Help for Novel

[Grenouille]

So in commercial diving operations, they're connected to what, a sub? Is there any technical advantage to using them, as opposed to independent tanks? Is it less expensive, less likely to be at fault?

 

[Rich Keller]

They are usually connected to the surface, to a volume tank that is pressurized from a low pressure compressor. The compressor, air in the tank and communications with the surface are all monitored by a tender. This is far safer then scuba as you have two sources of air plus you can also wear scuba tanks that are attached to the helmet as a third source of air commonly refereed to as a bail out bottle. If you do run into trouble you can communicate with the tender in two ways to let him know that you need to be pulled up to the surface. For long duration dives it is far more efficient and safer then scuba but it is not less expensive.

 

[Grenouille]

If they have tanks all the same, does that mean they could disconnect the cord and just use the tanks to get back to the surface, or would they need necessarily what the cord specifically provides? This cord is a pretty awesome info, thanks a lot!

 

[Rich Keller]

The umbilicals are not made to be disconnected by the diver. It can be done in an extreme emergency where the umbilical has been crushed under something that cannot be moved.

 

[Grenouille]

Right, but if one did so, would that mean they could get back to he surface without the cord, just with their tanks? Can we imagine they have no tanks at all or would that seem suspiscious?

 

[kaerius]

Umbilical divers do not have tanks as a rule of thumb, possible exception of an argon tank that is for their drysuit only.

At 1km depth, umbilical or rebreather diving are the only real possibilities, with a regular open circuit scuba cylinder, gas would be consumed at a rate 101 times that on the surface. A tank which would last 300 minutes at the surface would last for less than 3 minutes down there. Even a gigantic 50 Liter/300 BAR tank(529 cubic feet of gas at that pressure) would last less than 20 minutes, and the weight of that would be massive, it's just not a feasible alternative.

Also, to surface safely from 1km down would take years. The decompression obligation would be monstruous. The guy who did the deepest recorded dive, 701 meters, had 6 months of decompression. Decompression times get exponentially longer the deeper you go, not linearly, I wouldn't be surprised if the math showed someone saturated at 1km to need something like a decade of decompression.

 

[Grenouille]

James Cameron dived at 7 miles, if I'm not mistaken, in that little sub of his, breaking a record, how is that different from my situation? James Cameron's sub is like our shelter, why would they need years of decompression if he didn't, and he dived much deeper? Sorry if I misunderstand some basic things, it's still all pretty new to me
Oh and also, what kind of time would they have outside with an umbilical cord? A few hours? Unlimited?

 

[kaerius]

James Cameron's sub was a pressurized enviroment, a rigid container, he wasn't subjected to the pressure of that depth. A diver leaving the sub at that depth would no longer be in a rigid container, thus would be exposed to the full 101 atmospheres of pressure, and would start racking up decompression hours by the second. Max time with umbilical would depend on three things: how much decompression time they would be ok with, how large the gas supply is(since it is used at a rate 101 greater than at 1 atmosphere, note that the diving gasses required are rare, expensive, and in finite supply), and how good their thermal insulation is compared to the temperature of the water(for example even in a drysuit you wouldn't want to spend more than an hour in typical temperatures at those depths, because the temperature would be a uniform 4 degrees celcius unless it's near volcanic vents, that's the temperature at which water is heaviest).

PS: If the shelter is held at a lesser pressure than the ambient pressure at that depth, pretty much a requirement btw, then those month/year/decade long decompression times would apply to returning to the shelter as well as to going to the surface. Your divers would need to spend all that time in decompression chambers after a dive, if they wanted to return to the shelter.

 

[Grenouille]

Right right, but I thought I'd been told here that with the kind of suits we were talking about (the name escapes me now, but that's the ones looking like astronauts suits), it was like a mini-sub wrapped all around you, so you're not subject to pressure either.
So if I want my divers to be able to get out whenever they want to explore and get stuff, I'd need to pretend they can recreate the gasses that they need as much as they want... which sounds scifiish, from what you're saying.

 

[kaerius]

Right right, but I thought I'd been told here that with the kind of suits we were talking about (the name escapes me now, but that's the ones looking like astronauts suits), it was like a mini-sub wrapped all around you, so you're not subject to pressure either.

Those suits would have to made of several centimeters/inches of solid metal, or sci-fi unobtainium material to not be crushed at those depths. Military submarines often have crush depths in the 200-300 meters range... For a spacesuit: think tube of toothpaste under an asphalt roller and you're not far off...