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-capacitieshttp://californiadiver.com/back-to-the-basics-understanding-tank-capacities/