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Crunchy numbers for diving in Shadowrun

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Theoretical Anomaly

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« on: <10-31-20/2352:10> »
When reviewing the statistics on one of the pieces of cyberware under SR6e rules, I was appalled at the idea that someone could “hold their breath” for up to four hours from the air contained in the very tiny device shown as the “Internal Air Tank”:

(attached image by Mike Jackson, from Cybertechnology, FASA 1995.  Note that this image was from Second Edition SR, but the description never changed from then through Sixth Edition).

No matter how good technology has progressed, physics limits how much air can be put under pressure in a tank, and still be able to utilize it.  These days, an average external diving cylinder using modern technology, (say a standard aluminum 80 cubic foot (cuft) tank), is about 26.1" (66.3 cm) tall with a 7.25" (18.42 cm) diameter and will actually hold about 77.4cuft of air at 3000 pounds per square inch (psi) at roughly room temperature.  A steel high-pressure cylinder of approximately the same dimensions might hold about 100cuft at 3500 psi. These are not the crunchiest of numbers, but they’ll do for this article. (For metric users, those numbers for an aluminum tank are an 11.1 L tank with 207 bar maximum service pressure holds about 2210 L of air).

To calculate the “true capacity” of a diving tank, divide the ideal capacity by the Z Factor for the specific gas at the same pressure.  For imperial measurements in this article, I used 1.0320 as the “Z Factor”

● Ideal_Capacity(ft3) = Water_Volume(ft3) × Service_Pressure(psi) ÷ Atmospheric_Pressure(14.696 psi)

● Ideal_Capacity(L) = Water_Volume(L) × Working_Pressure(bar) ÷ Atmospheric_Pressure(1.01325 bar)

● True_Capacity ≈ Ideal_Capacity ÷ Z_Factor(gas & pressure@temperature)


Newer technology, carbon fiber tanks, allow greater pressures, up to 4500 - 5000 psi, but that still limits air capacity to about 130 cuft in the same sized tank.   While it is true that future technological advances could push the capacity even further, there are absolute limits that just cannot be surpassed (not to mention, who wants to install a 10,000 psi bomb in their chest cavity?)

The next thing to consider is air consumption.  Not everyone consumes air at the same rate, but there are definitely a few things that are going to be true across the board:  The larger the body, the more air it needs.  And the more athletic or experienced one is in diving, the less air one needs.  In Shadowrun, if you wanted extra-crunchy, you could give bonuses for athleticism or lower Body attributes or penalties for someone who is wounded, sick, stressed, or of a larger-than-average Body attribute.  I’ll post some sample numbers at the end.

SCR (Surface Consumption Rate) for an untrained human of average size is going to be at least 1 cubic ft/ minute.  This rate can double under stress.  Training, athleticism, increased or decreased body size will all contribute to increases and decreases of this average rate.  Note that this number was not achieved scientifically, but rather by ballparking based on a cross-section of reported SCRs.

To figure out how long someone could “hold their breath” (or not breathe external atmosphere), while using some sort of air tank, divide the total capacity of the tank by their SCR.

For underwater use, the rate will increase due to the decreasing volume of the air (because your lungs are under pressure from the water).  To determine the change in consumption for being underwater, first, figure the atmosphere rating (this is the amount of pressure exerted on the diver by the depth of their water).   In the following example, the diver is at 75 feet below the surface / sea level:

Using the ATM formula:
 
(Actual Depth + 33 Feet) / 33 Feet = ATM

(75 + 33 ) / 33  = 3.3 ATM

Now multiply their SCR by the ATM:  3.3 x 1 = 3.3.  The diver should consume 3.3 cubic feet of air per minute at 75 feet down.

Okay, so now let’s go back to the original device in question:  The internal air tank.  Now, no dimensions are given, but we can guesstimate based on the photo.  It looks to be about ⅓ the volume of a single lung.  Average human lung volume (for two lungs) is about 6 litres, so that means the miniature tank should have a volume of 1 liter (0.035 cubic feet).  To determine the actual air capacity, however, we would need to know the maximum pressure.  The best we can do with modern technology is 5,000 psi, but let’s go crazy for a moment and say Shadowrun technology in 2050 has increased that to 10,000.  That would give us an actual capacity of 23.8 cuft.  Going back to the original 2nd edition documentation, that’s actually pretty close to what is stated in Cybertechnology - about 20 minutes of air, give or take.  I don’t know if that was good math or just a great guess from someone in the writing department.  (As a side note, this has a side-effect of increasing the body weight by about 2 pounds.)

Forward to 2080, however, and now the technology can somehow allow from 1-4 hours of air, with no change in the description.  In 30 years, they have somehow figured out how to increase the pressure from 10,000psi to 30,000-120,000 psi.  While we actually do have some polymers and alloys capable of withstanding that pressure even now, I don’t know if we have valves and seals that can do so, and furthermore, the thought that someone would be walking around with 6-24 pound bomb in their chest that could explode when struck by a stray bullet is just frightening...

So now for a few game-related suggestions for those who want their game to be really crunchy…

Determining an individual’s SCR (surface consumption rate) in Shadowrun really only needs to check two things: Their skill in athletics or diving, and their Body attribute.  This can be used whether it’s for diving or just breath-holding or using air tanks under any circumstances.

SCR = ((Body/3) / Skill) in cuft/min.
True Capacity / (SCR * ATM) = how long they can breathe.


I hope you’ve enjoyed this exercise in futility as much as I did researching it.  I welcome input from anyone with greater experience in diving, Shadowrun or just holding your breath.

Theoretical Anomaly

Sources: Shadowrun (2nd ed) Cybertechnology, Shadowrun 6e, core rulebook
https://www.divegearexpress.com/library/articles/calculating-scuba-cylinder-capacities
http://californiadiver.com/back-to-the-basics-understanding-tank-capacities/


Xenon

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« Reply #1 on: <11-01-20/0419:06> »
Volume is actually not an issue here. Not as long as the internal air tank is filled with 100% O2. Reason why you want to regulate so you get the same air pressure as the surrounding atmosphere is to make it possible/easier to breathe as normal. And the biggest reason why we need so much gas is because during diving you typically exhale the gas in your lungs (and if it is pressurized then you are exhaling pressurized gas, which mean we are consuming a lot of volume each breathe we exhale).

But in this case we are not inhaling and we are not exhaling. The system just need to maintain enough air pressure so your lungs does not collapse and so oxygen deprived blood can release excessive CO2 to your lungs and take up new O2. The system also need to maintain proper O2 levels in the gas (not too high as O2 get toxic under pressure). There also need a way to scrub excessive CO2 from the gas.

While diving with a rebreather, scrubbing excessive CO2 is actually the biggest limiting factor. Not the volume of gas you bring with you (the human body typically only metabolise about 0.25 liter of oxygen per minute - while working hard you metabolise perhaps 4 liter of oxygen per minute). In a world of magic and cybernetic I'd imagine that they invented a more efficient method of scrubbing CO2 than we have today :-)
« Last Edit: <11-01-20/0439:37> by Xenon »

Theoretical Anomaly

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« Reply #2 on: <11-01-20/1157:29> »
Based on the description of being able to refill it through just regular breathing, however, this device is not an O2 tank.  It would definitely have to be a LOT larger to be a CO2 scrubber/rebreather system.  I think there is a rebreather system in Shadowrun, but I'm forgetting where it's listed.

Stainless Steel Devil Rat

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« Reply #3 on: <11-01-20/1406:23> »
It's very interesting to see the analysis.

Not to be dismissive of it, but I'm about 99% sure your suspicion that the writers/editors simply didn't bother to refer to real world physics is correct.

I mean, it wouldn't be the first time. (anti-grav grenades, etc).   Suspend your disbelief and have fun!


If you need some help suspending disbelief, here's a nugget I can suggest:

It's not a violation of physics.. it's that post-awakening physics have changed.  Literally, the laws of the universe are now slightly different as compared to how we understand them in the real world.  This potentially even includes phenomenae that don't directly relate to magic.  (note that is isn't complete BS... it's canon that the Sixth World weather is now unpredictable by pre-awakening meteorological techniques due to the weather being affected by astral/magical energies.)
RPG mechanics exist to give structure and consistency to the game world, true, but at the end of the day, you’re fighting dragons with algebra and random number generators.

Xenon

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« Reply #4 on: <11-01-20/1530:32> »
What if it just cycle regular air to and from your lungs?
According to wikipedia you can, on average, breathe same same air 10 times.

And according to wikipedia (again) you breathe 7-8 liters of air per minute.
That is ~450 liters of gas per hour. Or ~1800 liters during 4 hours.

If you can breathe the same air 10 times you need the tank to store 180 liters of air.
At 300 bars (which is what my regular diving tanks have) that give you a volume of 0.6 liters.
(edit, 300 bars = 4,350 psi)

Looking at the picture you provided, a 0.6 liter tank seems plausible, yes?
« Last Edit: <11-01-20/1534:30> by Xenon »

Reaver

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« Reply #5 on: <11-01-20/1931:27> »
Speaking as someone who as constantly raised issues about Power (as in Energy), Power transmission, and Shadowrun devices... I feel your pain in the, shall we say "Liberal" twisting of physics that happens in Shadowrun...

But SSDR is right also right... Sometimes you just gotta say "fuck it" to what you know, and roll with what has been handed to you, Or you go insane right quick...

Where am I going? And why am I in a hand basket ???

Remember: You can't fix Stupid. But you can beat on it with a 2x4 until it smartens up! Or dies.

adzling

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« Reply #6 on: <11-02-20/1012:15> »
Yes it's nuts and there are plenty more things in srun that do not hew to physics.

If you go down this rabbit hole you'll never stop... ;-)

for pure insanity lookup what happens when you speed up a moped as fast as possible with spirits and movement power and crash it into a city...

It is also true that it's gotten more out of hand as catalyst released more splatbooks for 5e.

The book with the anti-grav grenades was hot garbage because it pushed tech so far beyond anything that had been in shadowrun before.

Xenon

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« Reply #7 on: <11-02-20/1057:50> »
I still think this could be the answer:

What if it just cycle regular air to and from your lungs?
According to wikipedia you can, on average, breathe same same air 10 times.

And according to wikipedia (again) you breathe 7-8 liters of air per minute.
That is ~450 liters of gas per hour. Or ~1800 liters during 4 hours.

If you can breathe the same air 10 times you need the tank to store 180 liters of air.
At 300 bars (which is what my regular diving tanks have) that give you a volume of 0.6 liters.
(edit, 300 bars = 4,350 psi)

Looking at the picture you provided, a 0.6 liter tank seems plausible, yes?

And since the human body don't metabolize oxygen faster just because it is set under pressure (at least I don't think it does) I am pretty sure the gas would also last for 4 hours no matter if you are at sea level of 40 meters underneath the surface....

Theoretical Anomaly

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« Reply #8 on: <11-03-20/0232:32> »
What if it just cycle regular air to and from your lungs?
According to wikipedia you can, on average, breathe same same air 10 times.

And according to wikipedia (again) you breathe 7-8 liters of air per minute.
That is ~450 liters of gas per hour. Or ~1800 liters during 4 hours.

If you can breathe the same air 10 times you need the tank to store 180 liters of air.
At 300 bars (which is what my regular diving tanks have) that give you a volume of 0.6 liters.
(edit, 300 bars = 4,350 psi)

Looking at the picture you provided, a 0.6 liter tank seems plausible, yes?

I'm not sure where you got that wikipedia quote, but let's agree that Wikipedia isn't the end-all-be-all.  If you really could breathe the same air 10 times, I'm sure divers would have figured that hack out a long time ago to stay under far longer.  The dangers of rebreathing CO2 are pretty well substantiated, and the resulting damage would have to be accounted for in stun or physical resistance tests.

I agree that there's a lot of handwaving and saying "Because it DOES, okay?!" in Shadowrun, I was just having a lot of fun with something I actually do in real life and applying that knowledge to the game.  If nothing else, I think the SCR formula I came up with is neat enough to be useful if the situation ever came up (and who HASN'T had their runners dive off the end of a pier and try to hide underwater?!)

Xenon

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« Reply #9 on: <11-03-20/1429:20> »
I'm not sure where you got that wikipedia quote, but let's agree that Wikipedia isn't the end-all-be-all.  If you really could breathe the same air 10 times
https://imgur.com/a/J8vy0mG


I'm sure divers would have figured that hack out a long time ago to stay under far longer.
When diving with rebreather you are literally rebreathing the same air over and over.

You typically only have little more than one breath of air in the loop. You breathe in to fill your lungs. You breathe out to fill an "external lung". Over and over. Same air.

Since you only have little more than one breath of air in the system this will typically only work for 10 breaths until the CO2 levels become too high (causing hypercapnia) and the O2 levels becomes too low (causing hypoxemia) - in order to dive for more than 10 breaths using a rebreather CO2 is being scrubbed and O2 being added.

What I am suggesting is that you use the entire internal air tank as your "external lung". With a 0.6 liter tank at 300 bar this give you 180 liters of air. It will take you on average 4 hours to rebreathe all of it 10 times before the CO2 levels become too high and the O2 levels becomes too low ;-)

Theoretical Anomaly

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« Reply #10 on: <11-17-20/0312:38> »
When diving with rebreather you are literally rebreathing the same air over and over.

You typically only have little more than one breath of air in the loop. You breathe in to fill your lungs. You breathe out to fill an "external lung". Over and over. Same air.

Since you only have little more than one breath of air in the system this will typically only work for 10 breaths until the CO2 levels become too high (causing hypercapnia) and the O2 levels becomes too low (causing hypoxemia) - in order to dive for more than 10 breaths using a rebreather CO2 is being scrubbed and O2 being added.

What I am suggesting is that you use the entire internal air tank as your "external lung". With a 0.6 liter tank at 300 bar this give you 180 liters of air. It will take you on average 4 hours to rebreathe all of it 10 times before the CO2 levels become too high and the O2 levels becomes too low ;-)

A rebreather would be a little harder in the space available, because it requires three tanks: 1 to hold pure oxygen, one to hold "air" and a scrubber tank to cart off CO2.  Still, it does fit the space a lot better.  Ah well, if nothing else, I got some good numbers to play with if my players ever go diving or holding their breath.

Xenon

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« Reply #11 on: <11-17-20/0540:27> »
I was not talking about a rebreather that have an external lung, an oxygen tank, a dilute tank and a CO2 scrubber....

I was talking about just 'rebreathing' the air in the 'external lung'.
(but that the 'external lung' in this case is an internal tank of 0.6 liters that contains 180 liters of air)