Decompression Stop Questions

[mala]

a little off topic from your main questions, but is 52% permitted for extended range diver? I thought Advanced Nitrox certification was necessary for any nitrox mix over 40% O2

i guess the gas mix was just what it was on the day.

52% at the 21m gas switch would be 1.61pp02 -not the end of the world.

---------- Post added January 25th, 2013 at 09:26 PM ----------

thanks for clarifying...I'll shut up now

extended range is a 55m air qual

 

[JohnN]

From: Extended Range Diver | SDI | TDI | ERDI

Certain areas of the world provide spectacular dives at depths deeper than 39m/130 feet but you will not have access (or very limited access) to helium. The TDI Extended Range course teaches you the proper techniques for utilizing compressed air as a breathing gas and with a maximum depth of 55m/180 feet you won’t have to miss those dives. The Extended Range course will cover topics and skills such as

 

[rongoodman]

I'd recommend you pick up a copy of "Deco for Divers" by Mark Powell. The background you need is there. It sounds like you were shortchanged on the theory part of your classes. The current Deco Procedures and Advanced Nitrox books from TDI are much better than the old ones, and worth picking up if you don't have them.

 

[kierentec]

Hi

I am a TDI extended range diver. i have some questions about decompression and decompression stops or more accurately the physics behind them.

1) Why when making a deco stop must you remain at 9m for example and not 8.5m for example? Surely when you are at 8.5m you are off gassing just as you are at 9m?

2) How do you select a deco mix or deco mixes? when i did my courses i was told that we will use 52% o2 as that was what was available so how do you select mixes?

3) When diving with trimix as helium is absorbed when you ascend it is released just as nitrogen is so is it the same principle of residual nitrogen levels as residual helium levels and can they also be used to calculate the length of deco stops?


Craig Chamberlain

I know this is going to sound harsh, and I really don't want you to take it the wrong way. however, I believe there is something seriously missing here. Questions 1 and 2 should have been covered extensively in your training, with little or no confusion on the topics by the end of deco procedures at least, and certainly by the time you're through extended range. When planning dives, was there any discussion about gas choices other than "that's what we've got"?

 

[Dive Bug Bit Me]

I would also seriously recommend finding a mentor/instructor to guide you going forward. Your questions here will raise some eyebrows as the basics should have been covered in your extended range course. I have seen to many divers following a DC and not truely understanding the impact of what they are doing and ending up hurt!!! Decompression diving is serious stuff, make sure you know what you are doing. PLEASE!!!!

Craig, I would second what AJ has said. The Extended Range Course is a step up from AN/DP in the TDI system. I think that it is supposed to be conducted to the same standard as the basic trimix course in terms of the dives you conduct (except you breath nitrox mixes without helium and therefore dive a little shallower). This is why it is permitted to move directly from Extended Range to Advanced Trimix (which is what I did).

You should have the TDI manuals for Advanced Nitrox, Decompression Procedures and also Extended Range / Basic Trimix if you have completed these courses. You should reread these. I also second everything else that AJ has said. I don't want to sound harsh, but I might also consider contacting the instructor that certified you and asking him to walk you through technical dive planning again. I would also get him to walk you through using v-planner again.

I am worried about answering your questions as half the knowledge is a lot more dangerous than none of it. Hence my suggestion that someone walk you through technical dive planning.

However;
1. You plan your depths and deco stops in advance. If you plan 15, 9 and 6m stops, then that is where you stop. I could plan 16, 10 and 7m stops and the dive would work out fine. Note that this might impact my gas planning as 100% O2 now is not an option at my 7m stop. If the plan is a 6m stop, you should have the skill to do this. If not, do not do the dive. If you are bouncing between say 5.5m and 6.5m then you are probably going to be ok, but you shouldn't be on the dive. Bear in mind that deco is not an exact science, so you might miss your stops slightly and be fine, or you might execute them perfectly and bend anyway.

2. Mixes are a function of what is available, the preferred depths for gas changes, the CNS loading of the dive and also personal preference. I would play with V-planner and see what different permutations of dives and gasses do to your CNS and your length of decompression stops.

3. The physical laws that apply the nitrogen also apply to helium. They behave in the same way. They are, however, different gasses and the coefficients to the formulae change. Therefore you cannot take a nitrogen dive plan and apply it to Trimix, just as you cannot take a dive plan for 32% O2 and apply it to air. There are a few extra things to learn about helium that you should watch (ICD and HPNS spring to mind) just as there were extra things to watch when you moved from air to nitrox mixes.

 

[boulderjohn]

I would like to address the general issue of the seeming arbitrary nature of some of the conventions in terms of depth, mixes, etc.

Yes, they are somewhat arbitrary. You can do your deco on anything, including air, if you really want to. You can do stops at various depths if you want to. You just have to understand the consequences of those decisions and plan accordingly. One of the beauties of using standard depths and mixes is that people have pretty much thought out their relationship in dive planning. There are other reasons to go with the standard mixes and depths.

I am very used to using EANx 50 as a deco gas and switching to it at 70 feet. It is a common practice. The next time I do such a dive, I will go into a shop with my tank with a big 70 sticker on the side and ask for EANx 50. The attendant will attach the tank to a fill whip from the EANx 50 outlet and fit it up. (I could ask for EANx 46 and pay a lot extra for a custom blend, but I don't see the point.)Then I will input EANx 50 (and other gases) into my planning software and get a plan that will tell me what stops to take in standard 10 foot increments. Easy Peasy.

On the other hand, when I did some decompression dives in Cozumel a couple of months ago, the shop with which I did my business used EANx 36 as a standard decompression gas for that range of decompression for much the same reason--it was easy and relatively cheap for them to get it. EANx 50 would cost a lot more. I put that gas into the same software and got a dive plan. It had me switch to that gas deeper (as you would expect), and the stops were different (as you would expect), Easy Peasy.

When it comes to trimix, your TDI instruction will teach you a way to calculate the best mix for those dives as well. (I assume your Extended Range course used the combined Trimix/Extended Range book, so you probably already have that information at hand.) On the other hand, lots of people use standard mixes, like 21/35, for those ranges. (I am one of them.) They are not ideal in the sense your book teaches you, but they are better in many ways. For example, that same shop that will fill my EANx 50 tank can fill tanks more cheaply with 21/35 from their banked gas as well. If you want to use the other standard mixes, you will often find that shops that offer them a lot can do them more cheaply and easily than custom blends as well. (They can be made easily by putting in the right amount of helium and then topping off with banked EANx 32.)

 

[rongoodman]

As for why those specific depths in meters for the stops, they follow the English units of 10' increments in stop depth. Using a set of standard gases, both for deco and bottom mixes, keeps life simpler and allows schemes like Ratio Deco to be useful.This discussion of M-values should be helpful too.

 

[fdog]

I would like to address the general issue of the seeming arbitrary nature of some of the conventions in terms of depth, mixes, etc.

Yes, they are somewhat arbitrary...

<snip>

Meh. I was going to author a long involved reply for which I am possibly (in)famous for, but John put is succinctly and knowledgeably. So I'll cheerfully co-sign on everything he said.




All the best, James

 

[HowardE]

The basic theories of tissue compartments and basic offgassing of fast compartments vs slow compartments and the "virtual overhead" of the decompression ceiling should have been extensively covered in all of your training to this point. I am extremely surprised that you could have passed the final exams for any of the previous levels of training without understanding the how and why stops and virtual ceilings are prescribed for the decompression schedule, and what in theory is at risk for ascending above the ceiling before your prescribed time.

--

However... There is little difference between 9m and 8.5m, and knowing the actual ceiling vs making hard stops does differ, and if you were to hang at 8.5m instead of 9m when your ceiling is actually 6m isn't that much of a problem as long as you're in the decompression zone, and not too much deeper than the ceiling so that you were ongassing in another compartment.

--

Extended Range is the course that comes after Advanced Nitrox and Deco Procedures. Advanced Nitrox and Deco Procedures are prerequisite for Extended Range, and ER qualifies a candidate to take the highest certification in the TDI Open Circuit range, which is Advanced Trimix. Extended Range certifies a diver to dive Air to a depth of 180 FSW. Extended Range is basically the same course as Basic Trimix (they use the same book), except you don't actually get dive Normoxic Trimix, and don't get a card that says you can.

 

[Kevrumbo]

Hi

I am a TDI extended range diver. i have some questions about decompression and decompression stops or more accurately the physics behind them.

1) Why when making a deco stop must you remain at 9m for example and not 8.5m for example? Surely when you are at 8.5m you are off gassing just as you are at 9m?

2) How do you select a deco mix or deco mixes? when i did my courses i was told that we will use 52% o2 as that was what was available so how do you select mixes?

3) When diving with trimix as helium is absorbed when you ascend it is released just as nitrogen is so is it the same principle of residual nitrogen levels as residual helium levels and can they also be used to calculate the length of deco stops?


Craig Chamberlain

1) Simply a training exercise & motivation to be aware of where you are at depth in relation to body kinesthetics and breathing patterns; IOW, if you are aware & can fine control your deco stop depth oscillations with just nominal inhalation/exhalation. But if you have to, dump or add gas from your buoyancy wing and/or from your drysuit if you're using one. The challenge is when switching from backgas to deco gas & vice versa (especially in drifting deco scenarios in poor viz): can you hold that particular deco stop depth?

2) Try to select a dive operation that has the capability to blend standard deco mixes like Eanx50 and supply 100% Oxygen at 200 bar pressure. Additionally:

The choice of which deco gas(es) to carry is based on the following set of criteria. The first is that each gas is a standard deco mix for one of the deco segments listed [21m to 9m and/or 6m to the Surface] and it provides an O2 window to accelerate the deco. This will reduce in water decompression times and reduce the risk exposure to the environment. Another important consideration is the amount of backgas a diver needs to reserve in order to do the dive plus the ascent plus the deco. This backgas should be enough for two divers to airshare during the exit and the ascent and the deco. Therefore the deco gas is not only chosen to accelerate the decompression, but also to provide extra gas supply deeper so that the divers need not carry unreasonably large amounts of backgas. Very quickly this amount of gas becomes excessive for even the largest tanks and an alternative gas needs to be available deeper anyways. You need an additional bottle, and it is better to make it a Deco bottle rather than a stage bottle as you also get the benefit of accelerating the deco exposure. . . (see pp11-12 in attachment below, Ratiodecodone.pdf)

3) Yes, but be advised:
Shallow depth NDL's on Helium can be less than Nitrox, or even Air diving:
​

From Bruce Wienke, Technical Diving in Depth, Reduced Gradient Bubble Model (RGBM) In Depth:

Helium NDLs are actually shorter than nitrogen for shallow exposures . . . Reasons for this stem from kinetic versus solubility properties of helium and nitrogen, and go away as exposures extend beyond 150 fsw, and times extend beyond 40 min or so.

Helium ingasses and outgasses 2.7 times faster than nitrogen, but nitrogen is 1.5 to 3.3 times more soluble in body aqueous and lipid tissue than helium. For short exposures (bounce and shallow), the faster diffusion rate of helium is more important in gas buildup than solubility, and shorter NDLs than nitrogen result. For long bottom times (deco and extended range), the lesser solubility of helium is a dominant factor in gas buildup, and helium outperforms nitrogen for staging. Thus, deep implies helium bottom and stage gas. Said another way, transient diving favors nitrogen while steady state diving favors helium as a breathing gas.

http://www.scubaboard.com/forums/ad...aining-recreational-trimix-5.html#post6603329

The greater diffusivity of Helium from a free phase bubble model perspective (i.e. RGBM) means possibly loading idiopathic bubble seeds/bubble nuclei and having a Boyle expansion pathology resulting in DCS upon ascent --even if you were diving trimix within air or nitrox NDL's. . .​