Stage Dive Planning

[T-Dog]

With all due respect … read the damn posts before you type.

I read and re-read all posts before submitting anything to any forum. When I do post my sole goal is to provide safe recomendations for completing dives. I suggest you do the same since you repeatedly mis-quoted the original dive profile to trivialize this dive.

You take quotes from my post, but instead of responding to real issues raised, you
aim to dismiss them with a wave of your hand rather than using data & calculations.
Please use science and logic to justify your position for using an exceptionally slim gas reserve for decompression diving.

 

[limeyx]

Your first post in over a month and this is what I get?

Sheesh :mooner:

it was actually a reall good answer. Often the easiest way to do a long dive is actually to do two shorter ones and avoid the deco.

 

[limeyx]

I have done the class.

Have You?

No, I have not taken a class that certifies me to do a deco dive with a deco gas and a bottom stage. I don't think you have either because if you had, you would not have had to ask the original question.

 

[limeyx]

I read and re-read all posts before submitting anything to any forum. When I do post my sole goal is to provide safe recomendations for completing dives. I suggest you do the same since you repeatedly mis-quoted the original dive profile to trivialize this dive.

You take quotes from my post, but instead of responding to real issues raised, you
aim to dismiss them with a wave of your hand rather than using data & calculations.
Please use science and logic to justify your position for using an exceptionally slim gas reserve for decompression diving.

Actually, his posts were spot on. The Original poster and probably yourself would do a good thing by really reading and trying to understand them.

Simply reserving 1/3 of gass as bailout is evidence of not doing proper dive/gas planning.

1/3's is overkill for most open water dives (not all). You will be better served by considering

- what "kind" of dive -- all usable, 1/2 or 1/3's
- planning the gas appropriately considering not needed to violate the deco scheduled given a certain set of failures (usually 1 serious loss of gas on 1 diver for a team of two)

 

[Thalassamania]

Is it scientific and logical enough to just repost that which I posted before, and explain it step by step (as I did before)?

OK, 0.6 scfm for me and 0.6 scfm for my buddy.
[NOTE: 0.6 SCFM is a given from the OP.)

For a 110 ft dive for 40 minutes that’s 104 cubic ft for gas that I’d need.
[NOTE: 104 cubic feet needed to do the dive.)


50% reserve is overkill. To wit:

Gas for ascent for me and my buddy = 4 cubic ft.
[NOTE: 2 cubic feet needed for ascent.)

3 min deep stop at 55 ft for me and my buddy = 10 cubic ft.
[NOTE: 5 cubic feet needed for deep stop.)

10 min at 10 ft (7 at 10 required) for me and my buddy = 16 cubic ft.
[NOTE: 6 cubic feet needed for decompression.)

Reserve for an extra 10 min decom (in case of one step too deep or too long) for me and my buddy = 16 cubic ft.


[BOTTOM LINE: Carrying 160 cubic feet, 117 required to complete dive and deco, 43 cubic feet in reserve or 27%.)

Is that any clearer?

Have a great fourth.

 

[T-Dog]

Actually, I appreciated your last post. I'm in no way, shape, or form interested in slinging mud on the internet.

BTW thanks for the 4th of July sentiments. Unfortunately, I had to work all day, though it was in AC comfort all day. I'm a geologist, and tomorrow I'll be on a drilling rig morning at 6:30am w/ a 65-yr old ex-marine driller who will wear me out in the SC sun for 10-hrs. Hell this guy would put John Wayne to shame. I hope you had a great 4th holiday, and I really do mean that.

Back to the original thread...

"Dec to 110ft (2) Nitrox 32 50ft/min descent.
Level 110ft 37:48 (40) Nitrox 32 1.38 ppO2, 90ft ead
Asc to 80ft (41) Nitrox 32 -30ft/min ascent.
Level 80ft 10:00 (51) Nitrox 32 1.09 ppO2, 64ft ead
Asc to 20ft (53) Nitrox 32 -30ft/min ascent.
Stop at 20ft 6:00 (59) Nitrox 32 0.51 ppO2, 13ft ead
Stop at 10ft 11:00 (70) Nitrox 32 0.42 ppO2, 4ft ead
Surface (70) Nitrox 32 -30ft/min ascent.

OTU's this dive: 77
CNS Total: 30.8%

142.1 cu ft Nitrox 32
142.1 cu ft TOTAL"

If both divers have a SAC of 0.6 cfm, then they will each use ~15 ft3 of gas for deco. If these two divers are constrained to Al80 doubles, and do not take a stage, then theoretically they have enough gas for both divers to complete the deco portion of
the dive, even if one diver losses all remaining his gas @53 min into the dive, and they'll have 3ft3 to spare.

However, this is too slim a margin for me personally. I would either carry a stage to complete the dive, or dive an abbreviated schedule with just a set of Al80 doubles.
So I can't in good conscious recommend any other course of action to another diver.

If equipment availability wasn't an issue, and I really wanted to dive this profile, I would use a larger set of doubles and use a deco bottle (40 - 80 ft3) w/ 100% O2 at the 20-ft and 10-ft stops. There is a tech dive shop on Grand Cayman that may rent other equipment.

Safe Diving!

Terry

 

[Thalassamania]

I guess were just going to have to agree that we add things up differently. But I think that we are in agreement that a set of double 80s and a 40 full of 100% O2 might be the best way to cover all bets.

 

[JimC]

[BOTTOM LINE: Carrying 160 cubic feet, 117 required to complete dive and deco, 43 cubic feet in reserve or 27%.)

Is that any clearer?

Have a great fourth.

You'll note your deco profile looks nothing like what the poster plans to do. Although, I agree with the 1/3rds being complete overkill and a a copout of doing a real gas plan a set of double 80's does not have sufficient gas to do the dive the way the original poster wants to do the dive. So he needs to take more gas or change the plan.

 

[Rick Murchison]

even if one diver losses all remaining his gas @53 min into the dive, and they'll have 3ft3 to spare.
However, this is too slim a margin for me personally.

Hmmm... It's fine for me... as an absolute minimum. How much additional reserve over being able to handle both your buddy's and your needs from any point in the dive do you routinely plan for? My reserve is usually set as what I need to handle my needs with a failure of one gas supply - or - what I need to handle both my buddy's and my needs should one of us have a total gas supply failure - or - "X" (minimum 13, sometimes a bit more) CF back gas + 150% of my deco gas requirement, whichever of those three is the most gas. That covers the contingencies I want to cover; anything beyond that is a nicety, but not required.
Rick

 

[Thalassamania]

That may be, the gist of what I am trying to address is the blind adherence to a concept, such as the rule of thirds, in a situation where such adherence has the potential of getting you into more trouble than it solves.