Strategies for handling deco schedule deviations

[rjack321]

I guess I am still interested in the big question: why?

Interestingly enough, the Deco Stop thread you cite, which I read when it first came out, was in the back of my mind when I wrote the post. If you read the whole thread rather than that one part of that one post, you see that if there is any kind of a consensus, it is that more rapid ascents are acceptable on the deeper portion of the dive, and the percentage of change in ATAs is important to consider as well. I agree that they are talking about the deeper portion of the dive being a little deeper than this example, but not to the degree that it is out of the question.

Well some people agree with deeper = faster. But even those who agree with you that 60fpm ain't no big thing, the agreement is only up to the point of initial offgassing. Once you pass that point, 60fpm really is "too fast".

For the OPs dive, he blew by the depth where offgassing starts by about 50ft. Staying an extra ~3 mins once you are well into the offgassing zone is probably fine. Vs. what the bulk of that TDS thread was about which is overstaying on the deeper portions of the ascent.

I.e. if the error was a delay like this example:
140-2mins
130-6
120-2
110-1
100-1
90-1
remaining schedule etc.

That would dictate extending the intermediate and (mostly) shallow stops since beyond maybe 1 min at 130/140 you are creating extra slow tissue loading than your plan hadn't accounted for.

So my approach would be:
fast ascent from depth and/or blown deep stops = catch up to runtime where you are - do not ascend more or try to refigure a different type of profile.

slow ascent from depth and/or overstayed deep stops = add time shallow. How much I'd add would depend on the portion of the ascent I overstayed and by how much.

 

[Rick Murchison]

I don't get it... How?

Not relevant to the problem...

 

[Rick Murchison]

I'm with Rjack, I'd stay put and let my runtime catch up with me.

Agree... (that's what I did)... do you also agree with the shallow stops tack-ons? Or were those more penance than pragmatic?
Rick

 

[Thalassamania]

I'd also consider being quite agressive with any rich mix and oxygen, maybe to a ppO2 of 2.0 or higher, but that would depend on my OTUs and I don't have them running in my head.

 

[rjack321]

Agree... (that's what I did)... do you also agree with the shallow stops tack-ons? Or were those more penance than pragmatic?
Rick

5mins extra on O2 can "fix" alot of errors. I would add it like you did.

As I've said on some of our dives... I'd rather do 5mins extra in the water than spend 5 hours trying to get to a chamber.

 

[boulderjohn]

For the OPs dive, he blew by the depth where offgassing starts by about 50ft. .

Well, he went past his first planned stop by 30 feet, going from a planned 5 ATA to 4 ATA instead.

I assume you are basing your reply on a different system that calls for deeper stops, some of which would have put the first planned stop at 160 feet or more. Those systems are controversial, and the post you cited even mentions the recent research that is calling the deep stop phiilosophy into question.

Your statement that the off-gassing starts at that point reminds me of something I read from a deeper first stop proponent as part of his rationale for that deeper stop:

The 5 minute compartment will start to unload at 75% of your depth, and the time at fully loaded to offload the 5 minute compartment is 5 minutes."

That is not my understanding.

Any saturated compartment, especially the 5 minute compartment, will start to off gas ("upload?") the second the diver begins to ascend from the point at which that compartment has achieved equilibrium. Given a normal ascent rate, the 5 minute compartment will have dropped quite a bit by the time it reaches the 75% point. It will then start to off gas in relation to its half time in comparison to its new ATA. In 5 minutes it will have reached the half way point to equilibrium at that level.
Any compartments that were saturated at 7.5 ATA will have begun off gassing the second they began the ascent. Some compartments may reach equilibrium and then supersaturation during the ascent, and they will begin to off gas as soon as they do. I could dizzy myself trying to calculate what compartments became supersaturated at what point in the dive and ascent, but I don't think I will find any crisis points, at least in terms of M-values.

At least, that's my understanding.

 

[Meng_Tze]

....
So my approach would be:
fast ascent from depth and/or blown deep stops = catch up to runtime where you are - do not ascend more or try to refigure a different type of profile.

slow ascent from depth and/or overstayed deep stops = add time shallow. How much I'd add would depend on the portion of the ascent I overstayed and by how much.

Yup, probably the best approach.

 

[Rick Murchison]

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

 

[rjack321]

Well, he went past his first planned stop by 30 feet, going from a planned 5 ATA to 4 ATA instead.

I assume you are basing your reply on a different system that calls for deeper stops, some of which would have put the first planned stop at 160 feet or more. Those systems are controversial, and the post you cited even mentions the recent research that is calling the deep stop phiilosophy into question.

Your statement that the off-gassing starts at that point reminds me of something I read from a deeper first stop proponent as part of his rationale for that deeper stop:

That is not my understanding.

Try v-planner, 215ft dive. Near the bottom of the schedule it will tell you "offgassing begins at 150ft" (or very close, it will be darn close to 2atas off the bottom). First planned stop depth will be a bit shallower, in the OP's plan 130ft.

Any saturated compartment, especially the 5 minute compartment, will start to off gas ("upload?") the second the diver begins to ascend from the point at which that compartment has achieved equilibrium. Given a normal ascent rate, the 5 minute compartment will have dropped quite a bit by the time it reaches the 75% point. It will then start to off gas in relation to its half time in comparison to its new ATA. In 5 minutes it will have reached the half way point to equilibrium at that level.
Any compartments that were saturated at 7.5 ATA will have begun off gassing the second they began the ascent. Some compartments may reach equilibrium and then supersaturation during the ascent, and they will begin to off gas as soon as they do. I could dizzy myself trying to calculate what compartments became supersaturated at what point in the dive and ascent, but I don't think I will find any crisis points, at least in terms of M-values.

At least, that's my understanding.

M-values work "ok" but the reality is that those are just a dissolved model and deep stops are all working off of bubble concepts. Keeping critical radii small takes depth.

Extending time shallow shifts you from a bubble model to a "bend & mend" model. While bend and mend can work I suspect we all underestimate the really looooong time you need to spend at 30,20,10ft. Missing 5mins deep does not translate into an added 5mins shallow. Its more like miss 5 mins deep time = adding 15mins shallow (roughly, I'm at work with no access to a Buhlmann tool to mess with this).

 

[Thalassamania]

How high would you be willing to push your ppO2 in these circumstances?

What about "wetnote" level OTU calculations?