Deco stop question

[Spectre]

Originally posted by jbd
from the link provided.

GI says to begin the deco stops at 80% of your profile in atmospheres. Why couldn't you just use the depth in feet or meters instead?

Because Atmospheres takes into account the first atmosphere, depth does not.

How does this, "....rounded up." come into play? Would it make up the 7' difference you came up with in your example?

The difference is with atmospheres, you "add one" and "subtract one". If you do the same with feet you will get the same result:

E.g.

240 fsw = 240/33 + 1 = 8.27 ATA * .8 = 6.62 ATA = 185.4 fsw

[As shown already].

You can get the same thing with:

240 + 33 [for the surface level atmosphere] * .8 - 33 = 185.4 fsw.

Where does the 7 come in? 33 * .2 = 6.6 fsw.

And the other thing using ATA takes into account? fresh vs. salt water.

 

[jbd]

spoil the broth?

Thanks for all the input it makes sense to me now.

 

[Jeff M. English]

exact science. 80% of pressure at maximum depth is the starting place GI uses. He's got a lot of experience so his ideas carry a lot of weight. Don't get overwrought in the tiny details; try to see decompression as a process - slow ascent to the first place you want to start offgassing / reducing those tiny bubbles. Short stops while deep to allow the bubbles a chance to resolve, longer stops as you get shallow to allow the dissolved gas to come out, (although I guess WKPP drops much of the shallow time and just lets the lungs handle bubbles on the surface, I'm not there yet, believe me).

To the specific question of how come 80% of pressure instead of 80% of depth either would probably work. Hey Jeff, you said you were a math guy; won't 80% of actual depth always be deeper than 80% of maximum pressure with a 1ATA surface pressure? (We can discuss altitude effects later). Actually, as I think about it, 80% of depth should always place you 6.6 feet below 80% of maximum pressure, (actually, that's part of what Jeff showed a minute ago, 'scuse me amigo!)

You could probably just take 80% of your maximum depth and round to the nearest 10 feet. That should place you pretty close to 80% of max pressure rounded up. Personally I use 80% of max depth and round down, or the first 1 minute stop GAP gives me whichever is deeper.

DISCLAIMER: I'm still a newbie at this too, like TexasMike I've only completed an Advanced Nitrox and Deco, all the rest just comes from stuff I've read and stuff I've tried. Deep stops via GAP and a 20 second stop at each 10 feet (which makes 30'/min) ((which I borrowed from the WKPP web site)) seems to work pretty good for me. Your mileage may vary.

Jeff

 

[jbd]

In the GI article that we are discussing he mentions times in a general way 20 seconds minumum and 5 minutes maximum on the deep stops. I understand the point of a maximum time here but I'm fuzzy on how one knows when to use something between the minimum and the maximum. Is this a case where more is better, lets say a 1 minute stop instead of a 20 second stop? Then again maybe 3 minutes would be better.

 

[Cave Diver]

Originally posted by jbd
In the GI article that we are discussing he mentions times in a general way 20 seconds minumum and 5 minutes maximum on the deep stops. I understand the point of a maximum time here but I'm fuzzy on how one knows when to use something between the minimum and the maximum. Is this a case where more is better, lets say a 1 minute stop instead of a 20 second stop? Then again maybe 3 minutes would be better.

JB

Again, this is merely my *OPINION* as I have no where near the amount of training or experience as GI and others have in deco theory, etc. but to me it would seem to work like this. The idea behind a deco stop is to allow the nitrogen that has been forced into saturation under a higher pressure to come out of solution slowly enough that you may expel it through normal respiration. Staying too long at a deep stop could potentially counteract that in my *OPINION* as you are still breathing Nitrogen under a higher partial pressure. So if you stay too long at a deep stop then you are absorbing nitrogen in at a faster rate than you can off gas it. So you come up slowly, stop to allow some to come out of solution, come up a little more to help some more come out, etc. Your stops would get progressively longer at shallower depths because as you get shallower, your body is able to off gas at a faster rate than you are absorbing *up to a point*. The use of gases that contain lower amounts of nitrogen, such as pure O2, nitrox, trimix, heliox, etc. allow a more favorable exchange rate, but again, if you stay too long at any depth, you will be absorbing nitrogen, under pressure faster than your body can naturally release it.

Hope this is somewhat helpful to your question.

Dive often, but dive safe.

John

 

[jbd]

Cave Diver,

What I'm not understanding at this point is how does a diver know when 20 seconds is enough or 2 minutes or whatever at any given depth.

As Jeff M. English mentioned, "deco is not an exact science" But there has to be some sort of guidelines to follow or even GI would be bent by now. Something had to guide him to the methods he's using. Maybe its plain old experience, but when I read the article it sounds like there is more to it than experience.

Some one had to put some kind of information into the various computer based deco planners to work out the descent times at any given depth.

I know this rambled but I'm just thinking out loud with my fingers.

 

[Cave Diver]

JB

Most dive tables and ccomputer models are based on Buhlman or Haldaan algorithms that determine tissue loading of various compartments. The rate that these tissue compartments absorb or release nitrogen varies, which is why the stop times vary. From what I have read, GI has found methods that greatly accelerate the normal stop times, but I havent seen anything that describes exactly how is accomplishing this. Sorry I can't be of more help on this matter, but if I find anything I will be glad to post to the board.

Dive often, but dive safe.

John

 

[jbd]

have provided so far it would seem I need to learn more about half times and tissue saturation etc etc. If so, that gives me a good starting point. Thanks!

 

[Jeff M. English]

Sorry to take so long to post again.

Based on the article by GI and the email threads that it came from originally, (see quest or techdiver archives), I think what GI is doing is in large part just experience based. The parts that are not probably start with common deco tools.

As CaveDiver said most of the software out there today is calculating dissolved gas loads to figure decompression, but that's not why people have started doing "deep stop" type decompression. Dr. Deco has a few threads that cover this.

The reason to start all these little stops so deep is to allow tiny micro bubbles, which already exist in us anyway, to crush themselves down a little more before we ascend some more.

There are many available deco-software tools that will allow you to mess around with the addition of deep stops and many will even show you what compartment saturation is like at various points during the deco. The one I have the most experience with is GAP, see www.gap-software.com. Read the Decompression Theory articles by Erik Baker.

GAP and Decoplanner are both, I believe, Modified Haldane implementations of RGBM. AS such they are just REALLY conservative Haldane models for your first stop and then get progressively more liberal until they recover the allowable surfacing M-values of the original model for your last stop and cruise to the surface. There is a full-on non-haldane-modified RGBM model in use in some dive computers and you can buy the tables from NAUI, (Dr. Wienke is the developer, you can find articles by him as well.). The true RGBM is said to produce shallow stops even shorter than the mod-haldane algorithms.

As to the actual question of how long at what depth, I don't know how GI produces his schedules.

He may be doing something as simple as drawing out a graphical representation of the deco profile and adjusting the shape of that step-curve to match what he believes an efficient deco should be. He would have to have many years of trials during which he gradually changed the stops that he did and then watched himself and how he felt after the dive very carefully.

I think I've prattled on long enough, by the way the 20 second stop thing just enforces a 30ft/min ascent, if you spend 20seconds covering each 10 feet your doing 30'/min, if you stop for 20seconds at each 10' your going even slower which is probably better.

Keep reading jbd , and let me know if you DO figure out how Irvine does it, in a real practical sense, I'd love to know.

Have a good one,
Jeff

 

[jbd]

I found what I was looking for; actually very much more than I was looking for. In the Ask Dr. Deco section there is a link to deepocean.net. Don Acheson provided the link. Its no wonder computers are used to calculate this stuff. The amount of math required is absolutely amazing.

I think GI must be at least quite aware of this stuff and has probably made adjustments based on his experiences.

When you get to the deepocean website click on the science button.