid job of blood degassing. Assuming the society understands the cause of "caisson disease" (not mentioned in 20,000 Leagues because Verne didn't even know it existed, when he wrote in the 1860s), it shouldn't be hard for the magical/medical community to come up with a way to purge the excess nitrogen from the blood and tissues.

Breathing another gas mix won't really help, because other gases (helium, hydrogen, etc.) are also soluble in water/blood to some extent. Heliox is used for saturation diving at great depth, not because it greatly shortens decompression (it does shorten it, but not by even a factor of two), but because it doesn't become toxic in the blood as quickly as nitrogen (40-60 meters is deep enough for nitrogen to start to cause narcosis, so the underwater cities will have to breathe heliox anyway, assuming they know about and can extract helium from natural gas).

Hard hyperbaric suits are impractical because of weight -- 150 m is 15 atmospheres; a soft suit (like a space suit) won't come close to sustaining that level of pressure and allowing the wearer to move their major joints (movement is a problem even at 1 atm relative pressure). A hard suit good for 15-30 atm. will be heavy, however -- including its breathing gas supply, it'll likely weight above 100 kg. Not very practical.

The one kind of reasonably portable pressure support that might work is a skintight suit -- one that literally fits like skin, except over the torso, and provides enough compression to avoid edema and allow circulation to return to the core. The torso would be similar to a portable iron lung, pressurized, and a helmet supplied with the same breathing gas used below, plus pressure bottles and regulator, would complete the ensemble. Not terribly practical, but at least physically possible, in the absence of magical blood degassing.

Another option, possibly more attractive under the desire for steam power, is a hyperbaric perambulator. Basically, a decompression chamber on wheels with a small steam engine. This limits ability to manipulate the environment, but offers the ability to trail a small cloud of coal smoke...

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  • $\begingroup$ I like the idea of this I think. Sort of like Samstead Environmental Suits from Alan Dean Foster's Humanx Commonwealth series. Obviously, they will be steam powered... $\endgroup$ – Riddles Dec 20 '19 at 14:31
  • $\begingroup$ Why would it need power? Compressed gas, a regulator (known for welding before 1880), and careful assembly. The point of this is to be enough lighter than a full body hard suit to actually walk around in. $\endgroup$ – Zeiss Ikon Dec 20 '19 at 14:36
  • $\begingroup$ Brass isn't the lightest thing to be walking around in... $\endgroup$ – Riddles Dec 20 '19 at 14:37
  • $\begingroup$ But a brass (or steel and brass) torso sufficient for 20 atm shouldn't weigh more than 30 kg or so -- plus the breathing gas bottles, of course. $\endgroup$ – Zeiss Ikon Dec 20 '19 at 14:40
  • $\begingroup$ Fine if you're an adult, but I'm envisaging families living down there - imagine the havoc, small children running around a hotel (outside of their pressurized rooms) could do in an steam powered exoskeleton environmental suit... $\endgroup$ – Riddles Dec 20 '19 at 14:45
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Surface suits.

This is essentially a hyperbaric chamber that you wear. These suits are built like deep sea diver suits of the era but are for land use. Your characters breathe pressurized gas and wear a pressurized suit on land. They take the pressure with them and it slowly decreases as they move about on the surface.

If you have SCUBA it would be more convenient - those old time suits required someone to pump air down a hose.

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  • $\begingroup$ To hold anything up to 20-30 atm (1 atm per 10m sea depth), this would be a (very heavy) hard suit. Very steampunk, but not something you'd want to walk around on the surface in because it'll weigh a hundred kg. $\endgroup$ – Zeiss Ikon Dec 20 '19 at 13:55
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    $\begingroup$ Might be a bit more practical to have a simple hyperbaric chamber and have other people haul it around for you, palanquin-style. $\endgroup$ – Starfish Prime Dec 20 '19 at 14:18
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    $\begingroup$ I love this - "Yeah he's a Nemoan - walks around all the time in that weird Surface Suit they all wear, belching out smoke like it's going out of fashion!" $\endgroup$ – Riddles Dec 20 '19 at 14:34
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Blood degassing would also take time to prepare and process, and it wouldn't help much- blood is minor liquid in human body (4-10 kg out of 30-70) and will regase quickly enough.

The only plausable method - is to breath oxigen-helium mixture for couple hours before. It can be done without being stationary: just ware a gasmask (sort of firefighters use IRL).

Or your domes can be filled with such a mixture at first place. Than the only thing that will case "bends" would be immediate return after arriving to seafloor.

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    $\begingroup$ Heliox might be a bit out of reach of Victorian engineers, but a hydrogen/oxygen mix would certainly be doable. Adds a certain amount of tension when you're in room full of vapourised rocket fuel for a few hours, too... $\endgroup$ – Starfish Prime Dec 20 '19 at 11:45
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    $\begingroup$ We can handwave a lot, but a hydrogen/oxygen atmosphere would mean that a) there is no way you can use fire for anything, and b) there is no way you can allow something as simple as a stone or a steel knife into your habitat. Because if you did, sometime, someplace someone will collide two such objects fast enough to create one little spark and BOOM, there goes your habitat... Life would be pretty constrained in there. $\endgroup$ – cmaster - reinstate monica Dec 20 '19 at 12:09
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    $\begingroup$ Even heliox requires decompression, it's used to avoid nitrogen narcosis. $\endgroup$ – Zeiss Ikon Dec 20 '19 at 13:53
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    $\begingroup$ It is apparently possible to use a hypoxic gas mix with hydrogen as the inert component if you're 50m+ down... the O2 partial pressure will be high enough for it to be breatheable at depth, but the gas mix isn't explosive. There was some work done on this post-WW2 by countries that didn't have access to decent helium reserves. $\endgroup$ – Starfish Prime Dec 20 '19 at 14:17
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    $\begingroup$ @cmaster-reinstatemonica, hydrogen has an upper explosive limit, a point at which there's not enough oxygen to keep combustion going. Once you get below about 40 meters, a hydrox mixture is reasonably safe, simultaneously having enough oxygen to breathe but not enough to burn. $\endgroup$ – Mark Dec 20 '19 at 21:55
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You can have your characters stay in hardbody armor the whole time they are underwater. Their suits would need to be airtight and bulky, like space suits, so that the suit can maintain surface air pressure inside itself without being crushed by the surrounding air. It will disrupt your chase scenes: “Quickly! She’s getting away!” “I’m on it. One pace to her 10, if she runs around the city perimeter, she’ll catch up to us soon enough!”

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Genetically modified hemoglobin!

You want a couple proteins whose role is moving dissolved gases from areas of high concentration and releasing them in lower ones. This efficiently moves the gases to the lungs, greatly decreasing decompression time.

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