Analyzing a 90 m trimix dive in dahab egypt

[AbyssalPlains]

Seems like an awful lot of trouble for a two-minute bounce dive to 90 meters, for which an over-filled aluminum-80 with air would have probably worked just fine.

 

[Blackwood]

1000 cubic centimeters . :mooner:

what the hell is a cubic?

 

[Kevrumbo]

How much backgas gas did you reserve to get an OOA buddy from 90m up to 21m? ~14mins at an average depth of 40m with 2 stressed divers...?

Personally, I would not bother with 2min bottom time dives but to each his own.

The OP didn't say where the safety diver as meeting them and also didn't mention an OOA (backgas) scenario despite listing various lost deco gas scenarios. He asked for opinions and rock bottom (along with the role of a safety diver in it) seems to be lacking from his plans.

If you only do 2min dives in training does that mean you're only qualified to do 2min dives?

The way they executed the dive, it looks like the "Rock Bottom"/Out-of-Backgas contingency was figured from 90m to the 21m deco stop, without deepstops, at a constant 9m/min (30'/min) ascent rate. This would require 3100 litres or 110 cuft of reserved backgas per teammate. With only 11 litre twins (double AL80's) for bottom mix, I think they barely had enough for only one donor-recipient pair (although mitigated somewhat with a team of three and a safety diver).

As soon as their SPG read 140 bar (or 2100 psi), they better had had started their ascent whether they reached the planned maximum of 90m or not. . . 90m to 21m, a range of 69m/230' is a very long way to go sharing gas in an OOG emergency.

 

[Kern]

what the hell is a cubic?

:shocked2: You need to find a cubicle, & have a good lie down.

 

[Rhone Man]

what the hell is a cubic?

Isn't it a unit of length from Biblical times, roughly equal to the length of a grown man's forearm?

 

[GrimSleeper]

V-Planner Configuration:
We then looked at the configurations of the V-Planner. We choose a SAC rate of 20 liters/minute both for the bottom gas/travel gas and the deco. We all had better SAC rates than this, but better to be conservative when it comes to gas planning. The descent rate was set at 15 meters/min (also a fairly slow descent rate, but once again a more conservative one). Our ascent rate was set for 9 meter per minute for the ongassing phase of the dive and 6 meters/minute for the off gassing phase (see plan below, off gassing starts at 59 meters and that’s where we slowed down from 9 m/min to 6 m/min.). Our accepted ppO2 for theO2 decompression part of the dive was set at 1.4 ppOO2 for the mix with O2 up to 28%, 1.5 ppO2 up to 45% and 1.6 ppO2 up to 99%. We used conservatism "+2" in the V Planner Configurations.

Not sure why you'd think setting a slow descent rate is conservative: if you bust that rate, but leave the bottom on runtime, you've built up a significant extra deco obligation that your plan doesn't account for. Try running exactly the same dive again, but with a 30m-minute descent (easily achievable), and the profile runs 8 minutes longer. Surely it makes much more sense to plan for a descent rate faster than you can achieve, so that your actual inert gas load is lower than the software is assuming.

And I've gotta say... Why would you go all that way and not spend some time at the bottom? But that's just personal choice.

Anyway, happy diving...

 

[Kevrumbo]

Do a You Tube search on "Blue Hole of Dahab", and you'll see there's nothing much other than the macabre sight of divers' remains lying at the sloping bottom of the arch at 90m and beyond.

 

[GrimSleeper]

Do a You Tube search on "Blue Hole of Dahab", and you'll see there's nothing much other than the macabre sight of divers' remains lying at the sloping bottom of the arch at 90m and beyond.

Oh. Just did. Looks like... fun?