Can you dive again after a minor Deco dive?

[Genesis]

more or less what others have said, but perhaps a bit more condensed:

1. There is nothing wrong with "deco". All dives are in fact decompression dives - you just do (most of) your deco on the surface in the form of your surface interval.

2. If you violate a TABLE, that is, if you use the PADI RDP or something similar, and violate it, the TABLE says to stay out of the water. That's because it didn't give you a deco schedule, it gave you an "oh !@#$, I screwed up" procedure.

3. If you DO NOT violate a computer (that is, you go into deco AND FOLLOW ITS INSTRUCTIONS), then the schedule that computer gives you SHOULD be good enough that a repetitive dive ON THAT SAME COMPUTER should be no less safe than an original dive, or a repetitive dive WITHOUT a deco obligation on that same computer.

4. HOWEVER, some computers are less than reasonable in how they do their deco. There is a "shape" to decompression that is optimal for a given gas mix, and most recreational computers do not follow it. Most deco planning software DOES, to one degree or another (VPM-B, Decoplanner, etc.) BUT, if you understand the general shape of the curve, MANY computers will allow you to decompress on that curve, even though they don't give you the curve themself (Suuntos in particular work just fine this way.)

So in general, no, there is nothing wrong with repetitive decompression diving, provided that your repetitive dive (whether planned on table or computer) properly accounts for your residual nitrogen level in your tissues AND you properly decompress in the first place. Violate either of those rules and you are asking to get bent, and the computer part requires that you understand whether your computer works in a reasonable fashion if you do a proper "shape" deco. (FWIW, the current Suuntos appear to be just fine in this regard for "reasonable" exposures; I've not gone beyond about 10-15 minutes of required "hang time" though, and they do not accomodate Trimix at all.)

I do repetitive dives after a decompression dive all the time; nothing wrong with it at all, provided you understand what you're doing.

One word of warning - decompression diving is diving in an overhead environment, just as is cave or wreck diving, in that you are precluded from an immediate return to the surface at any point in time you would otherwise want to. A CESA or EBA is forbidden once you enter mandatory decompression; if you do one anyway, you are likely to suffer DCS or an AGE which can seriously injure or even kill you.

The MINIMUM requirement for such a dive includes enough redundant gas ON YOUR PERSON (NOT on your buddy!) to complete the deco obligation and make your ascent from the worst-case point in the dive. For MOST profiles, this is an AL30. I have an AL19 pony, and it can only support a few minute obligation from 100', as you simply don't have the gas in it to complete the deco otherwise. An AL30 gives you enough gas for most "light" exposures. Even better is a set of doubles with a proper decompression gas (e.g. 50/50) for that AL30. However, THAT requires even more planning, thought, procedures, etc - grab that 50/50 at 130' and you're very, very likely to be dead before you can say "lights out" from a tox hit.

As such it is mandatory to understand the manage the risks of overhead environment diving and be prepared to complete the decompression irrespective of problems that may arise. It is extraordinarily unwise to be doing overhead environment dives of ANY KIND without that understanding and an appropriate gear configuration - you can seriously injure or even KILL yourself that way.

 

[Dr Deco]

Dear SCUBA SOURCE Readers:

Repetition and Deco

One certainly can perform another dive if the previous dive was a decompression dive and the decompression procedure was followed. You must use the same decompression table (or meter) and cannot switch from one system to another. That would be analogous to gluing together two maps with different scales. Yes, they will match at one point but you cannot cross back and forth at others. You will end up being in the wrong place on both maps eventually.

Naturally you cannot use a table or meter that is not meant for decompression and then go into the decompression mode. The “road map” begins to fall apart. The same will be true if you do not perform the required decompression. The algorithm will not work if excessive phase separation (bubble formation/growth) occurs. Even two-phase models will not give accurate results if the conditions in the tissues begin to vary too much from the initial system. Blood vessels become blocked, perfusion changes, off gassing becomes indeterminant, and the system will behave chaotically until reset by time or pressure.

Dr Deco :doctor:

 

[DA Aquamaster]

Genesis once bubbled...
Even better is a set of doubles with a proper decompression gas (e.g. 50/50) for that AL30. However, THAT requires even more planning, thought, procedures, etc - grab that 50/50 at 130' and you're very, very likely to be dead before you can say "lights out" from a tox hit. [/B]

Good point on using the pony for a dedicated deco gas. A recreational dive computer, even one suitable for deco diving, will not accomodate a gas switch or give you credit for accellerated decompression when using a deco gas such as Nitrox 50. Using it does increase the saftey margin in terms of off gassing but also adds the requirement of manually tracking the 02 exposure throughout the day to ensure you stay within acceptable limits. It's a good idea, but one that requires proper training to safely utilize.

 

[Diver0001]

ScubaAaron once bubbled...
<snip>

And as a follow up: If I am correct and my buddy is wrong, is there any increased danger to going a few minutes into deco? As in increased bubbles? Other than if you do an emergency assend you will assend without doing the required stops.

Your question sounded hypothetical but just in case it wasn't I'd like to say this: There are ways to plan and execute deco dives safely. People do it all the time. However, it doesn't sound to me like you've been trained to do so and I would strongly advise against experimenting with it until you do.

R..

 

[Genesis]

Good point on using the pony for a dedicated deco gas. A recreational dive computer, even one suitable for deco diving, will not accomodate a gas switch or give you credit for accellerated decompression when using a deco gas such as Nitrox 50.

The Vytec does

 

[padiscubapro]

DA Aquamaster once bubbled...


Good point on using the pony for a dedicated deco gas. A recreational dive computer, even one suitable for deco diving, will not accomodate a gas switch or give you credit for accellerated decompression when using a deco gas such as Nitrox 50. Using it does increase the saftey margin in terms of off gassing but also adds the requirement of manually tracking the 02 exposure throughout the day to ensure you stay within acceptable limits. It's a good idea, but one that requires proper training to safely utilize.

using 50% at the 20ft or 10ft stop only only slightly raises the cns
at 20fsw the po2 is ony .8 which is only .22% per minute(for the 10-ft stop is about .17%), even a 15 minute hang (if required@20 fsw is not a slight deco dive)only raises the exposure by 3%(and in reality of a person is diving nitrox, the computer is adding up some exposure already so it really just the difference in PO2's exposure- we are estimating on the safe side).. a dive diving at 1.4po2 has a 150minute per dive/180 per day exposure limit, most divers even doing several dives a day would have trouble reaching this.. and doing so would require using the highest mix for the dive you are going to do.. even a diver just doing deco dives in the 60 ft range would have exposures upped to 210/240 (using commonly available 36%) would have to do at least 4 dives over an hour long each at the maximum depth..

Most dive computers that have CNS alarms start giving alarms considerably below the limit.. 3% additional per dive without giving credit for SI, can simply be added to the "calculated" exposure... even at 5 dives a day thats only 15% which would require diver to violate the suggested 80%cns loading limit advocated by many.

 

[Genesis]

use it RIGHT!

If you're only going to carry ONE deco gas, then for most recreational depth dives I'd argue that 50/50 is probably the "right" one to carry.

Here's the argument....

1. You can breathe it down to 70'. This means that when you are ascending, you make a gas switch stop at 70', stay there a minute or two to get the oxygen window benefit, then begin your ascent schedule from there. It also means that if the REALLY SERIOUS brown stuff hits the air mover (e.g. catastrophic manifold failure), you can bail to 70' and breathe it. This is unlikely to bend you even if you started at 100-130' (it MIGHT, but I'd rather take that risk than try a CESA from 130 with a deco obligation!) I don't like the odds on grabbing a bottle of 80/20 or 100% at 70' and surviving it!

2. IF something goes wrong on the ascent, you have done SOME of your decompression on accelerated PO2. This is, IMHO, superior to having done NONE of your accelerated deco. I'll take as much wash-out as I can get if for some reason I must abort my deco schedule (e.g. an aggressive shark that is threatening to have a snack.)

The "bad news" with this choice is that you will consume significantly more of the deco gas than otherwise, which means you may only get one dive out of an AL30 or 40, where if you stick with the backgas until you get to the 30 or 20' stop (for 80 ro 100%) you can typically easily get two dives off one bottle.

You have to weigh the arguments for 50/50 against the benefit of being able to spike the PO2 at shallower depths. But the only way you will get significant additional credit for doing that is to carry 80 or 100%; your longest stops are the real shallow ones - where it will benefit you.

The problem with carrying ONLY 80/20 or 100% is that you get no acceleration of the window down deeper. You also lose the bail-out option. In the ocean, at least, I personally believe that is a net lose.

Some folks would argue that you should carry TWO deco bottles, but for Nitrox dives (e.g. stuff above 130-140' or so) I tend to disagree. (Of course if you're one of those "Helium for anything under 100'" folks then this doesn't apply.)

 

[DA Aquamaster]

Genesis, you can dive with me any day.

Padi scubapro. I agree with you on your calculations and I agree with you overallto an extent, but those are choices and decisions that need to be made with training and with an understanding of what is occurring. Do I bother to track my CNS exposure during a diving day with short deco stops using a hotter nitrox mix for deco, usually not.

But there are limits to this. On a longer dive with signicantly more deco the CNS exposure becomes a concern As it does if you do it smart and do the same depth and bottom time on an apprpriate nitrox mix with less deco obligation. Training and experience become important in understanding and knowing when O2 exposure will be important. I think it is a little too risky to make a blanket statement that an inexperienced diver may read indictating that it is not an issue.

From a CNS versus reduced deco standpoint 80/20 is hard to beat and gives up very little to pure O2. And in general on longer dives with deeper stops the 80/20 will shorten the total deco time compared to 100% O2 as it can be used sooner and deeper.

I agree with genesis though that 50/50 is an ideal mix for many deco situations. On most dives I will, as he describes, switch to it at a deep stop at 1/2 max depth (assuming it is at or above the 70' MOD) and remain on it for the balance of the ascent.

If you are doing accellerated deco you are not giving up much compared to 80/20 on most profiles and it is far cheaper and easier to mix 50/50 if you are transfilling your tank and do not have access to a boost pump.

If you are using a computer that does not allow for the gas switch, you are still adding a significant saftey margin over air or a cooler nitrox mix and again giving up very little in terms of saftey to 80/20.

And as genesis states it gives you one more viable redundant gas supply to 70 ft and at worst would only require you to make an esa to 70 ft rather than to 20 ft if things really went wrong on the dive. And realistically, a single breath if you really needed it on the way up from 100 or 130 ft would most likely not result in a O2 hit. I am not comfortabel saying that about 100% O2 or 80/20.

 

[Atticus]

As we are offering advice to a recreational diver, and trying to increase his safety...

It seems to me that what he needs next is a fully redundant air source, and as such it would be best if that air source contained a mix that he could breath at depth.

When recreational divers talk to me about pony setups, I suggest that they fill them with air, or at most 28%.

This is a CESA avoider first and foremost. Any accelerated deco for a recreational dive is probably leading down the wrong path . . .

 

[ScubaAaron]

Well I must say I am quite surprised with the response to this thread. It has made great reading. From everything that has been said I now understand that it is indeed safe to dive after a minor deco dive should it ever happen. That being said I now also more completely understand the concept that going beyond the NDL in effect creates a cieling and that can cause major complications. That is why more training and redundant air sources / mixed gases are required.

I will stay within my gear and training and not use the deco feature on my computer unless it is an emergency.