Decompression Diving

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Basics on Decompression Diving

During your conventional diving career through PADI, SSI, NAUI and the like, you got to know No-Decompression Dives, how to plan and conduct them, and that they are ended via a direct ascent to the surface.

As well you learned the usage of Standard-Air Tables (RDP, US Navy Table, ... ) and how a dive computer can support you. As well you learned something about the maximum ascent speed and how to conduct a safety stop.

Thus in contrast to a decompression dive. This dive is ended with stops in certain depths for a definitive time.

Technically speaking: "Every Dive is a Decompression Dive!"

Why? An ascent means just a reduction in the ambient pressure: this per-se, is: De-compression, and as such valid for all these dives.
The difference is just the ascent procedure!

First of all, a couple of examples of No-Decompression-Limits (NDLs) and the maximum ascent speed:

Nice to know:
the value of 1 ft/sec developped historically as the arithmetically mean of the ascent speed of hard hat divers with that from combat divers: it should be uniform in all the navies manuals. So there is no unalterable physical law behind it.
As well the NDLs are subject to change in the course of time and differing by as much 20 to 25 %. So here as well there is no absolute truth behind it; instead a lot of statistics and ball-parks about human physiology.

The whole story becomes thrilling very quickly by operating repetitive dives!
We need the repetitive, or pressure group (PG ) and the length of the surface intervall (SI).
The paradigm is SI = 60 min, the results the adjusted NDLs for this 2nd dive to be also
a No-Deco dive.

Now we can see a little bit more clearly, how the different tables/models could pack a punch!
Now the variation in the adjusted NDLs can be up to 500 % !!!
And so does the safety?

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Table Handling

Here's a simple example: you make a dive to 37 m, 16 min.

You look up the DECO 2000 table from Dr. Max Hahn. (This table is in widespread use all over europe, it comes for: AIR, Nitrox32, Nitrox36, at sealevel and as well the whole bunch for mountain-lake diving!)

You round up and you will find the following entry under 39 m and 18 min:
6 m / 3 min, 3 m / 7 min, F


This means very simply you have to stay in 6 m depth for 3 min,
and as well in 3 m depth for an extra 7 min.
So your effective dive time (not your bottom time) has increased about 10 min.
Therefore you will need extra air.

The letter "F" simply designates your Pressure Group (PG) and is a coarse measure of a calculated (assumed) inertgas load in the body,
resp. in a so-called "compartment" (a mathematical model with a designated halftime in minutes for a piece of body tissues).
As well with these halftimes there is no flying magic: halftimes are basically just
the reciprocal of the blood perfusion in this model-tissue.

For the plan beeing on the safe side concerning air consumption and cold / exertion,
you look up the next greater depth or the next greater time or both.
i.e.: 39 m and 21 min or:
42 m and 19 min. This gives you: 9 m / 2 min, 6 m / 4 min, 3 m / 10 min.
The actual dive profile now looks like that:
after 18 m at 39 m you go up slowly to 9 m and stay there 2 min, then slowly to 6 m,
after 4 min you are allowd to rise to 3 m for additional 10 min.
For these additional 16 min of time spent under water you need approx. 800 liters.
(16 * 2 * 25, that is with an Surface-Air-Consumption (SAC) rate of approx. 25 l / min plus a little bit reserve.
So with a 12 l bottle you could hit empty ... How to avoid?

Now we check again the PG:

We see: this pattern is relatively coarse. Why so?

And: what's the story with cold water and the next greater time/depth?
Well, there is the funny Henry's Law and a change in the blood perfusion of your skin and muscles.

Surely enough, having another table or dive computer will yield other figures? Why this?

Now let us check a couple of these other tables:
In the rough data with 39 m / 18 min we will get:

As well there are various mindsets on how to deal with cold and exertion,
and, these mindsets are subjected to change in the course of time due to different approaches to the so-called "critical super-saturation".
Here a couple of examples, (the [No. #] is the reference in my Literature List):

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Computer Handling

A dive computer should support you on such a dive!
In order to get along, you should know all the displays of your computer by heart!
Normally you see the actual depth (DEPTH 39 m) and dive time (BOTTOM 17 min),
as well the calculated TTS, and where and how long the deepest deco stop is.

(Rem.: DEC = Decompression, P is the oxygen partial pressure, ASCENT displays the ascent speed)

Here we see the Ceiling at 12 m: so you ascent slowly to approx. 12 m, then starting your stop procedure!

Here we see the display during the ascent from the 6m stop to the next, the 3 m stop.
The remaining stop time being 7 min, the dive time increased already to 26 min.

Other computers may give additionally deep stop information:
so, for this dive with another computer we will have:
22 m/2 min, 14 m/1 min, 6m/1 min, 4.5m/10 min

More interesting question:

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Inertgasbubbles

Any reduction in ambient pressure, through ascent, flying, for astronauts or by driving to a mountain top will produce inevitably microbubbles.

To a certain degree the body can handle this and a lot of bubbles get stuck in the lung-filter and are down-breathed with no signs or symptoms. But if there are too much bubbles or they get too big then we are on the verge of decompression sickness (DCS).

The amount of bubbles and the bubble volume is mainly dependant on: dive depth, dive duration and ascent speed.

Dive Tables resp. Dive Computers should assist us to dive such profiles that DCS can be avoided.

Normally this is the rationale for No-Deco Dives.

As was indicated above, there are measurements, showing a lot of inert gas bubbles even for dives within the NDLs! Here we see the plot for a No-Deco-Dive (30 m, 20 min) in comparison with the same dives, but with safety stops:

Doing a safety stop reduces the bubbles, but the bubble reduction is even more pronounced with an additional deep stop at 6m

As well quite nice to know: the ideas of the "deep stops" has been published already in 1908 by Haldane. However, it went to historical oblivion, but a strict scientific reason has been proved already in the 60's by Hills, B.A.

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Ascent Pattern (© ALBI)

A couple of figures concerning No-Stop/No-Deco Dives:
in an investigation of 163,400 No-Stop-Dives according to the USN, Version 1983 table between 21 and 55 feet there have been 48 DCS incidents ...

This is exactly why a lot of diving pro's prefer the following ascent pattern, especially with long "No-Deco-Dives":

12 m / 1 min, 9 m / 1 - 2 min, 6 m / 4 min, 3 m / 2 min.

So any time your dive computer tells you to go straight to the surface:

slow down, young feller ... If your DC shows up some deco time already, you just add this deco time to the ascent pattern.

Interestingly enough, this resembles more or less the paradigm, developped 2001 by the DSL.
The DSL is the Diving Safety Laboratoy from DAN/Uwatec.

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a very special table

this is the contrast: a table which allows for max. 0,4 Bar super saturation

These tables have been provided by my late friend, Dr. Max Hahn in 1992 for a UHMS meeting:
here just 2 short parts for 39 and 42 m:

As well this table meets the requirements of the DSL - PMRC concept from 2001:
PMRC: Proportional M-Value Reduction Concept.

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the United States Navy Table, AIR, Revision 6 (04/2008)

The old table from Rev. 3 of 1993 was based on the proposed changes through the NEDU report #13 from 1983: there have been various corrections concerning wrong calculations, or slip-of-the pen. For the Rev. 3 there was the reduction of ascent speed from formerly 60 feet / min to 30 feet / min. This means, that the 1983 version has 25 years under the belt! (Which is, b.t.w., a lot if you take into account, that by today, air diving tables are just 100 years old!)

The unacceptable number of DCS incidents for longer and / or deeper dives had to be reduced. Starting point was the extensive research of Dr. Edward D. Thalmann, Captain, Medical Corps, U.S.Navy (now retired) and as well from Edward Flynn and Wayne Gerth.

This culminated in various sets of modified M-values, which have been tested in thousands of controlled dives in the NEDU chambers.

The system with the less DCS hits was the one which was called after the name of the FORTRAN source code variable: VVAL18.

What are, in brief, the highlites of the new Air Table from 04/2008, Revision 6?

And here is my "test"-dive: 42 m

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a little "examplay"

Let's take a quick look at a couple of figures from the very beginning.
We are planning 3 dives:
18 m / 45 min, 1 h SI, 18 m / 25 min, 1 h SI, 18 m / 25 min:
here "effective" means really in effect for the table.

Why this example/play?
Well, now you can see very quickly:

An all-is-easy happy bubbles No-Deco-Dive without any even recommended-only safety stops within one educational framework
could be simply forbidden / not possible within another framework or,
can imply within the next framework a dive with real serious decompression obligations!
Or, there can be a deco stop of considerable length with even O2 breathing!

The easy and very clear take-home-message here is:
the sharp separation between No-Deco and Deco dives, which certain educational frameworks are juggling with,
is not that sharp ....

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