Need info for a novel (deep diving information)

[Greg1034]

I'm not sure if anyone here can, or would even have the time to respond to this inquiry, or if this is the right forum topic to be posting this. I'm writing a novel and a portion of it takes place at an undersea shipwreck. I want it to have at least a basic sense of reality so I was looking for some simple tech info on deep sea salvage dives. I'd appreciate any help anyone could give me. I've done a small amount of research but I'm not completely familar with all the correct terminology so please bear with me if I get some of this wrong. And please remember, this will be a work of fiction so there is room for pushing the reality envelope to a certain degree. I'm looking for basic info pertaining to:

1) At a dive of approx 200ft, working with trimix tanks, how long could divers work and stay down safely at that depth? Is there a maximum safe time? If there is no limit, what would the answers be to the following questions based on an on-wreck time of 60 to 90 minutes? Keep in mind these divers are very experienced yet somewhat unorthodox risk-takers.

2) What would the time of ascent be with a dive like this if they stayed down the maximum safe time (or for 60-90min)?

3) How many stops on the way up would they need to make and what type of mixture level changes would they need to make during ascent?

4) How long would they have to wait before they could dive again to that depth, and if they did dive again, would their down time (time at site) need to be less and how would this affect their decompression ascent time back to the surface?

5) If someone had to do an emergency ascent, fully inflating their buoyancy compenasator, or some other means of very, very fast ascent, from approx 100ft, could they survive if the ship had a portable hyperbaric chamber on board and couldn't make it to a medical facility, and how long could it possibly take before they might regain some sense of coherent consciousness?

Again, remember there is room to push the reality envelope a little for this scenario.

If anyone is able to assist with any of this info, I thank you now, and if and when this gets published, a technical advisory credit would of course be given.

 

[devolution365]

I'm no help, but good luck with the book!
(And as an avid reader, I thank you for trying to get the technical stuff as close to reality as possible.)

 

[pennypue]

Greg, there are LOTS of people here who can help. Number one though pretty much turns the whole scenario into a body recovery though. You really want to talk to someone that dives trimix with decompression.

Reword number one to ask "what type of gas they should be diving if I wanted them to do a dive to 200ft for X minutes". You'll get a much better scene this way, and much better advice!! And try posting it in the technical forum.

 

[roakey]

1) At a dive of approx 200ft, working with "enriched air nitrox" tanks, how long could divers work or stay down safely at that depth? If this type of air supply isn't correct for this type of dive, any info about that would also be appreciated also.

I'll take this one first, because it'll change the rest of the answers. Nitrox won't work for this kind of a dive; above a certain partial pressure, oxygen becomes toxic, for this reason Nitrox (having more oxygen than "normal" air) is an intermediate depth gas, not a deep gas.

In fact, the oxygen in air (~21% O2, 79% N2) potentially becomes toxic after about 190 feet. So you actually have to REDUCE the amount of O2 in the breathing gas beyond this level. You could dilute the oxygen with more nitrogen, but you're already going to have a bad case of nitrogen narcosis at that depth anyways, and increasing the Nitrogen in the gas you're breathing will only make it worse, so you don't want to do that, so you'll have to dilute it with some other gas that isn't as narcotic at that depth -- enter helium.

So you're talking about trimix at that depth, a mixture of oxygen, nitrogen and helium. I'm sure others will begin to chime in, giving you answers to the rest of your questions.

Good luck on your book!

Roak

Ps. Yes, I know they're some nits in what I said above, I suggest everyone keep in mind that this is a book, and we should strive to give correct answers without getting all hung up in the nitty details that the autor isn't going to include anyways...

 

[Bicster]

I'd suggest getting open water certified as part of your research. It would be a useful gateway to understanding the more technical stuff. There are also some interesting articles on wreck diving (and accidents) at njscuba.net, particularly the Andrea Doria stuff.

 

[Goose75]

Check the Technical Diving Specialties forum for technical diving methods.
http://www.scubaboard.com/forumdisplay.php?f=43
Wreck diving is one of the sub forums there. There are two major camps as to agency, GUE(DIR) and TDI. The GUE webpage (http://www.gue.com) will have detailed information as to gear choices, methods and has video for sale. If your character is going to claim to be DIR, then it needs to be as close to reality as possible. The program is strict and no one in the dive community will find a character believable that is "DIR" and doesn't act like it. The information on their webpage should be informative whatever philosophy your character is from. A diver may use some or all of the DIR methods but they won't call themselves DIR unless they are. TDI divers have more variation and you can get away with a bit more flexibility for a novel. You don't have to mention philospophy at all for what you want, just make sure not to claim something that will turn off your readership as unbelievable.

Check the Marine Science and Physiology forum for the effects of a rapid ascent from 200ft. I'm sure you will get a gory and painful account of the bends
http://www.scubaboard.com/forumdisplay.php?f=94

 

[mart1]

1) OK, so you know now trimix (nitrogen, oxygen and helium is the preferred mix for this depth.
2) There is no maximum safe time down there, as long as you have mix in your tanks. Ascent time for half an hour at this depth would be about one hour, depending on the gas(ses) the diver will breath during ascent and the safety margin he wants. longer bottom time means slower ascent.
3) Stops would start at about half the diving dept, and be every 10 feet. Early stops would be 1 minute each, later stops would become progressively longer. Mix for the ascent could be 50% nitrox and 100% oxygen, gas switches would then be somewhere around 60 feet and 20 feet (as I work in metric normally and have to change this to feet now numbers are not exact)
4) Surface interval could be anything, but most divers will limit trimix decompression dives to one a day. Theoretically a diver could descend again immediately upon surfacing, again dive time has no limit as long as the diver is prepared to make a very slow ascent.
5) Two possible problems with this ascent: damage to the lungs caused by expanding air, worst case the diver would arrive dead on the surface. Decompression problems: if this emergency ascent takes place after the 200ft dive again chances are the diver does not survive, a lot depends on dive depth and dive time. Symptoms of decompression could range from skin bends (red spots on chest, belly, etc) to central nervous system (paralysis, speach impairment, blindness, anything goes here). Immediate recompression in a chamber is possible and as long as decompression illness has not set in no damage is done. In fact this is how a lot of commercial diving is done. Again all depends on how long, how deep, and how fast the diver gets in the chamber. Most of the time divers get to the surface without symptoms, decompression illness will set in after surfacing and get worse as time goes by.

ciao, maarten

 

[Ber Rabbit]

"Shadow Divers" and "The Last Dive" would be excellent references for you to read.
Ber :lilbunny:

 

[roakey]

to:1) At a dive of approx 200ft, working with trimix tanks, how long could divers work and stay down safely at that depth? Keep in mind these divers are very experienced yet unorthodox-risk takers.

"stay down safely" isn't easy to define, because in actuality there’s no limit. People can work at that depth an indefinite period if they do "saturation" dives where they either live in a habitat at that depth, or have a diving bell that keeps them at that pressure as it returns them to the surface, then they move through a lockout into a larger chamber that keeps them “at depth” while on the surface.

The other extreme is a “bounce” dive where the person reaches their maximum depth, performs a very quick task (like setting the hook on the wreck) and then ascends.

Decompressing from a 200’ saturation dive may take days, decompressing from a bounce dive may take tens of minutes, I’ll run some number later for you.

So let me redefine what you want to consider for “how long can you stay down safely.” Think in terms of the whole dive, surface to surface. This is called “run time” – the time for the descent (a couple minutes) the time at depth and the time for decompression.

So the question becomes, what’s a safe “run time?” Interestingly, the answer to that becomes more about what’s happening up top than what’s below you. If there’s a storm brewing, and the waves will become a problem in a couple hours, a 1.5 hour run time might be your maximum “safe time.”

There are some comfort factors too – you can pee in a wetsuit, and you can pee in a drysuit (men only) if you have a pee valve (condom catheter with an overboard dump valve) but pooping is a problem, for example. So that’s a limiting factor of run time as well.

Repetitive dives again don’t have a maximum safe time at depth, but the decompression phase of the dive becomes longer for the same amount of bottom time… I don’t have numbers right now, but taking a previous noter’s estimate of an hour of decompression for 30 minutes of bottom time (I don’t know how accurate this is, but I’ll use it as an example). If the diver went in for another 30 minute dive four hours later, he might be looking at two hours of decompression for the second dive.

Hold on, let me figure out a mix for this dive…

Roak

 

[roakey]

The mix. Since as you say these folks are “unorthodox” I’ll figure out what’s called “best mix” for that depth. Many people use standard mixes for certain depth ranges, because your gas is just another piece of equipment and you should know your equipment, and creating “best mixes” for certain depths violates this principle since there’s basically an infinite number of permutations and combinations, depending on depth.

But anyway, here are the goals: O2 exposure a maximum of 1.4 ATA (atmospheres absolute) at maximum depth. An END (Equivalent Nitrogen Depth) of 100’ (person will experience the amount of narcosis at depth as they would experience on air at 100&#8217. This is a 3.16 ATA of N2 at maximum depth. This will keep their minds quite clear for salvage operations.

I’m going to do my calculations for 198 feet, because that’s a nice, easy number to calculate – 198 feet is a pressure of 7 ATA (198 feet / 33 feet per 1 ATA + 1 ATA for the atmosphere sitting on top of the water column itself).

So, percentage of O2 = 1.4 / 7 ATA = 20% O2 (just barely less than air).

Percentage of N2 = 3.16 / 7 ATA = 45% N2.

The rest is Helium: 100 – (20 + 45) = 35% He

So they’d do this dive with 20/35/45 or 20/35 for short.

Given a mix, now we can calculate decompression times.

Roak