Decompression Theory

[Liquid]

Roaky-
I'm a bit rusty at the calc part, havent done it for some time now (hardly dove in the last year).

Let's see. first of all I belive I said 50 meters, which is a bit les than 190'. Now- we didnt take the 1.4 limit, which is very conservative. We took the 1.8, which for relatively short, and without any serious phisycal work is conservative enough. based on that I belive we used 30% mixture for the dive, and 50% for the stops, as we didnt want to use 100% after the exposure of the dive itself.

 

[roakey]

LY,

Yup, I got all three of the airspeed press books (Ok, there's four, but I don't need an aircraft preheater ).

Roak

Ps. My *only* claim to fame is if you look at the OHC page you'll see a testimonial from "R.O."

 

[Lost Yooper]

Liquid,

50M = 164'

190' = 58M

EAN30 at 164'? And people call me an unconventional nutt case! By doing that dive, you managed to knock off a mere 23' (7M) END which still puts you well into a significant narcosis depth of 141' (43M). By trying to fix a narcosis problem, you introduced yourself to an even greater problem of OX tox at considerable depth, in which case death would be likely had it occured (and it has occured).

Personnally, I would rather have subjected myself to the narcosis of 164' using air then take the risk of OX tox at that depth on EAN30. Ideally, of course, I would have used a trimix and reduced both the O2 exposure and the narcosis depth considerably for that dive.

Take care Liquid,

Mike

 

[Liquid]

LY-
There are several limitations regarding O tox (from now on OT). It depends on the kind of dive. I can tell you of friends that used 100% O2 up to 24 meters for short terms and were ok (military training). As a technical, trimix and EAN diver, I am sure you know there are a lot of aspect to OT. Starting with exposure-time, and finishing with ambient temperature. The 1.4 PPO2 limit is a very conservative one, and is intended for EVERYONE in EVERY CONDITION. Meaning, the fatest and the thinest, in coldest and wormest condition, in a peacefull dive or one that requiers a lot of phisical effort. It is good also for a situation that combines all the worst thing (unless you dive for over 2.5 hours or so, I don't have the book close by to be sure). A limit of 1.8 is still considered conservative, for dives that do not take you to the limits. Let's be frank, up to a PPO2 of 2ATM, if the conditions are good, and the exposure time is short, the danger is low enough to consider it a safe dive, if made by VERY experienced divers. I belive the limit for 1.6 PPO2 is 45 minuets, in the safest zone. It's more than enough to explor a nice wreck at 50 meters.

The main points here are few- Reduce significantly the time of the Decompression stops (while making it even safer by using a 50/50 mixtures for the shalow ones, I wont use 100% couse it's kinda teasing the OT). Improve the level of niterogen release, due to the higher portion of O2. And staying a bit more focused, than you would be with air, due to that wonderfull N2 intoxication.

To sum it, what I am saying is that dumping deep ean aside is not a good thing. I say EAN can be safely used up to certain depth. Whoever wants to get deeper, should than go use trimix, but throwing away everything but trimix is not a good move.

 

[Diverl]

Liquid

Please forgive me if I have misinterpreted your last post, but I want to clarify a few things.

For the slight if any reduction on END, you are willing to push you pp02 to 1.8 or more (basically to the limit) risking oxygen toxicity to avoid what maybe 10-15 minutes of decompression?

I was under the impression that technical diving was about reducing the risks, backup systems, lower END and lower pp02, so that you could safely return from whatever depth you have chosen to go. Not push the limits and risk everything for a couple of minutes of deco time! I have to agree with Lost Yooper, take the normoxic trimix course and become a whole lot safer. The dives might cost you more but at least you will come back from them. I guess that is why the GUE course has intrigued me; they teach you to use Heliox at shallower depths, and then have you move into the deeper dives.

Again if I have misunderstood then please except my apology!

 

[Lost Yooper]

Liquid,

I will concur to some degree that the 1.4/1.6 PPO2 is on the conservative side for some divers. If I remember correctly, the USN once used a max PPO2 of 2.0. As the story goes, the recommended max PPO2 was reduced as it became apparent that "recreational" divers (non military) were following that recommendation.

As far I as I know, every agency and exploration group out there is using 1.6 as the max -- even the unconventional guys of the WKPP. Based on what I have read and picked up over the years, oxygen tolerance is as complicated and individualized as narcosis. However, unlike narcosis, OT at depth is quite likely to be a death sentance and thus not worth the risk to most divers. From what I've gathered, for deeper diving it is best to keep the PPO2 as low as possible (realistically) during the dive and only spiking it to 1.6 at deco (70' with 50/50 or 20' with O2) to open the oxygen window for better decompression. This is how we do it, and I believe is the way that GUE teaches.

IMO, the 1.6 PPO2 is one of the few standards in the industry that almost everyone really seems to agree about. There are some closed circuit rebreather guys who are way out there, I guess. As I have heard it, long exposures to high PPO2 even over the course of days of diving play havoc on your lungs (sometimes referred to as "burning" the lungs).

Maybe Dr. D can offer some input on the subject of super high PPO2 diving and repetitive days of diving at high PPO2 and its effects on the lungs.

Mike

PS.

DiverL, I don't think you meant Heliox, right?

Oh, and Liquid, I never said to put aside nitrox and replace it with trimix entirely. Nitrox has its place -- (IMO) in depths less than 120' or so. After that, I'd like a squirt of helium.

 

[Liquid]

Diverl,

The PPO2 I used was not 2ATM. I spoke about using 1.6-1.8ATM. There is a concept, I don't know it's english name, but let's call it "Safety cycles". There are several cycles, like an onion. The further you go out from the center, the less safe you are. Using PPO2 of 1.6ATM (that's EAN30 at 44 meters), for up to 45 minuets, is considered still within the inner most cycle. It's still considered VERY conservative. going to PPO2 of 1.8ATM takes you one cycle out, and still considered "ok". PPO2 of 2ATM, like said is considered today dangerouse, though for short exposures it is still ok. When I plan a dive I never plan it for over 1.8ATM PPO2.

The reasons for using it IS safety. It depends on the way you plan the dive. You can use the mixtures while planning on air, making it one step safer. You can plan on a safety stop of air while using a 50/50 mixtures. It also gives you longer bottom time (which is, in other words, shorter deco stops)

L-Y,
About OT-
It is not 100% fatality, far from it. It is a good reason though, you should want to dive with a GOOD buddy, becouse your life may become epndant on him. A diver may be aware of developing OT before it get's too seriouse, and than end the dive as fast as possible, hoping to reach the surface before it get's really bad. If the situation get's bad, than all the deco procedures are abandoned, off course, but than, that's why we have HBO.

I know of someone that started developing OT simptoms, ascended, and they were gone after a very short time, while still diving. He did it fast enough, and was concience enough of his condition.

 

[Liquid]

Oh, Diverl, I also find it wierd about the heliox. Using it for shalow dives, beside being unnessesarily so expensive, is actualy somewhat danegerouse.

 

[Diverl]

Opps! sorry about the Heliox, reading and typing two different things, not good!

 

[Dr Deco]

Dear Lost Yooper and Readers:

When I worked in Germany, I was involved in a table-testing program where the oxygen partial pressures used were quite high. They were a time average of 1.9 atm. and were of great value in providing rapid decompression. [MR Powell, W Thoma, HD Fust, and P Cabarrou. Gas phase formation and Doppler monitoring during decompressions with elevated oxygen. Undersea Biomed. Res., 10 (3), 217-224 (1983).] They were safe from toxicity and downing because were all performed in a decompression chamber, however. None of these procedures, paid for by the Dragger Company, was intended for the diver in open water.

A recreational diver is putting himself at risk when not in the confines of a dry bell as are commercial divers.

When I left Germany to return to the United States (1980), I heard that these Dragger tables (or similar high-oxygen ones) were causing pulmonary oxygen toxicity when used on consecutive days. I have no doubt that this similarly would occur to recreational divers.

Dr Deco