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There is this community where people dive into the sea for precious stones. I need an oxygen cylinder sort of thing for divers. It can be plant based or it can be a dependency on some other species. There is no magic in the world. The world is an alternate earth in medieval times. How to achieve that?

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    $\begingroup$ Something like a diving bell, first described by Aristotle? $\endgroup$ – mouviciel Sep 11 '15 at 9:33
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    $\begingroup$ Honestly, given all the other issues with underwater digging, it might be better to build a dam around the area and use wind mills to pump it dry. $\endgroup$ – Ville Niemi Sep 11 '15 at 9:53
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    $\begingroup$ @abhithakur88, if the diving bell is weighted so that it's neutrally bouyant, then it should be portable. $\endgroup$ – Steve Bird Sep 11 '15 at 12:28
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    $\begingroup$ What depth do your divers need to go to? $\endgroup$ – DJClayworth Sep 11 '15 at 13:53
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    $\begingroup$ Oxygen cylinder, or air cylinder? Oxygen cylinders would be far harder, involving more chemistry, and are not typically used by divers unless they are using a rebreather. Also, as DJClayworth brought up, knowing the depths being targetted is essential. Going to 10m is very different than going to 20m. Beyond 30m you start getting into the region that is difficult, even with modern technology. $\endgroup$ – Cort Ammon - Reinstate Monica Sep 11 '15 at 19:06

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Leather, treated properly, can be very water tight. I would make a bell out of leather, large and with open bottom attached to a basket like contraption with stones tied to it to weigh it down. Sort of like the balloons we use to fly around with hot air, but this would go down into the water. The "basket" part of this would be attached closer to the balloon/bell part of it. I could then dive from the balloon/bell, or work directly from it once i'd set it down on the floor. I'd keep a rope or line attached to my dock or boat. Once done working or run out of air, release the weight stones and come back up to the dock or boat for more air and stones. If I ever saw the water level rising inside of my balloon/bell, while under water, i could swim outside it and look for air leaks (bubble streams).

Say I consume 30 liters of air per minute. And say a cowhide gives me 30 sq ft or leather (2.8 m2). That would make a sphere with a radius of about .5m. Which would give me about .44 m3 of air (440 liters), so almost 15 minutes of air. But because volume grows with the cube of the radius, while area only takes the square, it would only take about 10 to 11 cowhides to give me a balloon big enough to hold 8 hours worth of air.

The bell would sort of collapse some at different depths, kind of like an atmospheric balloon expands as it goes up and shrivels up when it comes down. The bell would have to be flexible to allow this, so the leather would have to be thinned down just right. But, this is a good thing. It means the air in your bells is getting pressurized by the water around it. You want that air to get pressurized, otherwise breathing would become impossible very quickly.

At depths of even 10 meters, you'd have to worry about compression sickness. The most you could spend down would be about 2 hours, without the need for a decompression chamber. So to stay that max, you'd need about 6 to 7 cowhides to make for 4 hours worth of air, compressed down to half volume - 2 hours. But different pressure affects how you absorb oxygen in your lungs so it would affect how much volume of air you would need per hour, and I don't know exactly what that would mean.

A problem you would run into is that a leather bag containing even just .44m3 of air, submerged in water, would have a strain similar to the same bag holding that much volume of water, hanging from a roof. The internal pressure would be trying to tear the seams apart. Even if you glued and super-stitched the seams, you would be likely to come apart at the seams, and all the more as the bell got bigger. This would be a bigger problem near the surface, where the bell is in full volume without as much external pressure to balance the pressure from the buoyancy of the air inside it. One partial solution would be that you would need to reinforce the bell with a netting of thick ropes that will be the ones that end up carrying the weights at the bottom. The leather bell sits inside the netting. Another partial solution would be to create a network of multiple bells, rather than one bigger bell. This will require more material but it may prove an absolute necessity. Another possible solution is to make a double or even triple layered bell, with the layers glued to each other, so that the seams of one bell never run too close to the seams of the other. It would be ok for them to intersect, but not run alongside. The bottom line is, imagine you were building a bag that you could hung from the roof and fill with water. Mind you, a cubic meter of water weighs a ton, literally a metric ton, 1000 kg or nearly 2200 pounds. The strains on the bag and its moorings would be pretty much the same. If you can build such a bag, you can use it as a bell, turned upside down, with air inside and water outside.

An interesting aside here would be that the people in the world doing this, would have learned the hard way about compression sickness, and would constantly have to deal with it. People being tempted to stay longer, jut to get a little more precious metal, and then ending up with the "bends" (compression sickness makes a person double up and bend their joins in pain).

This sounds so doable and fun (building the bell, not getting the bends), I might have to try it just for the heck of it. The balloon thing, without the compression sickness part. So maybe just the 15 minute version to start with.

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    $\begingroup$ The air inside the bell compresses as you go down. At 10m the bell is half full of water. At 30m it is 3/4 full. $\endgroup$ – DJClayworth Sep 11 '15 at 14:25
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    $\begingroup$ Running out of oxygen is less of a problem than being overwhelmed by increasing levels of carbon dioxide. $\endgroup$ – Ben Voigt Sep 11 '15 at 22:47
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    $\begingroup$ Estimations you use are unfit for this example. True, it's 30 liters per minute - if you can assume fresh air in every breath in, and used air to go away. In your situation you simply cannot. First - forget the oxygen. In air you breathe out there is still plenty of it. Carbon dioxide will kill you long before you will start to feel lack of oxygen anyway. 7% is pretty much instant fail of rational thinking. With every breath we turn 4 liters into 5% carbon dioxide. 20 breaths per minute are only at the start, later it gets faster. After 15 minutes you calculated, our diver is flat dead. $\endgroup$ – Mołot Sep 12 '15 at 8:28
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    $\begingroup$ Sorry for the long comment: but he is feeling dizzy way, way sooner than that, and can work on his full capacity only for a fraction of this time. That's why pretty much all caissons had air pumps working when people was inside. $\endgroup$ – Mołot Sep 12 '15 at 8:31
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    $\begingroup$ CO2 scrubbers are not particularly high-tech, and in theory, a mediaeval society could pump used air through quicklime to absorb the CO2. The problem would be finding a plausible way in which they could have found out that they need to do this, without knowing the actual chemistry involved. $\endgroup$ – Mike Scott Sep 12 '15 at 10:21
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Depending on how "alternate" this alternate world is, I have two recommendations:

If the world is very similar to Earth: The most grounded solution would probably just be a long, flexible tube. One end would be attached to something buoyant (like a leather sack full of air, or the side of a rowboat), and the other end would be taken with the diver. By only using it to inhale (and just exhaling into the water), he should be able to continue breathing for whatever depth he could safely traverse without any protection against water pressure. It's portable in the sense that he could coil it up to carry with him when he's not diving. The tube itself could just be a (fictional) hollow plant vine coated in preservative.

If the world is very different from Earth: A symbiotic relationship with a fictional animal would probably make for an interesting detail, and would also serve to differentiate your setting from reality ("here's why this works in my world, and didn't work in human history"). For instance, divers could have a carnivorous plant which attaches to their skin like a leech, and sucks their blood to feed off their CO2, and leaves O2 as a waste product, so the diver would be able to just stop breathing, but still maintain a relatively healthy oxygen level in his blood for a while (if he attached a lot of these leech plants).

Depending on the direction of your narrative, this could also give you an opportunity to introduce limitations on the divers. For instance, the instinctive imperative to breathe is extremely deeply rooted, and a diver would have to overcome that primal impulse in order to use one of the leech plants. This would limit the use of such creatures only to people who are either extremely disciplined, or completely insane.

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  • $\begingroup$ So, you can't breathe and you're covered in blood-sucking leeches? Sounds like fun. $\endgroup$ – DaaaahWhoosh Sep 11 '15 at 13:22
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    $\begingroup$ That's why I'd probably work at a general store instead. Adventurers will pay 5 gold coins for an apple. $\endgroup$ – Liesmith Sep 11 '15 at 13:29
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    $\begingroup$ Breathing tubes don't work in the real world, at a depth of more than a few metres. The pressure difference prevents you breathing. That's why you need a compressor for real life fixed divers. $\endgroup$ – DJClayworth Sep 11 '15 at 13:52
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    $\begingroup$ @DJClayworth - Try "more than a few feet". This Soviet diving manual books.google.com/… sets a pressure of ~65mm of mercury pressure difference as a limit, and that's 2.4 feet of water. Also, at any depth the tube becomes too long to allow stale air out of the tube during exhalation, so suffocation comes quickly. $\endgroup$ – WhatRoughBeast Sep 11 '15 at 14:12
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    $\begingroup$ @WhatRoughBeast Only breathing in from the tube and out elsewhere solves the stale air problem. The pressure one is still a killer though. $\endgroup$ – Tim B Sep 11 '15 at 15:10
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I don't think an oxygen cylinder is really the way you should approach this. Take a look at the history of freediving. In ancient greece they would use a large stone tied to a boat to help them get down 30m, and they would be able to stay there for up to 5 minutes. This could be used to get them down twice as deep with little difficulty.

Why is this better than some form of stored oxygen? At 60m down, you're already below what you can reach with recreational scuba gear. Your body can handle the pressure just fine, but at that pressure normal air becomes toxic - the partial pressure of nitrogen is too high.

Oh, and any air you breathe at depth has to be pressurized. If not, you simply won't be able to resist the force of water pushing against you in order to breathe in.

Oh, and the amount of air you use depends on its volume, not its weight. So if you have a balloon large enough to hold 5 minutes worth of air and take it down to 40m, it becomes only 1 minute worth of air. (FYI scuba tanks take advantage of this in reverse - if the air is pressurized at 2x what you need and you have a way to supply it at just the pressure you need, your tank effectively has twice as much air.)

So in order to breathe air at depth you need:

  1. Pressurized air. A balloon would allow the water to do the compression for you, but I don't know of medieval materials that can expand and contract as needed. If you don't have a balloon then you need
  2. A container strong enough to hold pressurized air. This is actually really important - the pressures involved would make most containers explode.
  3. A different mix of gases. Deep dives use heliox, a helium-oxygen mix, in order to get around nitrogen narcosis. FYI pure oxygen won't work, either.
  4. Something to make sure you get air at the right pressure. If your air is pressurized above the pressure of the water, you need something to make sure you don't burst your lungs when you breathe in.

If I haven't made it clear already, an oxygen cylinder is not a good idea. So why is freediving better? It doesn't have any of the above problems!


Something else to consider is how people first figured out that the precious stones are down there. They're not going to be discovered while fishing - you do not want your nets to hit the sea floor because that makes them likely to get caught on rocks and either get stuck or break. In order for them to be discovered by your divers with oxygen cylinders, you'd have to have a reason for those to exist and be in use prior to discovering the precious stones.

The most plausible explanation is that there are precious stones in shallower waters as well. At a depth 5m or less, it's reasonable that someone could spot it while fishing and retrieve it. As knowledge of the stones became more widespread, the stones that are easy to collect are all harvested. Over time, the people have to go deeper and deeper in order to find new stones.

So why would they use anything other than freediving? Having to go deeper only over time means that there is plenty of time for them to refine their diving techniques, such as using ballast to get down faster and breathing techniques to be able to stay down longer.

Also, consider the first person who tries using an external oxygen source instead of just holding their breath. They're very likely to die or be seriously injured. If you've taken scuba lessons, you'll understand why - it's really easy to mess up badly. Here are two mistakes you can make and why an untrained person is likely to make them:

  1. Holding your breath as you ascend. It feels pretty natural to continue holding your breath until you reach the surface, especially because it makes you more buoyant and helps you ascend. However,

    a diver who breathes at 10 metres and ascends without exhaling has lungs containing twice the amount of gas at atmospheric pressure and is very likely to suffer life-threatening lung damage. (Wikipedia)

  2. Ascending too quickly even while exhaling. When you free dive, you generally are going to try to come up as quickly as possible when you're done (in part so that you can go down again sooner). This is a good recipe for getting decompression sickness (the bends).

So imagine this - a group of divers, where one has figured out a way to take air down with him. He goes down and is able to stay down for longer than anyone has ever stayed down before. However, as soon as he comes back up it's clear that something is wrong. He is clearly in pain, and gasping for air. As his friends watch, horrified, he passes out and starts turning blue even though he's still breathing. He soon dies, having suffocated due to lung damage. As they mourn their friend, they look at each other and understand - man was not meant to stay underwater for that long. (Still works if he only gets the bends)

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  • $\begingroup$ "the amount of air you use depends on its volume, not its weight". Not exactly. The amount you inhale is by volume, yes. But at pressure, that volume contains a lot more oxygen / takes longer for the O2 fraction to decrease and the CO2 fraction to increase as a result of metabolism (which hasn't really changed), so your respiration rate could decrease, and the mass rate of usage would be back to normal. Except that it is partial pressure of CO2 that limits you, not O2 fraction, so the air becomes useless while there's still LOTS of oxygen. If you had something to absorb CO2... $\endgroup$ – Ben Voigt Sep 11 '15 at 22:52
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Stay Dry

You may try dry-digging, by placing a long, wide, tube made the same way a wine barrel is. However, this is a cylindrical tube weighted at the bottom and dropped into your area of interest. Get it nice and solid into the lake-bed.

enter image description here

Next, simply pump the water out, take a ladder, and climb down to bed. Then you can start digging for stones all day long. If you need more clearance, you can make your tunnel as wide and sturdy as a ship's hull, but just a long cylinder to the base of the lake.

This could work so long as you provide air ventilation when your miners start digging.

Please note that I am no artist. Also note that I'm nonetheless proud of the precious gems.

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  • $\begingroup$ I think you meant "pump the water out", not "pump the air out". It wouldn't be very hospitable if it was full of vacuum. $\endgroup$ – hobbs Sep 11 '15 at 19:05
  • $\begingroup$ @hobbs - bahaha nice catch; it has been edited. $\endgroup$ – Mikey Sep 11 '15 at 19:15
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    $\begingroup$ This is a cofferdam. It has been used extensively in our world - Romans were doing it 2000 years ago. Tried and true solution. Nice. And I am thinking that you would need two ways to mine the precious stones. One more exploratory, where you'd need to be able to move around to look for deposits or study the bottom for signs of the stones, my answer would work for that. But once you found a site you wanted to mine more in depth, this answer would be much better as you could work at the bottom for as long as the cofferdam held (no problems with compression sickness). $\endgroup$ – user11864 Sep 11 '15 at 19:47
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    $\begingroup$ You may need to keep the pump running if either the lake/sea floor is water permeable or the seal with the lake/sea floor is incomplete (either of which is likely if the bed has a covering of mud or sand). This approach is also less economic if the gems are distributed over a wider area because it would take (unproductive) time to relocate the caisson. $\endgroup$ – KillingTime Sep 11 '15 at 19:48
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    $\begingroup$ Check out the bit about cofferdams in this page: historyworld.net/wrldhis/PlainTextHistories.asp?ParagraphID=dol Apparently the Romans would drive two concentric circles of posts into the floor, then fill the gap between the circles with clay (watertight), then scoop out the water and mud from inside the inner circle. $\endgroup$ – user11864 Sep 11 '15 at 19:50
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You create a large ceramic pot (ceramic because it is water and air tight and easy to build into different shapes. The pot should be shaped like this:

/-----\
|     |
\     /
 \   /

Place weights around the neck of the pot so that it naturally floats this way up when filled with air.

You have large bundles of rocks on the floor of the ocean tied to ropes, you use the ropes to pull up the rocks into your boat and then attach them to the pot. You then stand inside the pot and release the rocks from the boat. The rocks then pull you down to the bottom.

The increasing water pressure will push the air up further into the pot but the same amount of air will stay inside. It will just fill a smaller volume.

Walk around on the sea floor with your shoulders and head inside the pot, you can duck out as needed to grab things then go back in to breathe. The weights and the air inside keep the pot naturally stable and the rocks keep it down by the sea bed.

/-----\
|     |
\  o  /
 \/|\/
   ^
  / \

When the air starts getting stale or you have what you came for then release the rocks (leaving them attached to the main line so you can pull them back up for next time) and the air inside the pot will pull you to the surface. Pull the pot back into the boat to refresh the air inside then repeat as often as desired.

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    $\begingroup$ This is basically the same as my answer, only with ceramic instead of leather. And trying to shape a pot with 2 meters diameter, out of ceramic, and bake it without cracking it - would be quite a feat. What about glass? Ooohhh, and then you'd be able to see out! $\endgroup$ – user11864 Sep 11 '15 at 15:30
  • $\begingroup$ @GiliusMaximus You were talking about the bell collapsing and changing shape, which isn't needed at all. $\endgroup$ – Tim B Sep 11 '15 at 16:15
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    $\begingroup$ Alexander the Great was said to have descended below the sea in a barrel made of glass. Aristotle described people using "cauldrons' as diving bells. $\endgroup$ – stib Sep 12 '15 at 12:34
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This world has a type of seaweed which grows deep under the water but still relies on photosynthesis. It creates bladders filled with oxygen (from photosynthesis) along its stem for flotation, and because of the square-cube law, making these bladders larger gives more benefit as the plant grows.

Divers discovered that these bladders could provide a limited amount of air: breathe out, then bite down on the stem and breathe in as much of the escaping oxygen as possible. Each one is only good for one or two breaths, but it's an extension of time underwater, and they can even be "farmed" under appropriate conditions. The bladders are also difficult to carry while diving and tend to burst if brought to the surface, so in preparation for a dive people would cut a strand of this plant while underwater, tie the base to a weight of some sort, and tow it over to their diving site. Then they could get a few breaths before coming back to the surface again.

The process of putting the cut plants near the dive site takes a lot of time, but it's necessary for safety. Some divers try to go down in a single breath to avoid all the laborious preparation. This limits their time and has a greater risk of decompression sickness (since they're going up and down more often) but means they can get down to a new lode faster than other, more careful divers.

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A bellows operated at the surface could force air through pipes (bamboo, intestine, rubber, etc). The pipes could connect to a watertight, inverted, bowl (like capsized boat) that could be weighted to sink close to where the gem harvesters would be working.

This would be far less portable that what you may be asking for, but it would allow for divers to remain below for lengthy periods of time.

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Instead of storing the oxygen under compression, you could generate it chemically.

One example of this is the 'oxygen candle'. From Wikipedia:

A chlorate candle, or an oxygen candle, is a cylindrical chemical oxygen generator that contains a mix of sodium chlorate and iron powder, which when ignited smolders at about 600 °C (1,112 °F), producing sodium chloride, iron oxide, and at a fixed rate about 6.5 man-hours of oxygen per kilogram of the mixture. The mixture has an indefinite shelf life if stored properly: candles have been stored for 20 years without decreased oxygen output. Thermal decomposition releases the oxygen. The burning iron supplies the heat. The candle must be wrapped in thermal insulation to maintain the reaction temperature and to protect surrounding equipment.

This seems within the range of plausibility for a gunpowder-level alchemist.

Breathing pure oxygen at the pressure a few tens of meters down is a Bad Idea for humans, but the physiology of your characters may differ.

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great place of ideas https://en.wikipedia.org/wiki/Timeline_of_diving_technology#Pre-industrial

DaVinci designed airtanks made of leather, diving suits, and breathing hoses connected to woden floats. Metal or wood diving bells and snorkels were common. goggles were also known. air pumps and hoses started showing up in the 16th century.

http://www.bl.uk/onlinegallery/features/leonardo/diving.html

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Balloons are one option. Rubber trees => rubber => balloons => fill with oxygen by keeping them airtightly wrapped around some greed plant's twig until it is inflated with oxygen. The balloon ought to be transparent (of course) for photosynthesis to work.

Another option is to use a large air-tight bag mixed with chemicals which slowly react together and produce oxygen. Such chemicals do really exist in nature too (Hydrogen Peroxide when treated with a catalyist -i forgot the name- immediately disintegrates into water and oxygen). The larger amount of chemicals in the bag, the longer you can stay underwater.

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While multiple people have tried to improvise oxygen storage devices from various medieval materials it's not going to work. None of the proposals work at extreme pressures and nothing else is viable due to the buoyancy problem.

A low pressure air storage system of sufficient volume is going to have an insane amount of buoyancy. It could be counterweighted enough to allow the diver to descend but you're going to have an insane mass to lug around--being at neutral buoyancy doesn't cancel inertia.

Using the numbers from the cowhide answer I get about 2 m^3/hr--and remember that a m^3 of water weighs about a ton. Thus his 8 hours of air in cowhides requires that he starts with a 16 ton counterweight--good luck hauling that around!

Furthermore, as the storage isn't rigid he's going to be losing buoyancy as the dive goes on, he must shed about 1 pound per second from his counterweight. How is he going to accomplish anything?!?!

Also, there's an additional problem with this answer--compressing the air into a smaller volume doesn't lessen the volume of air you breathe. Thus when you're 10m down you use twice as much surface-pressure air as you do on the surface. Your 8 hours at the surface is 4 hours at 10m, 2 hours at 30m and a mere hour at 70m--not counting what you need for decompression.

Lacking the ability to construct high pressure air cylinders the only options are the caisson approach (enclose the area, pump the water out) or air being pumped down to the diver.

How complex the pumping option is depends on how much pressure your system can handle. If your pumps and hose can handle the needed pressure, fine, it's easy. If they can't you have to break it up into stages. Lets say you're at 20m but your pumps can only deliver 15 psi. Build three pumps--one on the ship, two that are lowered into the sea and run off a steam line. Each one feeds air to the next, the diver breathes.

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