Help me understand the rule of fifths for IBCD

[RedSeaDiver2]

Hello again,

Essentially answered by boulderjohn. Shearwater never moved their default GFs as the debate about 'deep stops' evolved. However, based on recent discussions with one of their lead engineers I believe that they are going to do so soon.

Also, for further perspective on gas switches, I have attached an NEDU experiment which describes switches straight from pure helium-oxygen to air.

Simon M

Many thanks once again Simon - it seems pretty clear that gas switches shouldn't cause an issue for us. Have fun at Oztek - unfortunately it is just too far for me to duck over there from the Red Sea just for the conference, but if this shipwreck is as incredible as we think that it is going to be then I will probably get a speaking gig at the next Oztek!

 

[sierrakilo]

Hi all, thank you for an enlightening thread.

In relation to planning a few short bounce technical dives to 100-105m on open circuit, I have read various approaches to avoid ICD, or rather to avoid it causing Inner ear DCS. I think I'm up to date on current material (The old “rule” of fifth, the RAID rule of three, the GUE and NAUI approaches too) but I am honestly still not quite sure how I will wish to approach this. For context, I'm an instructor, and I'm fairly accomplished above 100m, but this will be my first time below 100m too (if only by a few metres though, but still...) This will be with a student, whom I have dived a lot of tec dives in the past, and who is absolutely ready for the challenge and dive, and who wish to break past the 100m "Mount Everest of Diving," once he's passed his hypoxic tmx course obviously.

I'd really like to solicit your opinion on the best approach to doing this.

For student comfort I’d like to limit gear to a max of three 80cu aluminium stages.

Assume dive is on GF 40/70. GF low can be altered, but I'd like not to go higher on GFhigh. I expect he will wish to do a modest first timer dive, of only 5-6 minutes at final depth. Backgas probably 10/65. (could be changed slightly if solid reasons)

With three stages, and not wanting to have too large jumps between gasses if there's any failures, these are the options I've considered. But how would you go about it? I would really appreciate input, (other than get a RB - I have one, thanks! . My student doesn't) as well as anecdotal evidence from people doing such bounce dives on OC. I'm leaning towards option 4, (and it doesn't solicit any warning in deco software, but what I'm using seems to be based on rule of fifth... so don't know how much faith to place in it.)

Option 1, Shallow Switches. Use EANX50 as travel gas, and switch to EANX50 as first deco gas at 21m. (2 stages EANX50 for redundancy, 1 stage 100%) Slow ascent from depth and stopping at 21m for a bit, before switch to reduce super saturation before switch. (in line with the reasoning of Dolette, suggested by Doc Simon above)

Option 2, He-intermediate gas. Use 30/30 as travel, and also use same 30/30 to decompress from 40m to 21m, where I switch to EANX50 for remaining deco. (1 stage 30/30, 2 stages EANX50 for redundancy)

Option 3, Mountains of Money Use 30/30 as travel gas, and use 50/20 to decompress from 21m. (1 stage 30/30, 2 stages 50/20 for redundancy).

Option 4, He-intermediate gas2. Use 40/30 as travel, and also use same 40/30 to decompress from 30m to 6m, where I switch to 100% for remaining deco. (2 stage 40/30 for redundancy, 1 O2)


Comments are very welcome!

Best regards,
S.K

 

[kensuf]

I don't want to sound like a jackass, but I'm going to anyway. If you're at the limit of your skills and abilities and pushing into a pinnacle dive, should you be teaching? What is your capacity to deal with a problem with your student while you're on the razors edge?

 

[grantctobin]

Hi all, thank you for an enlightening thread.

In relation to planning a few short bounce technical dives to 100-105m on open circuit, I have read various approaches to avoid ICD, or rather to avoid it causing Inner ear DCS. I think I'm up to date on current material (The old “rule” of fifth, the RAID rule of three, the GUE and NAUI approaches too) but I am honestly still not quite sure how I will wish to approach this. For context, I'm an instructor, and I'm fairly accomplished above 100m, but this will be my first time below 100m too (if only by a few metres though, but still...) This will be with a student, whom I have dived a lot of tec dives in the past, and who is absolutely ready for the challenge and dive, and who wish to break past the 100m "Mount Everest of Diving," once he's passed his hypoxic tmx course obviously.

I'd really like to solicit your opinion on the best approach to doing this.

For student comfort I’d like to limit gear to a max of three 80cu aluminium stages.

Assume dive is on GF 40/70. GF low can be altered, but I'd like not to go higher on GFhigh. I expect he will wish to do a modest first timer dive, of only 5-6 minutes at final depth. Backgas probably 10/65. (could be changed slightly if solid reasons)

With three stages, and not wanting to have too large jumps between gasses if there's any failures, these are the options I've considered. But how would you go about it? I would really appreciate input, (other than get a RB - I have one, thanks! . My student doesn't) as well as anecdotal evidence from people doing such bounce dives on OC. I'm leaning towards option 4, (and it doesn't solicit any warning in deco software, but what I'm using seems to be based on rule of fifth... so don't know how much faith to place in it.)

Option 1, Shallow Switches. Use EANX50 as travel gas, and switch to EANX50 as first deco gas at 21m. (2 stages EANX50 for redundancy, 1 stage 100%) Slow ascent from depth and stopping at 21m for a bit, before switch to reduce super saturation before switch. (in line with the reasoning of Dolette, suggested by Doc Simon above)

Option 2, He-intermediate gas. Use 30/30 as travel, and also use same 30/30 to decompress from 40m to 21m, where I switch to EANX50 for remaining deco. (1 stage 30/30, 2 stages EANX50 for redundancy)

Option 3, Mountains of Money Use 30/30 as travel gas, and use 50/20 to decompress from 21m. (1 stage 30/30, 2 stages 50/20 for redundancy).

Option 4, He-intermediate gas2. Use 40/30 as travel, and also use same 40/30 to decompress from 30m to 6m, where I switch to 100% for remaining deco. (2 stage 40/30 for redundancy, 1 O2)


Comments are very welcome!

Best regards,
S.K

I'm with Ken on this one.

If you're going w three bottles each, I don't see the need to all dupe the 70' bottle. On the upside, thanks for picking an appropriate bottom mix.

 

[sierrakilo]

I don't want to sound like a jackass, but I'm going to anyway. If you're at the limit of your skills and abilities and pushing into a pinnacle dive, should you be teaching? What is your capacity to deal with a problem with your student while you're on the razors edge?

I don't think you sound like a jackass. You can't know my background and I take no offense at all, please don't worry.

Now, I could wave good bye to my students after certification, remind them about Dunning Kruger and ask them to read their manuals, the bits about being patient and wary, and tell them to slowly go and expand their depth experience, level of comfort and skills on their own with someone their level. But I would trust them to do their research and run into exactly the same questions I'm asking in this thread, because the how-to's on handling open circuit IBCD, is mostly for sub 100m dives, and range from meh-don't-worry-about-it to preposterously-high-helium-until-pure-O2.

Which of course is why I'm asking, to see if there's things I haven't considered, or stuff I should read and dig into, anecdotal evidence, informed opinions, anything to learn more and to help my buddy and me do it right.

I'm not doing the 100m dive as an instructor. The formal teaching is over. I'm doing it as a buddy. I understand there's better open circuit mix divers out there, who's fought off more underwater nazi octopusses than I have.

There will always be.

But if I don't ask, I won't learn. And I will be a worse instructor for it.

Yes, as stated it's pinnacle depth for me, but I've done long dives to 85-95m, and done much more deco than this quick bounce. Aside from the IBCD issue, and aiming, if possible, to only do this dive with three stages, I've got no reservations about going a few meters deeper. And I have no worries or concerns about being "on razors edge" or being unable to respond as a prudent buddy. I have been diving trimix for years before getting here, and if I am ever going to be doing slightly deeper dives with a peer, in my opinion I could pick no finer buddy then the guy I intend to do it with. Of the hundred of tec divers I've trained, he comes in in the top bracket. So enough about me and what lead me to asking. Bottomline, I appreciate you are trying to give some needed tough love by mentioning a troll lives under the bridge. I appreciate that, and I understand that, and I feel ready to cross it anyway.

I would still like to hear opinions on how you would go about gas planning this dive with regards to IBCD.

 

[tbone1004]

I'm with Ken and Grant on this one though with your last post I'll make some clarifications. When you are an instructor, you are pretty much always an instructor in the mind of your student. It sounds like this is basically a graduation dive which is fine but you should never be diving with a student on a dive that you can't essentially conduct in your sleep without thinking about it. I understand this isn't officially part of the class but it is difficult to make that differentiation with the student.

Based on the research and recommendation I'd personally dive a higher GF-lo around 50, but fairly inconsequential to dive plan.

Personally I'd go with your Option 1.5. Run the dive plan and see what makes the most sense, but I would have 3 distinct gases for this dive. Getting from 100m up to 21m for the 50% switch is a LONG way to come up before you can hop onto a different bottle IMO and while you aren't blowing off any significant deco, it's a hike to get up there and I'd certainly feel better with 21/35 to be able to get off the bottom mix in an emergency if I had to since you can make that switch at 60m if something goes sideways. Carrying O2 is always a good idea for something that deep and with a 10min bottom time you can push up to 6m and it gives you a large quantity of 100% O2 as an emergency solution at the surface.

I don't tend to do short bounce dives to 100m, and very rarely on OC. If I'm going to 100m it's usually a working dive in the bottom of the lakes with a 30min+ bottom time on a CCR so having to rack my brain on which of the 4th deco gases I would shed for a quicky.

 

[kensuf]

I'm not doing the 100m dive as an instructor. The formal teaching is over. I'm doing it as a buddy. I understand there's better open circuit mix divers out there, who's fought off more underwater nazi octopusses than I have.

Got it!

I would still like to hear opinions on how you would go about gas planning this dive with regards to IBCD.

IMHO, isobaric counter diffusion from a bounce to ~110m shouldn't be a real concern. The bigger concern would be narcosis from getting off trimix; I've seen people completely space out after switching to nitrox at the shallow depth of 36m.

The gases I would take in an ideal world for a dive like this would probably look something like:

100% - to 6m
50% - to 21m
30/30 (or 32/25) - to 40m
10/70 - to 110m

You will have stops deeper than 40m, those will be on 10/70, that'll need to be part of the plan. There's a risk/reward in taking additional deco gases, I don't think the reward is worth the management of four deco bottles in this particular case. If you were doing a longer dive then you'd want a 4th bottle, for that I'd probably back 18/45.

FWIW, even though I've got lots of experience in the 75m to 90m range, I've never been deeper than 120m, so we're at the limit of my personal experience in discussing this.

 

[Nick_Radov]

The gases I would take in an ideal world for a dive like this would probably look something like:

100% - to 6m
50% - to 21m
30/30 (or 32/25) - to 40m
10/70 - to 110m

Could you explain the reasoning behind using 30/30 as the deep deco gas (and travel gas) instead of something like 21/35? I assume that 30/30 would allow a slightly faster deco. But on the other hand, 21/35 would give you more margin on the minimum bottom gas, so in an emergency you could quickly ascend to a depth where it's safe to breathe and switch.
(I'm not arguing with you here, just trying to understand the planning process for dives in that range.)

 

[boulderjohn]

Could you explain the reasoning behind using 30/30 as the deep deco gas (and travel gas) instead of something like 21/35? I assume that 30/30 would allow a slightly faster deco. But on the other hand, 21/35 would give you more margin on the minimum bottom gas, so in an emergency you could quickly ascend to a depth where it's safe to breathe and switch.
(I'm not arguing with you here, just trying to understand the planning process for dives in that range.)

I agree with this and would use something like 21/35 myself.

I used to give my trimix students and assignment to experiment with dive plans with different mixes and talk about the pros and cons. You see a lot of options when you do that.

 

[kensuf]

Could you explain the reasoning behind using 30/30 as the deep deco gas (and travel gas) instead of something like 21/35? I assume that 30/30 would allow a slightly faster deco. But on the other hand, 21/35 would give you more margin on the minimum bottom gas, so in an emergency you could quickly ascend to a depth where it's safe to breathe and switch.
(I'm not arguing with you here, just trying to understand the planning process for dives in that range.)

Honestly, it's because when I first ran the numbers something with 32% oxygen had shorter times in the 24, 27, and 30m stops and the stops on 10/70 amounted to a total of 4 minutes.