Strategies for handling deco schedule deviations

[rjack321]

In summary... situation 1: Blown deep stop, looks like the general concensus is to hold at whatever depth you do manage to stop at until your schedule catches up with you, and to add time to the shallowest one or two stops.
Is there some point at which you'd descend to intercept your schedule?
Suppose I'd not gotten the ascent stopped until 50'?
My thoughts here are to have a "back pocket" Buhlmann or Navy Table schedule, and if I blew the deepest of that table's stops I'd descend (if possible) to at least the first Buhlmann stop until intercepting my original plan. If we use the same schedule to begin with on this one, for example, I might have a "back pocket" Buhlmann schedule that looks like this on the deep end...

First Stop at 70ft 1:00 (20) Trimix 18/45 0.56 ppO2, 15ft ead, 24ft end
Stop at 60ft 2:00 (22) Trimix 18/45 0.51 ppO2, 11ft ead, 18ft end
Stop at 50ft 2:00 (24) Trimix 18/45 0.45 ppO2, 6ft ead, 13ft end
Stop at 40ft 3:00 (27) Trimix 18/45 0.40 ppO2, 1ft ead, 7ft end

In this case I'd descend immediately to at least 70', or deeper, until intercepting my original run-time for the depth - 26 minutes at 70', for example, rather than just the 20 minutes called for by the Buhlmann table, then run the original plan from there and still tack on an extra 5 at both oxygen stops.

What do y'all think?

Rick

Eek! Good question. Assuming the ascent to 50 was short and quick (e.g. I didn't hang out there for 8mins)...

In this case you've blown past your first gas switch (well it would be for me, 215ft is a 2 deco gas dive for me). I would descend to 1 or 2 stops below the switch and try to get back onto schedule. So back down to 90ft. Start the clock all over again. Catch up on deep stops there then progress through the EAN50 and O2 schedules. I'd add some intermediate time at 70,60 on the EAN50 to try and clean up any bubbles I formed with the rapid ascent/missed stops.

 

[boulderjohn]

In summary... situation 1: Blown deep stop, looks like the general concensus is to hold at whatever depth you do manage to stop at until your schedule catches up with you, and to add time to the shallowest one or two stops.
Is there some point at which you'd descend to intercept your schedule?
Suppose I'd not gotten the ascent stopped until 50'?
My thoughts here are to have a "back pocket" Buhlmann or Navy Table schedule, and if I blew the deepest of that table's stops I'd descend (if possible) to at least the first Buhlmann stop until intercepting my original plan. If we use the same schedule to begin with on this one, for example, I might have a "back pocket" Buhlmann schedule that looks like this on the deep end...

First Stop at 70ft 1:00 (20) Trimix 18/45 0.56 ppO2, 15ft ead, 24ft end
Stop at 60ft 2:00 (22) Trimix 18/45 0.51 ppO2, 11ft ead, 18ft end
Stop at 50ft 2:00 (24) Trimix 18/45 0.45 ppO2, 6ft ead, 13ft end
Stop at 40ft 3:00 (27) Trimix 18/45 0.40 ppO2, 1ft ead, 7ft end
-- etc --

In this case I'd descend immediately to at least 70', or deeper, until intercepting my original run-time for the depth - 26 minutes at 70', for example, rather than just the 20 minutes called for by the Buhlmann table, then run the original plan from there and still tack on an extra 5 at both oxygen stops.

What do y'all think?

Rick

I do know that the Navy protocols and TDI (following the Navy) allow you to get back in the water and return to your deco schedule if you had to leave the water and omit some of your decompression. I assume that if you can go back down after exiting the water and resume your schedule, you should be able to drop back down to a missed stop as well.

 

[Rick Murchison]

How high would you be willing to push your ppO2 in these circumstances?
What about "wetnote" level OTU calculations?

I don't think we've reached a stage that needs to get into any exceptional exposure yet... indeed, if we normalize the dive to 'equivalent inert gas depth' and apply the Navy Standard Air tables (yeah, I know they're not designed with Helium in mind, but it helps lend reasonability to a SWAG) we're not even close to being outside the tables.
However... I'm sure we can come up with a scenario that would dictate/suggest pushing the PPO2 or OTU limits, so it's worth considering as a question of its own...
My quick reaction is "it depends."
Looking at (NOAA) OTU allowances and realizing that published daily doses are based on the 300 OTU long term daily exposure limit and that there's a first day limit of 850 OTUs I'd be willing to press things a good deal more if this were the first dive after a two week layoff than if it were in the middle of a multi-day excursion.
But it'd take some pretty dire circumstances, I think, to get me higher than 2.0 for anything over a few minutes.
Rick

 

[Thalassamania]

I done enough chamber runs to 60FSW on oxygen that I'm comfortable going a little above 2.0, but I really don't recommend that generally.

 

[rjack321]

But it'd take some pretty dire circumstances, I think, to get me higher than 2.0 for anything over a few minutes.

Pretty much agree
If circumstances are that dire I'm probably in no shape to be diving at all. Keep me on the boat on 100% O2

I would have no concerns diving to a decent depth (like 90-100ft) on back/bottom gas and then redoing deco with all scheduled switches above. Extending time a little, not changing the ppO2s used. I don't see a need to push beyond 1.6ppO2s in the "bubble crushing" phase of omitted deco or IWR.

Bent is fixable, toxing and drowning essentially aren't.

 

[Thalassamania]

True, true.

 

[Meng_Tze]

....

Bent is fixable, toxing and drowning essentially aren't.

Many people miss this point.

 

[karstdvr]

I done enough chamber runs to 60FSW on oxygen that I'm comfortable going a little above 2.0, but I really don't recommend that generally.

For some reason this doesn't work real well in a wet environment.
I would agree with pushing the ppO2 on the stops with oxygen,and personally I would eliminate 10' stops in favor of 20' on oxygen.

 

[Rick Murchison]

Pretty much agree
If circumstances are that dire I'm probably in no shape to be diving at all. Keep me on the boat on 100% O2

I would have no concerns diving to a decent depth (like 90-100ft) on back/bottom gas and then redoing deco with all scheduled switches above. Extending time a little, not changing the ppO2s used. I don't see a need to push beyond 1.6ppO2s in the "bubble crushing" phase of omitted deco or IWR.

Bent is fixable, toxing and drowning essentially aren't.

Well reasoned; well said.
Rick

 

[rjack321]

personally I would eliminate 10' stops in favor of 20' on oxygen.

I do this all the time anyway. I do 4-8min ascents from 20ft after all my O2 is done and this seems to work well for me. Just this small amount of padding on otherwise normal dives has made the difference between niggly hits and feeling 110%.

If I were doing an omitted deco schedule for a major schedule deviation (60fpm minus any deep stops all the way up to 50ft from 200ft for instance) I would probably extend my EAN50 stop at 70ft by a 3-5 minutes to try and encourage any bubbles made deep to shrink. And also extend my 20ft time by ~3-5mins.

Mostly just padding since its fairly well established that omitted deco need not be dramatically extended over the anticipated schedule (unlike IWR).