Dive Planning Excersize: Check My Work?

[lamont]

I did an excersize to plan a non-penetration wreck dive sitting in 130 fsw of water. I'd appreciate any critiques of this plan that you can give. From my understanding of rock bottom, if the situation is that you'd like to return to the up line but you can shoot an SMB and surface then you'd be diving halves. That would give 136 cu ft of gas needed for this dive -- which is difficult to get in a single tank. Could you also use an AL40 slung as a stage and an E7-100, or is the only acceptable two-tank solution going to be doubles?

dive to avg 120 fsw on EAN28:

max PO2 @ 130 fsw hard bottom = 1.38302

Descent to 120 fsw @ 60 ft/min:
- Air = .6 SAC * 2 mins * [ ( 60 fsw / 33 ) + 1 ]
- = 3.38181 cu ft

16 mins @ 120 fsw - 4.63636 atm
- PO2 = 1.29818
- PN2 = 3.33817
- Equiv Air Depth = 106.44 fsw
- NDL: 16 mins
- Air = .6 SAC * 16 mins * [ ( 120 fsw / 33 ) + 1 ]
- = 44.50905 cu ft

Ascent with .6 SAC:

Ascent to 0 fsw @ 30 ft/min:
- Air = .6 SAC * 4 mins * [ ( 60 fsw / 33 ) + 1 ]
- = 6.76363 cu ft

Stop @ 60 ft for 1 min:
- Air = .6 SAC * 1 mins * [ ( 60 fsw / 33 ) + 1 ]
- = 1.69090 cu ft

Stop @ 20 ft for 2 mins:
- Air = .6 SAC * 2 mins * [ ( 20 fsw / 33 ) + 1 ]
- = 1.92727 cu ft

Stop @ 10 ft for 2 mins:
- Air = .6 SAC * 2 mins * [ ( 10 fsw / 33 ) + 1 ]
- = 1.56363 cu ft

Total ascent with 0.6 SAC = 11.94543 cu ft
Total ascent with 1.0 SAC = 19.90905 cu ft
Total ascent with 2.0 SAC = 39.81810 cu ft

projected gas = 11.94543 + 44.50905 + 3.38181 = 59.83629 cu ft
rock bottom = 39.81810 + 44.50905 + 3.38181 = 87.70896 cu ft
rb w/halves = 39.81810 + 2 * (44.50905 + 3.38181) = 135.59982 cu ft
rb w/thirds = 39.81810 + 3 * (44.50905 + 3.38181) = 183.49068 cu ft

 

[lamont]

(incidentally, this is an intellectual excersize, i'm not planning on diving this profile -- also, i know this would be better planned with mix, but i don't have any mix training, so i don't know how to do that...).

 

[MikeFerrara]

Don't have much time but real quick...

First of all icksnay on the EAN 28. I'm no DIR expert but if I'm not mistaken they use 32% to 100 ft and/ or 30/30 to 120 ft and 21/35 from 120 ft to 150 ft.

For a DIR 120 ft dive you need some helium.

As far as gas management you must start the ascent before rock bottom (I didn't check your calculations) but you have some options as to how you use the useable gas.

If you would like to get back to the entry point but it isn't critical you could turn at 1/2 or 1/2 + 200 of the usable gas.

If getting back to the entry point is critical then you apply the rule of thirds.

If you can ascend any place then you can use everything except for your rock bottom reserve.

And yes, it's hard to do much of a dive at 120 ft on a single 80 and maintain adequate gas reserves.

 

[bwerb]

I'd echo Mike's comments and add...I do rockbottom calculations to see what the absolute "rock bottom" minimum gas required to safely ascend from maximum depth at an elevated SAC for two divers...rounding upwards each time. This is an ascent with a totally empty tank at the surface and one full minute at depth to "solve the problem".

You also missed the deep stop at 80% ATM's and the one minute ascents from 40 feet. I'm not sure what you used to determine one at 60, 2 at 20, 2 at 10.

You also need to take into account what your buddy is diving in terms of tanks and their SAC rate. If they are a heavy breather. I'd assume you would need to increase your RB accordingly to account for it for the team.

I'm sure someone else will come along and add some more, that's all from me.

 

[H2Andy]

First of all icksnay on the EAN 28. I'm no DIR expert but if I'm not mistaken they use 32% to 100 ft and/ or 30/30 to 120 ft and 21/35 from 120 ft to 150 ft.

what does the 30/30 and 21/35 refer to? oxygen and helium percentage, respectively?

 

[bwerb]

Yup!

 

[lamont]

hmm... i guess my calculations may not have been entirely clear...

for the planned ascent with a single diver with .6 SAC I got 11.94543 cu ft.
for the rock bottom time for two divers with 2.0 combined SAC I got 39.81810 cu ft

then I planned to use 44.50905 + 3.38181 = 47.89086 cu ft doing the descent and
on the bottom.

so, planning for rock bottom + halves would be 39.81810 + 2 * 47.89086 = 135.5998 cu ft

or turn this around:

dive with 135.59982. the rock-bottom is 39.81816 for SAC 2.0, so you have 95.78166, by rule of halves you can only use half of this before turning the dive, so you get 47.89083 for the descent and bottom at your normal SAC.

-----

Again, sorry about the EAN28 at 130 fsw which makes this a bad DIR example.

But since we're talking about mix, doesn't 21/35 give you really short NDLs in the 120 to 150 ft range? Does that effectively limit the utility of rec triox to just the 30/30 mix to 120 ft?

-----

As far as the deco schedule goes, I don't know what GUE teaches in DIRF about this. My 1 min@60, 2@20, 2@10 schedule was a guess based on what I'd pieced together from what I've learned. I'd appreciate knowing what the rules are for DIR schedules.

 

[bwerb]

-----

As far as the deco schedule goes, I don't know what GUE teaches in DIRF about this. My 1 min@60, 2@20, 2@10 schedule was a guess based on what I'd pieced together from what I've learned. I'd appreciate knowing what the rules are for DIR schedules.

This is why you shouldn't try to learn decompression theory from the internet.

If you have taken a course and are trying to discuss what you have learned in person...then by all means, let's talk. If you are trying to piece together information from all over to come-up with what it means...then you end-up with your examples posted above.

Seriously, this is best learned one on one with an instructor or with someone who is extremely well versed in the calculations who can help you through it step by step.

(Oh...you don't do NDL dives on 21/35...but it has nothing to do with the gas and everything to do with the depth...)

 

[MonkSeal]

As far as the deco schedule goes, I don't know what GUE teaches in DIRF about this. My 1 min@60, 2@20, 2@10 schedule was a guess based on what I'd pieced together from what I've learned. I'd appreciate knowing what the rules are for DIR schedules.

GUE uses DecoPlanner. AFAIK DIR-F doesn't cover deco dives.

 

[deep_6]

Ok you're scaring me. I don't have my OW yet. Is this what I'm supposed to expect when "planning" a dive?

I don't mean to be sarcastic but whenever I hear or read about dive plan what comes to mind is...I plan to dive on Saturday at bla bla bla with bla bla bla.

Wow. Diving sure is heavy if its really like this. Do I have to be the one to make a this kind of dive plan or a DM does one for me?

Thanks in advance for the answers.