Deep stop question

[Artimas]

Can someone direct me to any threads here or internet articles that discuss deep stops or other new and/or controversial techniques to reduce the risk of DCS?

I did a search on here and not much came up.

I am interested to learn more about the topic mostly out of academic interest.

Thank you.

 

[Thalassus]

DAN Divers Alert Network : Deep Stops: Can Adding Half the Depth of A Safety Stop Build in Another Safety Margin?

Tales from the Wet Side - Deep Stops

One Minute to Safer Diving - Scuba Diving Magazine

Scubaboard thread already on this:
http://www.scubaboard.com/forums/ask-dr-decompression/194058-deep-safety-stop-question.html

 

[Artimas]

Thanks for those links. Just what I was looking for!

 

[Gene_Hobbs]

and some more papers on the topic here.

This includes the final paper by Marroni et. al. (from the DAN link above) but one should also read the letter to the editor that followed that work by Risberg and Brubakk.

Hills' work on the Torres Strait diving techniques is summarized in his SPUMS article (from this thread): Hills. (1978) A fundamental approach to the prevention of decompression sickness. South Pacific Underwater Medicine Society Journal 8(2): 20-47. RRR ID: 6176

But the almost nothing beats the readability of Rich Pyle. (Here)

I hope this stimulates some good thoughts and consideration for the upcoming Deep Stops Workshop in Salt Lake City.

 

[Artimas]

Gene-

Thanks for those references. I'm starting to develop a small library of articles to study, and there are some great ones listed in your post .

Regards, and thanks!

 

[Kevrumbo]

Deep Stops

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You specifically asked about the purpose of deep stops. Put most simply, the deep stop is a mechanism for slowing the ascent rate early during decompression. The purpose of this is to allow inert gas which may present in bubble form to exit from bubble seeds before the bubbles can grow beyond a certain size. The size is described as the Laplace cutoff and marks the radius at which a bubble will tend to either grow or contract. If bubble seeds grow larger than the Laplace cutoff, they will tend to continue to grow as you ascend-bad. If they remain smaller than this critical radius, the surface tension of the bubble will tend to be sufficient to cause a sufficient pressure gradient forcing inert gas back into dissolved form for transport to the lungs for elimination-good.

Dual phase decompression models like RGBM and VPM recognize the pre-existence of bubble seeds alongside dissolved inert gas and so incorporate deep stops by nature. Haldanian models only recognize inert gas in dissolved form and seek to prevent the spontaneous formation of bubbles by controlling the critical tension (M Value) of inert gas on ascent. Put another way, Haldanian models don't really explain theoretically why someone who follows the table should have post dive venous gas bubbles. Nevertheless they do and most neo-Haldanian models have been tweaked (scientific term) based upon Doppler studies to reduce this bubbling.

Rich Pyle's 1995 article on deep stops recommends inserting a stop half way between the max depth and the first required (Haldanian) stop. This was a good start, but not very "computational." For example, dual phase models suggest the first deep stop occur between 4/5 and 2/3 of the max depth for helium based mixes, depending on the nature of the exposure and the gasses used. Helium mixes generally involve deeper stops than nitrogen only mixes. Helium and nitrogen involve different diffusion and solubility characteristics.

For recreational "No Decompression" exposures, the Pyle method is probably fine. In essence, this is what most people do anyway on a typical warm water reef dive-start deep and work your way into the shallows. The dive involving much more deco stress is the traditional square profile. You know, 60 feet for 60 minutes and head to the surface at 60 fpm. For an air dive to 60 feet for 60 minutes, I would do something like 40x1, 30x2, 20x3, 10x6 and take a full minute from 10 to the surface. While either ascent profile probably won't bend you, listening with a Doppler, particularly in the context of repet dives, would not take long to illustrate which schedule leaves you with fewer post dive bubbles.

Above explanation by Kendall Raine and summarized nicely this thread awhile back. . .

 

[redrover]

I’m so glad this has come up again. I’m embarrassed to admit, understanding deep stops is not easy for me, there was a discussion a year or more maybe that I really struggled with. I still may have no clue really. There are a lot of factors and I’m not sure I can correctly weigh in just the ones applying to me. I’ve been winging it since then and found, shall I say a form of deep stops beneficial. My biggest concern is the ongassing at stops, or what are more of a pause for me.
May I present some personal info of my diving and get some thoughts of how to better apply deep stops?

53 YO ocean Rec diver, no deco, computer, air, with reef diving’s frequent seesaw profile. Rare to go below 100’ and down to 80’ is my comfort zone and most common dive.

Common at shore dives is the gradual work back upslope vs direct descend down to say a wreck then straight back up – and loads to see right up to the exit.
Am I better off sticking to creeping up the reef to surface? (I still do stops at 20’ and 10’ with the 10’ and up as long as I can manage to stay off the surface.)
Or better to set rigid depth ascend as stops? Say from 80’ to 60’ for specific minutes, up to 40’ specific minutes etc? My problem with that is I’d rather stay deep for less time and spend more time around 70’ to 50’.

On boat dives I’m stuck with the group (none of my buddies want to pay to dive .) It is usually 30’ down to buoy anchor then down to dive depth from 60’ – 100’.
With a boat dive to 100’ we pause a while at around 70’ then gradually work back up to the 30’ buoy anchor and call our stops as we wish. As the group is showing up at 30’ when the faster breather needs to be there, I’m hanging out at 30’ for quite a while, well most of the time. Bad vis here is 50’ so I do hang back at the tail of the group deeper for a while when the buoy is in sight when I can, topography doesn’t always allow that.
So when I get to 30’ I’ve been debating if it’s ok to putz around there for 15 minutes (almost always lot’s to find there) or if I should really cut that down to a form of deep stop and go up to my next stops and spent all that ‘free’ time there in off gassing (the allotted stops time as well as say another 10 minutes.)

I’ll bet I’m not asking this very well…I’m trying to figure out how much ongassing I get by hanging out on deeper stops or time spent there. And how to figure what I think of as a penalty for having worked up a ridge then over and down the other side when something really cool is there. Maybe I’m making it more complicated. It seems having the whole floor as a dive site is different than go down and just come up directly having planned stops in a straight vertical plane.
And any math equations need to be at a dummy level.

 

[Artimas]

A kind of related question that I hope makes sense. Since the idea of a decompression stop is to encourage offgassing, is there a specific depth (perhaps different for each individual) at which ongassing exceeds offgassing? A stop too deep (or too long?) won't allow offgassing, and may still cause ongassing.

In other words, how deep must one dive to begin ongassing? It obviously won't happen at the surface, and will at 40 ft. There must be a depth in between where "the lines cross". At what depth is that, where the ongassing exceeds the offgassing?

Theoretically, would it be possible to dive above that depth indefinitely without there being a risk of DCS?

I hope this made sense...

 

[ianr33]

A kind of related question that I hope makes sense. Since the idea of a decompression stop is to encourage offgassing, is there a specific depth (perhaps different for each individual) at which ongassing exceeds offgassing? A stop too deep (or too long?) won't allow offgassing, and may still cause ongassing.

For any profile there is a depth above which you need to ascend to start offgassing.It varies with depth and time (and O2 %)
V-Planner gives this information. e.g. for a 100 foot air dive for 20 minutes:

Dec to 100ft (1) Air 60ft/min descent.
Level 100ft 18:20 (20) Air 0.85 ppO2, 100ft ead
Asc to 30ft (22) Air -30ft/min ascent.
Stop at 30ft 0:40 (23) Air 0.40 ppO2, 30ft ead
Stop at 20ft 3:00 (26) Air 0.34 ppO2, 20ft ead
Stop at 10ft 6:00 (32) Air 0.27 ppO2, 10ft ead
Surface (32) Air -30ft/min ascent.

Off gassing starts at 68.7ft

There is not much point in going to the offgassing depth,you need to be 20-30 feet above it to start offgassing to any significant amount.

Many,many deco dives have a first stop about 60 feet shallower than the bottom depth. If I was winging it that is where I would do a first stop. That is not very different to a Pyle stop at 1/2 maximum depth for deepish recreational dives. None of this is an exact science. For the dive above I dont think it matters much if you do a first stop at 60 feet or at 30 feet,so long as you do something.

In other words, how deep must one dive to begin ongassing? It obviously won't happen at the surface, and will at 40 ft. There must be a depth in between where "the lines cross". At what depth is that, where the ongassing exceeds the offgassing?

Theoretically, would it be possible to dive above that depth indefinitely without there being a risk of DCS?

I hope this made sense...

You start ongassing as soon as you drop below the surface but if you are shallow it will not be very much. Above about 25 feet it is not possible to get bent,even if you stay there a week. (But you could still embolise if you held your breath)

 

[raymond phule]

Have some questions about off gasing related to the above post.

1. I believe that the off gasing depth is defined (atleast in Baker's work and V-planner) as the depth where the nitrogen (or helium) partial pressure is equal to the ambient pressure for the most loaded compartment.

2. But a tissue start to off gass, i.e. the partial pressure in the tissue decrease, when the nitrogen partial pressure in the tissue is larger than the nitrogen partial pressure in the breathing gas.

1 and 2 dont happen at the same depth. Why are #1 called the off gasing depth and not #2? Why are #1 important?

edit: wrote less insted for the correct larger.