Which deco stop to skip?

[Rhone Man]

First off, let's try and be grown ups. Anyone who answers the question: "if you have to skip deco, where should you cut from"? With the answer: "never skip deco," is being a little childish (not to mention patronising) about the question. Yes, I think we all understand the very basic fact that is is never a good idea to skip deco. But equally we all recognise that not all dives go to plan.

I recall a story a couple of years back about an experienced tec instructor who suffered a catastrophic drysuit tear during a dive in the Great Lakes, and he had to make some very real decisions about how much deco he could afford to do and where he would do it. I am not sure that recalling discussions repeating like a drone: "never skip deco" would have been helpful to him. So let's try and actually discuss the hypothetical.

My inclination, if I had to cut deco stops, would be to cut the deeper stops and keep the shallower ones. There are a lot of variables in this, but my basic reasoning is that if you are consciously going to do too little deco, then you need to take an aggressive approach to your offgassing, and then means reducing your partial pressures of gases more aggressively and (frankly) hoping for the best.

If your status is such that you are basically acknowledging that you are going to take a hit, then really what you are talking about is what you can do with your profile to mitigate the damage as much as possible. Unless you are doing some seriously extreme diving, you are very unlikely to have a hit whilst conducting deco. Again, skipping stops and going shallower sooner will means that you get more gas out of your tissues before you go aboard and wait for disaster to strike. And presumably having lower gas saturation in your tissues should equate to a lighter hit (everything else being equal).

However, I am a deep air guy. People who dive helium may feel that because of the different characteristics of helium make staying deeper longer the safer option.

It may also depend upon the reason for the cropped schedule. If it was catastrophic gas loss, you get more bang for your remaining gas buck at shallower depths. So that would also incline me towards pushing shallower sooner. If it is hypothermic exposure, then that might push you towards deeper stops rather than (longer) shallower stops.

 

[Tortuga68]

Deco stops cannot be missed with an illusion of retaining safety

I understand your point, but disagree

I can cut a table on any different number of algorithms and get different profiles & different plans for the same dive; even the same algorithm with different conservatisms with/without deep stops, and have wildly different deco schedules & run times. That doesn't mean I'll get bent if I don't follow the "most conservative" (sometimes unclear anyway) one, any more than it means I will get bent if I follow the least conservative one.

I've seen people get bent on rec NDL dives, and I've seen people blow off 20 mins of accelerated deco after a 70m dive and not get bent. So much for "dive your plan".

So I agree it's complex and there isn't a simple answer but DON'T DO IT doesn't contribute much to what is a worthwhile topic for theoretical discussion IMO.

To put it another way, if you're wearing two dive computers, which one should you follow? Is there a wrong answer?

Someone posts seeking discussion and gets told they're stupid for asking the question, welcome to scubaboard

 

[DevonDiver]

I understand your point, but disagree

I get where you're coming from - however, "retaining safety" is not the same as "getting away with it". Blowing deco isn't an absolute guarantee of DCS, but it is a likelihood - dependent, of course, on the dive undertaken.

I've tried to clarify my initial response, but done it badly. What I meant (and forgive my lack of clarity, suffering a slipped disk and laryngeal infection this week, so the painkillers, antibiotics etc are taking their toll) is that 'shaping the curve', especially when 'on-the-fly', has to be a product of a certain clarity of understanding about what you are trying to achieve through decompression and why...

If you're just trailing an m-value to the surface, then skipping the deeper stops is likely to be the most forgiving. However, if you're stopping deep to prevent initial bubble growth, with subsequent reduction in shallow obligaiton, then blowing the deeper element could be far more harmful than shortening the shallow stuff. You'd end up with short deep and short shallow stops.

Gas choice is critical. Are we assuming missed deco stops through lost deco gas...or some other factor. If we've got our gas, then I'm going to maximize my stops at the highest PPO2 and gas gradient....at the expense of less efficient stops, if absolutely necessary.

If you're pre-dive risk assessment identifies any hazards that may force a premature exit from the water (sharks/burst drysuit etc), then it'd pay dividends to have a contingency plan for precisely that scenario. Cut a table with absolute minimal conservatism to get you up as quickly as possible. No need to miss stops - just run the quickest ascent profile that you are comfortable to cut in advance. Keep that 'get out of Dodge' table in your wetnotes as a contingency only..

 

[Bombay High]

To put it another way, if you're wearing two dive computers, which one should you follow? Is there a wrong answer?

I would follow the more conservative computer, if my dive plan was to follow what a wrist computer says.
I think, as has been said before, that how you choose to modify your deco would depend on many factors, and most importantly, by what the driving reason is, for shortening your planned deco in the first place.

 

[dumpsterDiver]

I would probably cut a little off the deeper stop(s) and then try to do as much of the shallower stops as possible. Saying don't ever think about this because you will never do it is very bad advice. Thinking about it now, may make you much calmer if/when it DOES happen and this MIGHT reduce your breathing rate enough to prevent a DCS incident.

 

[Dive Bug Bit Me]

I tend to dive 30/80 gradients on Buhlman for non helium dives. This keeps me safe while balancing my conservatism with time in the water.

I will also run a 100 / 100 profile which acts as an emergency get out of the water fast plan. This plan is pushing the envelope, but not into insane territory. This cuts the deeper stops (cause thats how Buhlman works). It also gives a much shorter last stop.

If I do inadvertantly miss a stop on my 30/80 plan (it hasn't happened yet), I know where I am in the context of the aggressive plan. If I am still within the aggressive exit plan, then I don't pannic for oxygen on the surface and call a helicopter to fly me out. To be honest, more likely I just pad my oxygen time in the 3m to 5m range for a minute or two and breath out of my O2 regulator on the surface while I wait for the boat to pick me up. Then be a little more attentive towards what your body is telling you after the dive. Odds are still in your favour.

 

[CamG]

This topic is a critical one and should be well considered.
I appreciate the comments on all sides of this thread.
Deco obligation is just that and to cut or remove any of it puts the diver at risk.
Helium adds it's own characteristics to any dive and if you are using it you need to know them in and out.
Just because you complete a deco schedule or blow one off means you will suffer a hit.
However I have been horrified by the numbers of UN-deserved hits among those I know.

Conservative divers following a deco plan STILL getting hit with skin and even more serious levels.
From day to day, dive to dive we never know just what our physiology is doing and how it is going to off gas.
We try to eat, hydrate, do everything right but it still can not be enough.
Lets face the fact emergencies happen and we all have heard or been a part of dives that did not go as planned.
YOU WILL PAY THE PIPER! Get away with it or not.
What I am saying is you do what you have to to in an emergency but face the facts survival favors the prepared.
I know my deco theory has changed a bit in recent days not as brave as I once considered myself to be.
More gas reserves, conservatism, and way slower ascents with more time @ last stop / in water conditions permuting of course.

One of my good friends is no longer a Tech diver because of one of these incidents of which he did all stops even followed the in water rule.
It saddens me to know he did everything right and still took a major hit.
This is the reality of Tech diving like it or not We all face it!
I try like HXXL to make all stops and then some even if it means bagging the dive for another day!
Life is to short and I love diving to much to end it like that!

CamG

 

[ajduplessis]

There are many possible situation to cover your question. Couple of thoughts. Firstly the fact that you need to cut stops imply that you have no buddy (spare gas) bad dive planning, or this day day is going to get a lot worse. Accept this, clear your mind and live with your decisions.

It's all going to depend on where the pooh hit the fan. If you have no/little backgas and still need to travel to next stop, you will have to exclude/shorten the stops getting there; or switch earlier to the next deco gas. This is a simple call, OOA or high PPO2! If you don't have enough "next stop" gas I suggest a prayer or 2.

If you loose % of gas I would adjust the deeper stop in a % approach (example take off % of stop time at each stop, more aggressive deep). If you loose hard deco or % of hard deco gas you have use back gas and suck it dry sand take what is coming your way.

If you stayed to long and need more gas to completed the dive I slice off the deeper stops depending on how much you need; or skip the deeper stuff and try and make up for it shallow. Either way, you are having a bad day and need to live with the outcome. Don't panic, it will reduce your limited gas consumption.

 

[Diver0001]

.

My inclination, if I had to cut deco stops, would be to cut the deeper stops and keep the shallower ones. There are a lot of variables in this, but my basic reasoning is that if you are consciously going to do too little deco, then you need to take an aggressive approach to your offgassing, and then means reducing your partial pressures of gases more aggressively and (frankly) hoping for the best.

^^ This ... and the fact that on the whole you'll be able to spend more total time offgassing at shallower stops, assuming gas supply is the problem and not something like a flooded suit/hypothermia or an acute medical problem. My approach would mirror Rhone Man's. I'd get right to my ceiling and hang on as long as I could there.

R..

 

[boulderjohn]

I had an interesting experience this past fall that might be helpful.

I joined a group of three divers who were in an advanced trimix class for a dive to 300 feet. They were planning the dive using V-Planner, and I obviously agreed to go with that plan as well. They were using Liquivision X-1 computers with the same V-Planner software as backups to the plan. Not having that computer, I used a Shearwater Predator with its Büulmann algorithm as the backup. I wrote the V-Planner schedule in my wet notes, and then I programmed the same gases into the Shearwater and used its dive planning mode to see what kind of a schedule it would have wanted me to do if it were not being used as a backup. As one would guess, it was completely different. The run time was about the same, but its first scheduled stop was much shallower, and it put much more time at the shallower stops. Consequently, after programming in the SAC rates, it suggested that I would be using much less deco gas than V-Planner predicted for its schedule.

We followed the V-Planner schedule perfectly. My Shearwater was slightly upset that I was not doing what it said, but it kept recalculating the plan as we ascended. I stayed with them the whole way. Their computers cleared them for final ascent within one minute of completing the final V-Planner stop--and so did mine. It adjusted to the reality of my dive perfectly.

To answer the question then, if I were planning an ascent using a schedule with a lot of deep stops and was unexpectedly low on gas, I know enough about other kinds of plans to know that I could shave time off of those deep stops and add them the shallower stops. That could allow me to complete a perfectly acceptable decompression process with the amount of gas I have.

Moreover, if you are following a planned ascent and using a computer as backup, if you do shave time off your deep stops, the computer will adjust for you as you ascend. The computer can thus become your primary guide rather than the backup. On the other hand, if you hold to the deep stops and risk running out of gas on the shallow stops, the computer can do you no good whatsoever.

Whatever you may feel about the superiority of one decompression theory over another, there is no research to suggest that the one you rely on is safer than any of the other common theories. None. If you are using one that stresses deep stops, it seems to me that in a gas-challenged dive, it would be wise to change to another one.