Emergency Descent During Deco

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Diving in shipping zones is sometimes unavoidable. Mainly cause that's where you are likely to find ship wrecks.

Maybe it's a Cape Town thing, but if you are in a shipping zone, instead of only one SMB for the buddy pair, everyone shoots one. It is possible for the boat to SLOWLY tow you out of the way of the on coming ship. Maybe practice this without a deco obligation first because you will tend to drift upwards as you get towed.
 
Bottom line (for me): Having dived both ways, I prefer using computers. Still cut tables and have them and a BT in my pocket, but computers are the tool to have when the unexpected happens. Real-time PPO2, depth & mix calculations offer options.

I totally agree, that's why I now use a computer after starting with cut tables - much more flexibility
 
If you have to spit your 02 reg and go to back gas and dive rapidly, the computer is the last thing you are going to worry about. That means you have to know decompression theory and how to do deco on the fly. the computer has you on the wrong gas and will not compute correctly. The tables in your pocket did not contemplate this event at that time. Deco on the fly is actually quite simple if you understand the three stages of a deco dive and the ratios of bottom to deco for each one. Moreover, you can then pad the deco so as to give yourself an extra margin of safety as you come up your alternate bag. Arguing that oh...you should have it in your tables or oh you should use your computer, simply ignores reality.

As for the integrated rebreather computer, if that works for you then it is fine...until it fails. (they do fail). Nevertheless, it is probably a better bet that at the end of a dive if you had to do an emergency descent the rebreather computer will give you good advice. I found that the Vision Electronics on the Evo+ were a bit too liberal for my taste, but I would trust them to get me out of the water safely.
 
I ran this interesting scenario on Multi-deco.

So, I swagged a dive to 150 for 30 minutes on 26% with 50% deco gas. Came up with this:

Depth Stop Run Mix pO2 EAD
Des 150 - 3 26 - -
Lvl 150 27 30 26 1.41 138
Asc 70 - 32 26 - -
Stp 70 0:20 33 50 1.53 31
Stp 60 1:00 34 50 1.38 25
Stp 50 2:00 36 50 1.23 19
Stp 40 3:00 39 50 1.09 12
Stp 30 4:00 43 50 0.94 6
Stp 20 7:00 50 50 0.79 0
Stp 10 17 67 50 0.64 0
Sfc - - 67 50 - -

Dive# 1, VPM-B + GFS/85
Elevation = 250 ft
CNS = 28%
OTU's = 72
Gas 26 = 227 cuft.
Gas 50 = 56 cuft.
Decozone start = 99 ft


Then I punched all those levels in, with only four minutes at 20, leaving 20 minutes of deco, and sent the plan back to 100 for four minutes. Not much of a hit.

Depth Stop Run Mix pO2 EAD
Des 150 - 3 26 - -
Lvl 150 27 30 26 1.41 138
Asc 70 - 32 26 - -
Lvl 70 1:00 33 50 1.53 31
Asc 60 - 34 50 - -
Lvl 60 1:00 35 50 1.38 25
Asc 50 - 35 50 - -
Lvl 50 2:00 37 50 1.23 19
Asc 40 - 37 50 - -
Lvl 40 3:00 40 50 1.09 12
Asc 30 - 41 50 - -
Lvl 30 4:00 45 50 0.94 6
Asc 20 - 45 50 - -
Lvl 20 4:00 49 50 0.79 0
Des 100 - 50 50 - -
Lvl 100 2:24 53 26 1.02 91
Asc 70 - 54 26 - -
Asc 20 - 56 50 - -
Stp 20 1:00 57 50 0.79 0
Stp 10 20 77 50 0.64 0
Sfc - - 77 50 - -

Dive# 1, VPM-B + GFS/85
Elevation = 250 ft
CNS = 47%
OTU's = 83
Gas 26 = 244 cuft.
Gas 50 = 87 cuft.
Decozone start = 39 ft
 
I think I said this before, but it bears repeating in the light of what some people have posted. Once you drpart from the overall profile assumed by the people who developed the algorithm of your tables or computer, eg. you suddenly decide to descend to 100ft from an ascent through 15ft, you have entered untrodden territory. The tables/computer may well tell you numbers/depths/times, but these are no longer on a solid foundation and at best can be treated only as a general indication. All you can now do is guesswork based on your experience/knowledge/training and whatever these devices are telling you. Your guess may or may not be good enough. The very experienced diver I mentioned who dropped down to maybe 50ft from his final "safety" stop did the best he could - he wasn't short of air - yet ended up spending most of four days in the chamber with a severe bend and has been advised he should never dive again. Quoting lists of numbers given by tables or computers in these circumstances is grossly misleading and to my mind foolish in the extreme.
 
The reality is that if you are using a computer OR tables to control your dive/ascent, you will be in complete breach of the built-in algorithm by suddenly making a radical deviation from the plan. I think there's a very good chance you'll end up on oxygen and then in a chamber, but that's still preferable to the alternative in the cited scenario.

I watched a diver do something very similar some time ago, having run pretty well up to the limits on a recreational dive he dropped back to maybe 50ft from his final stop ("safety stop") to photograph something he'd just spotted on the bottom. That was a costly mistake. He returned home about a week later than he had planned, and some $40k poorer. He hadn't thought he needed dive insurance - he was wrong.

---------- Post added July 12th, 2013 at 09:00 PM ----------



Your comment was far from a general one. You implied that using a computer was a cop-out and a real "tech" diver wouldn't need/use one. That is supreme arrogance. Whatever a "tech" diver is, (s)he should be equally comfortable in either case, but interested in using a computer because of the greater flexibility it permits. I was using myself as an example of many experienced divers out there - I have neither the need nor the desire to justify myself on a message board.


What you are describing with the photographer does not relate to the Dumpster diver incident....
In your story, the recreational profile which we can assume had been "maxxed", got interupted at the final stop--and the diver in question was potentially at a borderline condition of bubbling at this moment--and needing to avoid a cascade effect.

If this photographer is going to shoot something , this usually means he is going to spend at least SEVERAL MINUTES in setting the shot up, arranging strobes, trying for multiple pictures and angles. While he was doing this final shooting, this photographer actually extended his bottom time --making the dive that many minutes longer, and I think it is safe to say that he turned it into a decompression dive--made worse by the bubble growth he had from the bubbles that had already been created at his initial safety stop. From your account if the incident, it does not sound like your diver treated the dive as a real deco dive, with a real decompression stop at 30 feet or 20 feet, after the final shooting.....

In the DD example, he would have needed a single minute or even less to have the big ship pass--and then he would go back to a REAL DECO stop.....which ever one he had been in before....if he had pure Oxygen( which would have been smart on a deep dive scenario as DD had suggested), then the bubble dynamics would be handled by doing the full stop duration at 20 feet on O2. If no O2 had been taken by DD( or his stand-in), then one stop depth higher might well be the solution---ten minutes at 30 feet, and then back to the 20 foot stop. Also keep in mind that the deco time was for a relatively short duration 160 or 200 foot deep dive...and that at 50 or even 70 feet deep, this is still technically a decompression stop depth, and the gas dynamics important to consider at this time-- are the issues with dissolved gas from 200 feet deep, not from a minute at 70 feet.

This is not about complex math...it is really more about common sense.
 
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Like Stewart said, there are plenty of technical divers that never us a computer, just a BT (and backup BT/watch), running cut profiles. I did all of my tech dives that way, down to 100m, until I got an X1 earlier this year. Depending on the nature of teh dive (square profile or not) I usually still cut a run time (on Decoplanner) and will often follow that with the X1 as a 'backup' incase the dive turns out not to be square or the dive plan changes to something I don't have/know a run time for.

I don't have a computer, neither do the guys I dive with. Why would you need one?

I guess I was just taught with a different mentality. I know you don't NEED to have one but I would rather have one than not...
 
I HOPE the boat would protect me, but a 25 ft boat versus a 450 ft ship moving 30 mph is not something I can "expect" to protect me.

Why shouldn't you? There are clear maritime laws on the matter. Diver down and/or boat drifting... vessel size is irrelevant. Assuming, of course, radio carried and appropriate flags displayed.

On cut tables (assuming little detail needed for computers, which should resolve a course of action), I'd apply a 'Delay in Ascent' protocol from whichever depth I had returned to, repeating the deco profile from that depth upwards:

Delay in ascent
1. A delay in your ascent between stops is not usually a major issue.
2. If used, your dive computer will calculate the changes in your required decompression, if any.
3. If using a tables, it is not critical if the delay is short (2-3 min or less)
a. Don’t count the delay as decompression time.​
b. Extend your last stop as much as practicable, gas allowing.​
 
Why shouldn't you? There are clear maritime laws on the matter. Diver down and/or boat drifting... vessel size is irrelevant. Assuming, of course, radio carried and appropriate flags displayed.

That's a good one! Wait... you're serious? Still funny, but for a different reason. Ask around a working harbor about how often they've seen a freighter come into port with the rigging of a 40-50' sailboat hanging off the anchor and the crew none the wiser.
 
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That's a good one! Wait... you're serious? Still funny, but for a different reason. Ask around a working harbor about how often they've seen a freighter come into port with the rigging of a 40-50' sailboat hanging off the anchor and the crew none the wiser.

Are you saying many freighters don't run a radio watch?
 

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