Deco with too less air, options from the book

[orange_diver]

Hey KR

Using Vplanner if you compare a 150' for 20 mins BT dive on air and then on 21/35, both decoing out on ean50, conservativism +2, the air dive requires a little more than 4 mins deco than the He dive.

Are you padding your "RD" air dives appropriately? An extra 5 mins seems to be needed, probably from 20 up to use that gradient. You're in some amazing water anyway, why not?

 

[Bismark]

Fair enough guys . . .it's tough being a "documentary guinea pig".

I call it Ratio Deco Method because that's how I applied the algorithm per AG's monograph & PP presentation in the lecture/seminar taken two years ago. I came to the understanding that because of RD's (ie. Ratio Deco's) overall strategic conservatism and tactical profile shape for a trimix deco profile, the same application can be made to a deep air dive.

As an example, I derived the following simple table derived from RD on successful deep air dives during my Dec '07 Truk Trip:

Couple of observations/questions: Not trying to challenge you, just trying to understand.

1) I am curious as to why you thought/think that you could use RD for something other than standard gases. What makes you think it will/would work with air when it was intrinsically designed to work with a specialized set of gases, air not being one of them. What is your rationale for this?

2) Do you believe your sample set of dives from the first trip was enough to extrapolate further and make a general rule based on what seems to be a very limited number of examples?

Like I mentioned earlier, it is your body but since many others read these posts, I think these answers would be useful.

 

[Kevrumbo]

Hey KR

Using Vplanner if you compare a 150' for 20 mins BT dive on air and then on 21/35, both decoing out on ean50, conservativism +2, the air dive requires a little more than 4 mins deco than the He dive.

Are you padding your "RD" air dives appropriately? An extra 5 mins seems to be needed, probably from 20 up to use that gradient. You're in some amazing water anyway, why not?

Yes . . .I'm now using an ascent rate of 1meter/min from the 6meter (20') stop, so it takes me 6min to surface.

Couple of observations/questions: Not trying to challenge you, just trying to understand.

1) I am curious as to why you thought/think that you could use RD for something other than standard gases. What makes you think it will/would work with air when it was intrinsically designed to work with a specialized set of gases, air not being one of them. What is your rationale for this?

2) Do you believe your sample set of dives from the first trip was enough to extrapolate further and make a general rule based on what seems to be a very limited number of examples?

Like I mentioned earlier, it is your body but since many others read these posts, I think these answers would be useful.

Simply by running profiles with various deco software programs, and comparing with a properly applied RD schedule for a particular depth and time, I saw the total times in general were similar, but of course expecting the RD shape for certain segments like the eanx50 depth range, or deep stops --for example-- to be very different. And yes, I was satisfied with my sample of dives to even try a nearly 2x bottom time extension while recalculating on-the-fly three months ago (and we all know now how that turned out)

Would I try it again? Probably not for a radical bottom time extension like 2x, and definitely not anything deeper than 60m. . .

 

[Bismark]

Yes . . .I'm now using an ascent rate of 1meter/min from the 6meter (20') stop, so it takes me 6min to surface.

Simply by running profiles with various deco software programs, and comparing with a properly applied RD schedule for a particular depth and time, I saw the total times in general were similar, but of course expecting the RD shape for certain segments like the eanx50 depth range, or deep stops --for example-- to be very different. And yes, I was satisfied with my sample of dives to even try a nearly 2x bottom time extension while recalculating on-the-fly three months ago (and we all know now how that turned out)

Would I try it again? Probably not for a radical bottom time extension like 2x, and definitely not anything deeper than 60m. . .

I just ran V-planner for both an air dive to for 30 min to 150' and a 21/35 trimix dive for the same depth and time. Your deco time for 21/35 was was not surprisingly 32 minutes while the air dive deco was 41 minutes. However, when you double the bottom times, the air dive deco obligation increased to a whopping 103 minutes while the 21/35 dive for the same 60 min was 74 min of deco. Again, these numbers were generated by V planner. These numbers would seem to suggest that following RD for an air dive that was doubled in duration from 30 to 60 minutes for the same depth, would leave you roughly 30 to 40 minutes short on your deco obligation. What software were you using that allowed you to make the decision that RD would generate the same type of profiles on air as it did on He for a 2x bottom time?

 

[Bismark]

I forgot to mention that the amounts of gas required to do this longer diver make it pretty silly to try without stages and multiple deco bottles.

Even if the dive was 40 minutes, you would run in to gas problems and at a minimum a bottom stage and at least an 80 of 50%. Doubling your bottom time from 20 to 40 also sees an additional third added to your deco.

Is that about right from your end?

 

[Blackwood]

Even if the dive was 40 minutes, you would run in to gas problems and at a minimum a bottom stage and at least an 80 of 50%. Doubling your bottom time from 20 to 40 also sees an additional third added to your deco.

Is that about right from your end?

Screw the 80. Add a 40 of O₂ at that point. That would give you
A. cleaner (though, per ratio deco methodology, not faster) deco, and
B. deco gas redundancy.

 

[orange_diver]

Yes . . .I'm now using an ascent rate of 1meter/min from the 6meter (20') stop, so it takes me 6min to surface.

that's not enough. you're supposed to essentially do that anyway on a He dive. plus you are not spending enough time at 10' and even 5'. you're not using the gradient to full effect by spending only 6 mins from 20' up on air.

 

[MXGratefulDiver]

that's not enough. you're supposed to essentially do that anyway on a He dive. plus you are not spending enough time at 10' and even 5'. you're not using the gradient to full effect by spending only 6 mins from 20' up on air.

From my memory of this discussion on another forum I believe Kevin was using air as bottom mix, and using EAN50 and O2 for deco ...

... Bob (Grateful Diver)

 

[Bismark]

Screw the 80. Add a 40 of O₂ at that point. That would give you
A. cleaner (though, per ratio deco methodology, not faster) deco, and
B. deco gas redundancy.

I hear you and that is what we did when we were there. 18/45 dives on those same wrecks, 50%, 100%...........no drama.............

However, that is not what he was talking about so just trying to keep apples and oranges in their respective crates.

 

[Bismark]

From my memory of this discussion on another forum I believe Kevin was using air as bottom mix, and using EAN50 and O2 for deco ...

... Bob (Grateful Diver)

I thought so also but I was referring to his post just a few back where he was talking about 2x bottom time for a 50% deco dive on air to 150' or something.

Getting things kind of all lumped together at this point.

For us, we had a pretty good idea of the depths we would be at but we wanted the flexibility of changing that depth if video required it. We calculated our min gas for the max depth, worked up a couple of RD profiles on the boat to make sure we were all on the same page, did the dive and turned it at min gas, calculated our average depth, did our RD on the fly and presto........50% deco, 100% deco and repeat as necessary......... We always knew our max depth for min gas but sometimes we would average as much as 20' less for the dive. Hence we would redo our deco using the same rules that everyone knew and understood on the team and it was really too easy. On one or two of the dives, we got what we wanted for video in shorter than the max time so since they had a booster pump in the dive shop we called the dives, recalced the deco and got out of the water with more trimix than we had planned to leave with and then reused it. By using some well thought out planning regarding stages, etc., we were able to maximize the amount of diving with trimix and keep the costs down as much as we could as HE was about $4.50 a cuft. Pricy but we were working and using the right tools and didn't take any shortcuts. I would say it was a picture perfect example of having the right skills and training and using the right equipment and gases and getting exactly done what we wanted to accomplish. (I want to add that I am not trying to toot our own horn here, this wasn't rocket science and most likely anyone can do this if they are committed to the proper training, practise, acquiring the knowledge and then practise some more. Of course this is not for rookies but we weren't) No drama, no hint of even the mildest subclinical DCS and we did a lot of diving. All in all a super awesome way to explore the deeper wrecks in Truk within what we considered acceptable levels of risk.

Wanna go next time?