Can you do too much deco?

[RedSeaDiver2]

I just scanned through this thread quickly, so I may have misunderstood. It seems to me that the one clear message you have received in this thread, including Tom's quoted here, is that you are planning your first stops too deep. As I read it, you do not respond to these clear warnings, but just keep saying that is what you will be doing.

Perhaps you will take a moment to 1) correct me if I misunderstood or 2) explain why you are so adamant about using sch deep stops.

There are other deco methods, but the two most common seem to be -
1) the GF 50/80 or similar "bend and mend" method which accepts that some bubbles will form early in the (deeper) deco stops but nothing that will hopefully cause you too many issues.
2) Deep Stops - which prevented bubbles forming during the deeper deco stops but had the unintended consequences of allowing slow tissues to continue to ongas during the deep phase of the deco, this additional ongassing during the deep stops went unrecognised and so the shallow stops weren't lengthened to account for this (and some probably planned shorter shallow stops thinking that they had less deco to do there due to starting their deco deeper).

The GF 50/80 bend and mend method might be considered appropriate practice due to its convenience (gets you shallow quicker and out of the water quicker), and reasonable safety record, but I'm not convinced that a plan that knowing allows bubble formation is world's best practice.
Obviously the traditional Deep Stops method isn't world's best practice either due to the obvious problems in its past with DCS hits.

We will be using relatively high helium mixtures (65%) for both thought clarity and a nice low density gas (4.1g/l) but that means a lot of helium to offgas as well. Using deeps stops gives the helium a bit longer to offgas - and remember that MultiDeco is only giving us two deeper stops of 40 seconds at 39 metres and 2 minutes at 36 minutes when compared to a GF 50/80 plan, and we are overcoming the problems associated with deep stops (the extra ongassing of the slow tissues) by using a second GF of 60 instead of say 80 which extends out the shallow stops to allow the slow tissues to appropriately offgas.
This obviously gives a longer dive profile than say a GF 50/80 plan (adds in 27 more minutes) and that wouldn't work for those that are impatient, may not work for charter boat diving in Florida, and wouldn't be much fun in the Great Lakes, but here in winter the water is 21degC / 70degF in the middle of winter so cold isn't a massive issue, there is no rush to get out etc so we can do this.

At the end of the day if @Dr Simon Mitchell isn't seeing any issues with it then I'm comfortable with it, but I'm happy

 

[tbone1004]

Indeed! Sorry to hear that you have had things go so wrong! The length of the project and the amount of decompression diving is the main reason for the three team rotation plan meaning that the time between the day 1 morning dive and day 2 afternoon dive will give a surface interval of about 26 hours, and then the dry day 3 means that each diver will then have a surface interval of about 40 hours before they do their day 1 morning dive again. Hopefully this combined with GF's of 30/60 will keep everyone safe, but you never know who has an undiscovered PFO waiting to rear its ugly head etc.

I would not rely on 30/60 to "keep everyone safe" given the recent research. I would seriously re-evaluate those gradient factors based on the recent research, none of which provides any evidence that those gradient factors would reduce decompression stress, quite the opposite in fact. Even something like 50/60 if you follow one of the recommendations for a GF-Lo about 80% of the GF-Hi does not necessarily mean that risk is reduced because you are increasing the exposure limits. Higher risk for something to change at the surface, reduced off-gas efficiency as the body cools down, etc. There is a fine balance and the more we get into your plans the more strongly I feel that 30/60 is not a profile I would agree to run if you asked me to be part of that dive. 50/70 is as conservative as I would ever run and since we are in the ocean I would personally feel much better running 60/80 and getting out of the water as soon as possible and taking more precautions at the surface.

 

[boulderjohn]

There are other deco methods, but the two most common seem to be -
1) the GF 50/80 or similar "bend and mend" method which accepts that some bubbles will form early in the (deeper) deco stops but nothing that will hopefully cause you too many issues.
2) Deep Stops - which prevented bubbles forming during the deeper deco stops but had the unintended consequences of allowing slow tissues to continue to ongas during the deep phase of the deco, this additional ongassing during the deep stops went unrecognised and so the shallow stops weren't lengthened to account for this (and some probably planned shorter shallow stops thinking that they had less deco to do there due to starting their deco deeper).

The GF 50/80 bend and mend method might be considered appropriate practice due to its convenience (gets you shallow quicker and out of the water quicker), and reasonable safety record, but I'm not convinced that a plan that knowing allows bubble formation is world's best practice.
Obviously the traditional Deep Stops method isn't world's best practice either due to the obvious problems in its past with DCS hits.

We will be using relatively high helium mixtures (65%) for both thought clarity and a nice low density gas (4.1g/l) but that means a lot of helium to offgas as well. Using deeps stops gives the helium a bit longer to offgas - and remember that MultiDeco is only giving us two deeper stops of 40 seconds at 39 metres and 2 minutes at 36 minutes when compared to a GF 50/80 plan, and we are overcoming the problems associated with deep stops (the extra ongassing of the slow tissues) by using a second GF of 60 instead of say 80 which extends out the shallow stops to allow the slow tissues to appropriately offgas.
This obviously gives a longer dive profile than say a GF 50/80 plan (adds in 27 more minutes) and that wouldn't work for those that are impatient, may not work for charter boat diving in Florida, and wouldn't be much fun in the Great Lakes, but here in winter the water is 21degC / 70degF in the middle of winter so cold isn't a massive issue, there is no rush to get out etc so we can do this.

At the end of the day if @Dr Simon Mitchell isn't seeing any issues with it then I'm comfortable with it, but I'm happy

I suspect that your reasoning is best exemplified by this phrase: "GF 50/80 or similar 'bend and mend' method...." This pejorative phrase indicates that you are still devoted to old thinking favoring deep stops, despite your willingness to later indicate that you are at least somewhat familiar with newer thinking on the matter.

At the beginning of the thread, you indicate that you are glad to have the input from Dr. Simon Mitchell. Why are you glad to have his input if you don't think he knows what he is talking about?

 

[boulderjohn]

We will be using relatively high helium mixtures (65%) for both thought clarity and a nice low density gas (4.1g/l) but that means a lot of helium to offgas as well. Using deeps stops gives the helium a bit longer to offgas

You may be interested in knowing that research a couple years ago indicated that helium does not need any more time to off-gas than nitrogen does. In fact, in light of the research, some people argue there is no need to include helium in dive calculations. The argument against it is that the same research that said helium off-gases at the same rate as nitrogen also suggested that we should be doing longer deco than standard algorithms anyway, so it does not hurt to plan for that extra time. There is no benefit, however, to doing that extra time during deep stops.

 

[Wibble]

1) the GF 50/80 or similar "bend and mend" method which accepts that some bubbles will form early in the (deeper) deco stops but nothing that will hopefully cause you too many issues.

50:80 IS NOT bend and mend.

As pretty much everyone on here said, deep stops are now discredited.

The most common gradient factors are around 50:80, frequently 50:70.

 

[JonG1]

Eliminating The Helium Penalty - Shearwater Research

Our understanding of decompression science has undergone dramatic revision in the last two years thanks to decompression physiologist David Doolette, Ph.D. and the team at the Navy Experimental Diving Unit (NEDU).

www.shearwater.com

 

[leadduck]

Your choice of GF_low was a reason why I brought up the little test with work load and extending bottom time in MultiDeco. If you assume faster on-gassing in your medium-fast tissues by exertion at depth, then you want to pull your first stop a bit lower and GF_low=30 is not as over-conservative as it would be for a relaxed drift dive. Surface oxygen can't prevent a DCS hit that you got already in the water during deco. I'm not suggesting a number here, there are so many other factors. Maybe 50 is just fine.

 

[Trace Malinowski]

Last night, I figured out an excellent method for determining what gradient factors one should use during a dive. I was watching Netflix. You know that "surprise me" option? That would be great in a Shearwater. Hit the "surprise me" option and the computer will select from among several popular deep stop or post deep stop GF's.

 

[Wibble]

select from among several popular deep stop or post deep stop GF's.

Like that term. Nicely describes where we are.

 

[Wibble]

Your choice of GF_low was a reason why I brought up the little test with work load and extending bottom time in MultiDeco. If you assume faster on-gassing in your medium-fast tissues by exertion at depth, then you want to pull your first stop a bit lower and GF_low=30 is not as over-conservative as it would be for a relaxed drift dive. Surface oxygen can't prevent a DCS hit that you got already in the water during deco. I'm not suggesting a number here, there are so many other factors. Maybe 50 is just fine.

Standard question about lower GF-LOW settings: why would you want to on-gas more? The point is to get up shallower where you're not on-gassing.

Also, why such a high helium content, 65% for a 60m/200ft dive? ~45% would be more normal to give an EAD of 24m/80ft -- and cost less. Gas density is 4.0g/litre compared with 5.3g/litre

According to MultiDeco, for a 60m/200ft dive with 30 mins bottom time (CCR with setpoint 1.3):

• 15/45 gas with GF 50:80 is a dive length of 94 mins
  • OTU's: 136, CNS: 51.3%, Gas density: 5.3g/l
• 15/65 gas with GF 30:80 is a dive length of 111 mins
  • OTU's: 161, CNS: 60.8%, Gas density: 4.0g/l

AIUI this is an additional 17 mins in the water at the cost of higher CNS/OTUs which becomes an issue on repetitive dives.