YD Scuba Diving Forums banner

1 - 16 of 16 Posts

·
Registered
Joined
·
2,839 Posts
Discussion Starter · #1 ·
Imported post

If something occured during the course of a dive that meant you did your deco deeper than planned - eg if you planned [email protected] and [email protected], but the surface conditions were so bad when you ascended that you wanted to do as much deco as possible @9...

How would you alter the amount of time spent doing deco? (Assuming a table dive and not a computer)

Obviously this is assuming you're using the same gas at all stops - back gas, or a stage of 50%, or some such.

Just curious...
 

·
more weaselly than a weaselly thing
Joined
·
3,628 Posts
Imported post

If its bad enough that you cant pull a 6m stop......I'd start the telepathy to the coastguard ;)

Simple answer, just take all the stop time from higher and add it into ya 9m stop. I wouldnt do it for anything deeper than 6m myself.
 

·
A short fat well off crap cave diver. Likes wrecks
Joined
·
15,343 Posts
Imported post

If you use GF's to plan deco and lok at a 20/80 profile you are of gassing at 71% GF on the 9m stop and 75% on the 6m stop. So adjusting the GFs to get the off gassing occuring at 71% you need to alter the profile to 20/75. Thiss will roughley give you the additional deco required to be off gassing at 71% at 9m.

End result for a 55m dive for 45mins on 21/30  32% and 80% is an aditional 6mins of deco on an existing deco schedual of 80mins.

In short, forget it unless you were pushing long or very deep dives with 100/100GFs then it might become an issue. Just get as shallow as poss.

ATB

Mark Chase
 

·
To dive or not to dive - that's not even an option
Joined
·
2,998 Posts
Imported post

I'm not into big deco anyway, but if it was the kind you give as an example, I would try to do double at 9 than I was missing ar 6 as you don't off gas as well at 9. But as Mark said, just get as close to your stop as possible. Too rough for 6m? brave man.

James  
 

·
A short fat well off crap cave diver. Likes wrecks
Joined
·
15,343 Posts
Imported post

[b said:
Quote[/b] (narkedat50 @ Mar. 10 2004,02:29)]I'm not into big deco anyway, but if it was the kind you give as an example, I would try to do double at 9 than I was missing ar 6 as you don't off gas as well at 9. But as Mark said, just get as close to your stop as possible. Too rough for 6m? brave man.

James  
You couldent do it. The 6m stop is 42mins and the 9m stop is 9 mins so you would be doing 93mins at 9m


I doubt if you would have enough deco gas and I definatly wouldent have the patiance


The good thing about running old fart 20/80 profiles is you can be quite abusive with the profile and still not get bent. If you are running 100/100 bhulman type profiles you have to be much closer to the intended profile to get it to work.

ATB

Mark Chase
 

·
Registered
Joined
·
24 Posts
Imported post

In terms of dissolved gas models; for air and Nitrox a 3m stop takes about 1.5 times as long to clear at 6m and about 2 times as long at 9m.  Trimix behaves differently because the gas controlling the decompression may not be Nitrogen.

The problem with pulling the last stop deeper than 5m is that it is theoretically possible to on gas a slower compartment to the point that it exceeds its M-Value on final ascent.  I have a profile somewhere which came from an AirX.  The initial 3m stop was completed at 6m but a further 3mins at 3m was added by the computer just as the diver makes the final ascent.

Some other computers are just as fussy about stop depths.  They can be very slow to clear if the stop is pulled deeper than the computer indicates.

As far as your body is concerned, you are probably better off completeing a relaxed and steady stop a little deeper than bouncing around in the surf a little shallower.

Regards
Matt
 

·
A short fat well off crap cave diver. Likes wrecks
Joined
·
15,343 Posts
Imported post

[b said:
Quote[/b] (msssltd @ Mar. 10 2004,09:51)]
[b said:
Quote[/b] ]In terms of dissolved gas models; for air and Nitrox a 3m stop takes about 1.5 times as long to clear at 6m and about 2 times as long at 9m.  Trimix behaves differently because the gas controlling the decompression may not be Nitrogen.
?
this dosent make sense. If I add a 3m stop to the above profile it only subtracts 1min  from the deco time. If it took 1.5 times as long to compleat the dive with a 6m stop it should add 16mins to the 6m stop. Obviously it dosent.

Are you just basing your argument on the BAR presure of water on the body?



[b said:
Quote[/b] ]The problem with pulling the last stop deeper than 5m is that it is theoretically possible to on gas a slower compartment to the point that it exceeds its M-Value on final ascent.
The rate of on gassing at 6m is not worth talking about. On air it goes off the scale let alown 80% Nitrox. You would have to be at 6m for several hours before significant on gasing occurs.

[b said:
Quote[/b] ]I have a profile somewhere which came from an AirX.  The initial 3m stop was completed at 6m but a further 3mins at 3m was added by the computer just as the diver makes the final ascent.
Safety stop


Mark Chase
 

·
Registered
Joined
·
24 Posts
Imported post

Mark you may have misunderstood me.  What I meant was the whole profile on Air or Nitrox, not just the deco phase.

35m 30mins .21
9m 3
6m 9
3m 19

9m 3
6m 38
So take the original 9mins off and the 3m stop has taken about 1.5 times as long to clear

9m 71
Take off the original 3 mins and the 6m stop has taken very roughly twice as long to clear.

This is pretty much what you would expect to see considering the gas expansion going on and the effect it has on the tissue / inspired differential.

Trimix profiles are just about impossible to estimate.  Before a Trimix dive you are saturated with Nitrogen at .79 bar.  Trimix at the surface has a PPN2 of < 0.79 so if you breathe trimix at the surface you off gas N2 and on gas He.  Once you descend it all gets very complicated very quickly as the He on gases faster than the N2 off gases.

[b said:
Quote[/b] ]The problem with pulling the last stop deeper than 5m is that it is theoretically possible to on gas a slower compartment to the point that it exceeds its M-Value on final ascent.
[b said:
Quote[/b] ]The rate of on gassing at 6m is not worth talking about. On air it goes off the scale let alown 80% Nitrox. You would have to be at 6m for several hours before significant on gasing occurs.
I will do my best to explain it.  

Keep in mind you have 16 half-times, 16 M-Values and 16 pressure differentials to think about, any of which can control the decompression.  Lets say that on reaching 6m two of the compartments are close to their surfacing M-Value, call these C1 and C2.  The faster compartment C1 is a little over M and is controlling the indicated stop. The slower compartment C2 is a lilttle under M.  At 3m both C1 and C2 would be off gasing.  But at 6m C1 is off gasing while C2 is still on gasing.  Once C1 has off-gased below M, the computer indicates it is safe to ascend, however C2 is now almost at M and still on gasing. In the time it takes to reach 3m C2 exceeds M and an additional stop is indicated.

You are unlikely to see this behaviour in a VR3 or any other computer or software package that models all 16 compartments.  It can show up on models quantized to 8 or 12 compartments due to the increased gap between the compartments.

This is all theoretical.  The biggest fudge factor is our personal physiology, possibly the last word in adequate decompression.

6m was thought to be the maximum depth you could become completely saturated at and still surface without decompression.  Shortly after the 88s were compiled this was revised to about 5m.   Personally I make the last stop nearer 5m than 6m, although on one dive last year the sea was snotty enough to persuade me that 7m was a better option than bouncing in the swell.

Regards
Matt
 

·
Registered
Joined
·
18,557 Posts
Imported post

So does this mean that even in terms of "safety stop" type stops we should really be doing a 5m rather than a 6m?  I used to do 5m stops but pushed it down to 6 to make it that bit easier and more comfortable.  If your argument follows through though I really shouldn't do this, or if I do I should add another stop in shallower to be safe.

Interesting stuff.

Lou
 

·
Just not enough dive time.
Joined
·
9,135 Posts
Imported post

Lou
I try to do a 6mtr stop for 3mins then additional 1min stops all the way up at 1mtr intervals. I get out feeling much better because of it. The Padi stops seem to be geared again to warm water non-aggresive conditions. I wonder if Bsac use 5m as its a reasonably straight conversion from an imperial measurement like 15ft?
Anyone know.

Matt
 

·
Registered
Joined
·
18,557 Posts
Imported post

I thought that PADI did 5m stops and BSAC 6m!!  Anyway, it isn't from any particular allegance that i do it, just that 5m stops when I first went in the sea and it was a bit rough were more uncomfortable and difficult.  I therefore pushed it down to 6m stops as I knew others did these and we have never changed.

We always do a slow ascent from there and feel fine, but the question was based on the statement that at 6m it is now considered that you could still be ongassing in slow compartments and hence a 6m stop might not be ideal.  Nothing to do with who does what!

I suspect hat it isn't really a problem as the question was dealing with some heavy profiles and for a safety stop type stop it shouldn't make much difference either way.

Lou
 

·
Registered
Joined
·
2,839 Posts
Discussion Starter · #13 ·
Imported post

I can't believe that on-gassing at 6 is a problem worth considering - if a tissue can take over half an hour at 20-30+ meters and STILL not have taken on enough gas to be off-gassing at 6, then it sure as hell isn't going to take on enough at 6m to worry about.

The reason for a 5m stop, AIUI, is that even if you become totally saturated at 5m, you can still ascend straight to the surface without breaking any M-values. Deeper than that and saturation would start to break M values if we went straight to the surface.

But hey, that's why we all do a nice slow ascent from our safety stop, right?
 

·
"Three sheds"
Joined
·
12,688 Posts
Imported post

Hmmm. Strikes me that there must be a way to work this out, although I've just had a good glass of La'frog so brain may be being a bit tricksy.

Anyways,

If you were decoing on (say) 50% and had to pull your stop at 9m, then pp of Nitrogen is 0.95 bar. Now, assuming that off-gassing is only a function of the ppN2, then this would be like doing a 6m stop on 40%

So if you run that through your tables, then with luck and a following wind,  that should give you the answer.

I hope  


Laters,
   Janos
 

·
Registered
Joined
·
2,839 Posts
Discussion Starter · #15 ·
Imported post

Oh, it's easy to do it on tables - just tell the software your last stop depth is at 9 instead of 6


I'm just curious how people would deal with changing stop depths "on the fly", when they DIDN'T have the option of checking their answers
 

·
A short fat well off crap cave diver. Likes wrecks
Joined
·
15,343 Posts
Imported post

[b said:
Quote[/b] (Dominic @ Mar. 12 2004,08:27)]Oh, it's easy to do it on tables - just tell the software your last stop depth is at 9 instead of 6


I'm just curious how people would deal with changing stop depths "on the fly", when they DIDN'T have the option of checking their answers
Before every deco dive I run loads of little scinarios through the computer just to see what could happen. I take the normal three sets of tables with me but I have also just fiddled about to see what diferance a few meters here or a few minuits there would do and then i get in the water with a good mental picture of what I need to do to get out again.

With Deco planner its easy and quick to do which helps. When I plan my 100/100GF major sh:t fan interface plan it always makes me realise just how much tolerance I have within my normal profile any way.

IE:

60m for 40mins 18/40 32% and 80%  20/80GF  84mins deco

60m for 40mins 18/40 32% and 80% 100/100GF 60mins deco

atb

Mark Chase




ATB

Mark Chase
 
1 - 16 of 16 Posts
Top