Reverse Profile.

[madmole]

I think what George Irvine argues is the the slow tissues do not become any more saturated on a deeper second dive than they would have become if it was the first dive, hence you can ignore the first dive. Kinda makes sense.

Seems to work for him, but we all know his drysuit has a big "S" on the front and he wears his red swimming trunks on the outside.

As a hobby I write deco software and compare different models. When I get to England at the weekend I'll run this through the Suunto RGBM, VR3, Abyss and my own models to see what convental models actually do and if the higher level of PPN2 already in the tissues slows down the absorbtion on the second dive to make the assumtion true. Me I believe that you end up almost with the N2 form dive 1 plus the N2 from dive 2. Irvine would argue that the fact that the tissue PPN2 is high lowers the transfer gradient thus slowing the uptake on the second dive until it equals what it should be if it was the only dive. I can see that only being true if the dive is a near saturation dive (which may well apply to the WKPP dives)

Also WKPP dont have a model to predict this. I think they just lop 5 mins of the deco each dive until folks hurt and then go back a bit

 

[rcohn]

Originally posted by Dr Paul Thomas
Without going into scientific detail, which is well beyond my limited understanding, this is the way I see it.

If the deeper dive is first the decond dive is equivalent to a deep stop on the first so the slower tissue compartments will effectively be offgassing during the whole of the second dive.

I don't believe the physics will support this argument. The slow tissues require hours of bottom time to saturate and on any normal recreational dive they would have a low load of dissolved gas. Also during the surface interval they wold have been continuously offgassing. So for all but the most shallow depths I would expect the slow tissues to be absorbing additional N2 (or He) during the second dive.

Originally posted by Dr Paul Thomas
As stated above, this must add considerably, to decompression penalties. More importantly you are entering the unknown, as there is little eveidence on the safety of reverse pofile dives from field evidence. So few divers do them!

While recreational divers have not been doing many reverse profiles there has been no such prohibition for commercial and military divers who have experience with reverse profiles. Also the US Navy table testing and I believe the PADI RDP included reverse order dives in their validation testing without problems. I'll see if I can pull a reference on this tonight from the proceeding of the Reverse Profile Workshop.

Ralph

 

[Dr Deco]

Dear Readers:

Any second dive that is deep and long will load the body with gas and "cover" the presence of the first dive - if it is shorter and shallower than the second. How much shorter and how much shallower is necessary needs to be determined with the decompression devices. These are what I refer to on this FORUM and in my talks as “road maps” since they tell you where you are in the “world of tissue pressure.”

These road maps indicate the gas loads and what is a good course of action. Virtually all divers, however, take these and use them as “bends prognosticators.” They expect that the tables (or meters) will tell you if you are in “safe zone” or the “bends zone.” Such “zones” are not physically real and do not exist. The individual differences between divers (such as micronuclei formation) is negelected in these devices.:nono:

It certainly is possible to design “rule of thumb” tables. The caisson workers on the Brooklyn Bridge used these more than a century ago. In many – but not all – cases they worked. Deco procedures for specialized uses are developed from time to time. When they are analyzed in accordance with Haldane gas loading concepts, they are found to be within the safe zone. Extrapolation can be dangerous however.

Dr Deco
:doctor:

 

[Liquid]

madmole-

If I understand you correctly what you say will not alwais work.
If you are saturated to a DEEPER depth than your current, It may impractical to get youself saturated to a depth much shalower (very long time).

 

[madmole]

Not me that says it. Its George Irvine, the demigod of DIR. and if its DIR then it must be right

Personally I can see all sorts of problems with it, but the WKPP folks seem to survive on it

 

[The Iceni]

Hi all,

Following my naive comments about reverse profiles and on-gassing of the slower compartments I took a close look at two sample dive profiles using my old edition of Proplanner, in order more fully to understand the consequences of reversed profile diving.

Although it does not include the more modern theories of decompression, since it uses the symmetrical Buhlmann ZH16 algorithm alone, one very useful feature of the programme is it provides a graphical display of the relative saturation of the sixteen theoretical compartments at each stage of every dive.

I looked at two air-only dives, the first to 50 metres for 30 minutes followed by a two hour surface interval and a dive to 30 metres for 30 mins.

As expected the faster compartments saturated first on each dive and at the end of bottom time on the first, deeper dive, compartment Nos 1 to 9 were supersaturated, with the slowest compartment No. 16 (with a half time of 635 minutes) 28% saturated.

62 minutes of decompression from 15 metres are required in order to allow off-gassing of the faster, saturated compartments during the ascent from the first deeper dive. On surfacing compartments Nos. 7 and 8 contained the most gas but, of course, the slowest compartment continued to take on gas during the staged ascent and contained 6% more nitrogen than at the end of bottom time, reaching 34% saturation.

After the two hour surface interval most remaining nitrogen was found in the mid range compartments, Nos 9 & 10 being 57% saturated, while compartment. 16 was still 31% saturated.

At the end of bottom time on the second, shallower dive, compartment Nos 1 to 10 were supersaturated, with the slowest compartment No.16 being 48% saturated.

30 minutes of decompression from 6 metres are required for the ascent. On surfacing compartment No. 8 contained the most gas and the slowest compartment reached 49% saturation.

I then looked at these profiles with the dives reversed.

At the end of bottom time on the initial 30 metre dive, compartment Nos. 1 to 6 were supersaturated, with the slowest compartment No. 16 being only 17% saturated.

11 minutes of decompression from 6 meters are required for the ascent. On surfacing compartment. 5 was most saturated and compartment. 16 contained only 1% more nitrogen than at the end of bottom time, reaching 18% saturation.

After the two hour surface interval most remaining nitrogen was found in the mid range compartments, Nos 7 & 8 containing 40%, while No. 16 still contained 17% .

At the end of bottom time on the second, deeper dive, compartment Nos. 1 to 11 were supersaturated, with the slowest compartment No.16, 46% saturated.

However, 96 minutes of decompression from 15 metres are now required for the ascent from the second deeper dive. On surfacing compartment No. 9 contained the most gas and the gas content of the slowest compartment had increased to 51% of saturation.

The total decompression penalty for the reversed profile is 117 minutes, compared with 92 minutes with a standard profile, an additional deco penalty of 27%. However, probably more importantly the residual gas load in the slower compartments is higher (51% compared to 49% in compartment 16).

In summary, as I see it, reversed profile diving not only attracts added deco penalties, it leaves an appreciably greater residual inert gas load in ALL the slower compartments.

(In graphic form this is much, much easier to see.)

 

[madmole]

Thanks Doc, thats what i would have figured as well.

Now who's going to phone up George Irvine at WKPP and tell him he's wrong??????

Now the $64,000,000 question is does the Buhlmann gas uptake model accuratelly reflect the bodies real gas uptake or is it the fact that WKPP are doing deep and MUCH longer dives leading to high saturation levels and the models break down here. For example the traditional model just keeps on taking up gas even when the compartment is saturated. But real tissues do saturate at some point and dont take on any more gas. Hence, especially for fast tissues the Buhlmann model can force more deco than needed.

It would be interesting to change the buhlman model so that gas uptake stops when the compartment is at 100% (110%, 120%????) (I know this is a simplistic model) and run the same profiles again

The fact is that WKPP ARE doing reverse profiles and ARE only decompressing for the last dive and ARE getting away with it. This suggests that their particular profiles (and they aint ordinary ones) are not modelled correctly by current theory.

I might ask Mr Irvine, as to why he believes this to be true and what model he is assuming is in operation. Better hide my wings first though, they have bungie on them

 

[The Iceni]

Originally posted by madmole
Thanks Doc, thats what i would have figured as well.

Now who's going to phone up George Irvine at WKPP and tell him he's wrong??????

Err, not me. Thats for sure! :baby:

 

[rcohn]

Originally posted by madmole
Now the $64,000,000 question is does the Buhlmann gas uptake model accuratelly reflect the bodies real gas uptake or is it the fact that WKPP are doing deep and MUCH longer dives leading to high saturation levels and the models break down here. For example the traditional model just keeps on taking up gas even when the compartment is saturated. But real tissues do saturate at some point and dont take on any more gas. Hence, especially for fast tissues the Buhlmann model can force more deco than needed.

Actually the compartments in real models saturate just like the actual tissues do. They use exponential equations. They don't continually take on additional gas.

Ralph

 

[The Iceni]

Originally posted by rcohn


Actually the compartments in real models saturate just like the actual tissues do. They use exponential equations. They don't continually take on additional gas.

Ralph

Yes, confusing isn't it.

I think the confusion arises because, as in Proplanner, they show supersaturation in terms of percentages. I may be wrong but compartment "3" shows a saturation of well over 400% at the end of bottom time on my first theoretical deep dive, clearly this is not physically possible.

I interprete this to mean that under pressure it contains over four times as much gas in solution as it could hold at surface pressure. For example a saturation dive to 50 meters would top out at 600% and never exceed it. (six times the surface saturation because it is exposed to six times the pressure). This necessitates a deco stop on the way up to bring its contents down to below 100% of the surface saturation before the diver can safely surface.

I will try it out later this evening.

Or am I up a gum tree?