Which deco stop to skip?

I am now taking an Advanced Nitrox course and came into a discussion about possible consequences of skipping parts of a deco schedule. More specifically, the argument I was presented with was as follows: in the event that you find yourself in a situation in which you will not be able to complete all your decompression obligation, you should try to prioritize the deeper stops.

The reason for it is that the deeper stops mainly control the formation and growth of bubbles in the fastest compartments. These compartments, on their part, represent tissues such as blood and nervous system. Therefore, if you choose to skip the deepest section of deco, you risk DCS affecting directly the nervous system. On the other hand, if you happen to skip part of the shallowest stops, you would be risking DCS on tissues such as cartilage, which wouldn't be as bad.

Personally, I found this correspondence between deco phases and body tissues a little suspicious.

Can anyone shed any light on the issue?

Thank you in advance for your time.

It'd depend greatly on what algorithm you were using to calculate the deco. Different models place emphasis on different phases of the ascent. No stops are surplus though.

Whichever algorithm the diver places their faith in, it remains critical that they complete the full obligations of the decompression profile they calculate. Proper planning and preparedness should render this a moot point - a theoretical discussion only.

I recommend reading Mark Powell's 'Deco for Divers' to gain a better understanding on the finer points of decompression theory and modelling.

Whilst I understand the logic of thinking of what if situations, my immediate response was to miss NON. Plan your dive and dive your plan.

Rule 1, take enough gas for the whole dive with deco and also to supply gas for your buddy from the deepest and latest part of the bottom time. If you both do this, you each have enough gas to do the dive and an emergency supply to allow travel to the surface and all deco obligations for your buddy. They are carrying your emergency supply if you lose your gas totally and you theirs.

Rule 2; Always CARRY all the necessary gas, if you leave it somewhere you might not get back to it or find it.

Rule 3; Plan the dive and dive the plan. NOT CHANGE YOUR MIND BECAUSE YOU GOT ALL EXCITED AND THEN OVER EXTENDED OR DIVED DEEPER THAN PLAN THUS NEEDED TO USE YOUR EMERGENCY SUPPLY.

Rule 4; Carry an emergency safety sausage (yellow) with slate attached. Talk about it in the dive brief (with the boat captain), if they see the yellow sausage, they come immediately over and read the slate "Need gas for deco at 10m", they can then lower a cylinder to say 10m.

PLAN YOUR DIVE.

With deco dives we also have an emergency cylinder under the boat. Its there in case you come up and everything else has failed you. Backup for the backup.

As cave divers say "Running out of gas is NOT an option, and we don't plan for it".

Do you know your SAC rate, normal work part of the dive, deco SAC and also when things get bad SAC. If you don't know them you should. Otherwise you cant plan your dive accurately. Do you know your turn around pressure regardless of the dive plan time to turn? If you don't you haven't planned your dive, etc etc. PLAN YOUR DIVE AND DIVE YOUR PLAN.

Translation "We don't know"

Translation: We don't care.

1) Don't miss stops, ever. Deco obligation is overhead environment.
2) All calculated stops considered equally critical.
3) Manifestation of DCS too unpredictable to link to omission of stops deep/shallow.
4) Discussion of missed stops versus DCI manifestation requires deeper theoretical understanding than OP has demonstrated.

If you don't care, don't reply

Don't get too caught up on which compartment represents what. None of that has been demonstrated; half time 'x' corresponding with 'cartilage' for example. Its more complicated than that, and you can't really predict what type of hit you'll get depending on what stops you skip. I wish it were that simple, but it isn't.

Time on o2 is good for you, though. Hanging out at 20' for a good while can prevent or fix a lot of screwups. If it were me, and I HAD to skip something (which is unlikely but conceivable), I would move my schedule up to the next available gas and sit there for a while. For instance, if I had stops at 40' or 30' on 50% for a few mins, but o2 is right above me, I'd get on o2 where I've got the biggest benefit.

Don't skip deco stops though. That's bad. You might get bent. Maybe severely. Don't do it. Plan your dives and contingency plans right and don't cut any corners. The entire ascent is important, and straying from the schedule could put a hurtin' on you.

Did I mention DONT SKIP DECO STOPS?!

I can think of a very good reason to miss deco. A friend was diving a deep wreck off Sydney (60 m - 200 feet) and doing deco when a tiger shark came by. Luckily it just looked at them and kept going, but they did cut short their deco a little. They had done more than enough based on some tables but their computer was still showing some.

We talked about this after and agreed that in the case where a tiger or great white started acting aggressively, we would seriously think about abandoning any remaining deco and immediately start oxygen while at the same time calling for emergency assistance.

As to the actual question, I have no idea.

Tortuga68:

If you don't care, don't reply

Click to expand...

The point being, as I mentioned, to discuss the issues raised by the OP, a higher level of deco theory knowledge is required. That's why I pointed him towards Mark's book.

His question relies on many variables; gas/es used, deco algorithm used etc... Actively nominating a stop/s to miss for decreased risk of DCS...or to increase chance of a less severe DCS hit... cannot be answered unilaterally as a general rule. Likewise, I question whether it can be answered at all (certainly the issue of 'choosing' your DCS hit type/manifestation).

For example, are we using Haldanian tables? Dual-Phase? Buhlmann model with what Gradient Factor? Are we using 50%, 80%, 100% etc for our deco? One gas or more? Are we using helium?

Understanding what you are doing, and why, is a prerequisite for the sort of improvisation/adaptation being raised for discussion...

Add to that a dozen other variables; surface support, water conditions, temperature, access to medical or hyperbaric treatment...

Deco stops cannot be missed with an illusion of retaining safety. Choosing to miss a deco stop because of another pressing safety issue is quintessentially a gamble between the lesser of two evils. That gamble is, again, never going to be clear cut or definite.

Nirvana:

I am now taking an Advanced Nitrox course

Click to expand...

Which one? IANTD, TDI or A.N other?

Nirvana:

and came into a discussion about possible consequences of skipping parts of a deco schedule. More specifically, the argument I was presented with was as follows: in the event that you find yourself in a situation in which you will not be able to complete all your decompression obligation, you should try to prioritize the deeper stops.

Click to expand...

Deeper stops or deep stops?

Deep stops are countraindicated for dives less than 50m, according to DAN

Deeper stops. Well on an advanced nitrox profile how deep are your actual stops?