Gas choice for 100' deco dives

[ianr33]

And yet another vote for O2.

Many Florida caves are around 100 feet most of the way. No one uses 50%

 

[rjack321]

100%.

GUE does a piss poor job teaching deco on this gas. Play with software till you feel comfortable with it (there are some "on-the-fly rules" you'll be able to deduce). Don't forget that min gas must still cover the intermediate back gas deco on the way to the switch at 20'.

These longer dives in shallower water are a lot of fun.

You could just take the UTD ratio deco class and actually learn a way (of many) to approach these dives, but I don't see Nick doing that. I'm all for self-teaching, but they really need to improve their course to address these common profiles.

Here's my simple version:
Take the time "over" the MDL, if short ~10mins add in the MDL ascent (5mins from 100ft) If longer, 60mins at 100ft on EAN32 = 30mins over there's no need to add the 5mins of MDL time. This is your total backgas deco time.
divide in 1/2 = 10ft stop
divide remainder by 1/2, this = 20ft stop
divide remainder by 1/2, this = 30ft stop
continue until you have allocated all the deco time.
fill down with deep stops proportionate to BT (e.g. 30mins over would lengthen <75% of depth deep stops to 2mins each)

combine the 10 and 20ft stop times
divide by 2, this is your O2 time

example
65mins dive, 100ft, EAN32, -20% EAD
35mins over MDL of 30mins

35/2 =17
18/2=9
9/2=4
5/2=2
3/2=2
2/2=1

10-17
20-9
30-4
40-2
50-2
60-1 (lengthen to 2mins due to exposure as deep stop)
70-1

17+9 = 26/2 =13

Final deco schedule, feel free to compare and contrast with other methods
10-slow ascent
20-13 on O2
30-4
40-2
50-2
60-2
70-1

14mins to get to O2 from 100ft with an average ATA of ~2.5 is still do-able from a rock bottom perspective. You can figure out how much gas you want to reserve normally.

Oh and agree, for a long shallow dive O2 is the way to go.

 

[sea2summit]

Blend as you go ~45% on the bottom up to high 90s during deco:cool2:

Sorry couldn't resist.

 

[Kevrumbo]

Normally just Oxygen as deco gas --however, if you're in an open ocean environment where the possibility exists that you may be forced out-of-the-water early (i.e. aggressive sharks), you may want to consider additionally bringing the nitrox 50 deco bottle and start your inert gas clean-out early. . .

At least you'll have some deco & fast (neural) tissue wash-out should big "Mr. GrayWhiteSuit" come by for a bite, and you have to abort the 20'/6m oxygen switch and/or the slow ascent to the surface. . .

 

[StreetDoctor]

Thanks for all the replies. Richard your method is exactly what I was looking for, and I just might surprise you with that ratio deco class. I just have to find the time to do it, lately I've been swamped with work.

Chris, I agree with you too.. I even went back and reviewed my notes and I didn't have anything about 100%.

 

[liuk3]

100% O2. That being said, you can always run the profiles on Deco Planner to check of you uncomfortable or want to double the profiles, but Richard's provided a nice methodology. Ultimately, you have to evaluate how your own body personally tolerates whatever deco methodology you choose to do.

 

[karstdvr]

80% aka 80/20 has been labeled as the "stoke gas" with the Baker's dozen diatribe,but it does share pretty close wash out ratios to oxygen. If you are using AL40,and have a hard time getting a 3000psi O2 fill,then 80/20 can be a great solution.

 

[Doppler]

I'd like to throw something else into the mix for dives to 30 metres or so and situations where surface conditions may be tricky... try 80/20 to 85/15 heliox.

 

[rjack321]

Thanks for all the replies. Richard your method is exactly what I was looking for, and I just might surprise you with that ratio deco class. I just have to find the time to do it, lately I've been swamped with work.

Chris, I agree with you too.. I even went back and reviewed my notes and I didn't have anything about 100%.

I actually picked an odd number on purpose, evenly divisible by 2 is easy peasy. That approach is kinda a non-European mathematician's way of creating a Fibonacci sequence. Side benefit to the "time over MDL" approach is that your lost deco gas plan is right there (although I probably wouldn't try to do quite so much time at 10ft). It works with air or 21/35 etc too as long as you use the correct MDL time. Honestly I have no idea what 30/85 GF Buhlmann would expect on that dive but I know ~9 mins to get to 20ft and 13mins on O2 is in the ballpark of "doable".

Actually I shouldn't pick on you :mooner: I don't think its explicitly in UTD's ratio deco course either. Back when I took GUE Tech1 the whole ~110ft dive was kinda skipped over as well.

 

[rjack321]

80% aka 80/20 has been labeled as the "stoke gas" with the Baker's dozen diatribe,but it does share pretty close wash out ratios to oxygen. If you are using AL40,and have a hard time getting a 3000psi O2 fill,then 80/20 can be a great solution.

Not sure how much BT you're budgeting for, but I can do ~25mins of O2 time on a 2000psi O2 transfill (25cf). This is more than I'm able to cover in the (cold) ocean with backgas if I lost the O2 (its plausible if I were diving a bunch of stages with near full backgas tho).