Deep-stops vs shallow-stops: an interesting read.

[Seameadow]

And all I can do is sit here and wait for the epic battle between SM and RH to start,,,, again

 

[boulderjohn]

In the strictest of terms, there are two kinds of dives--dives with required decompression stops and dives with no required decompression stops. Despite what the "all dives are decompression dives" population says, there is a difference.

• Doolette is talking about dives with significant decompression, where the deeper decompression stops are causing a buildup of nitrogen in slower tissues that must be eliminated in shallower stops. Extra time at depth demands extra time on deco. The GF low number makes a difference.
• On NDL dives, that does not appear to be the case. As long as you do not go into deco, you can spend loads of extra time on ascent and still go to the surface without a stop. There is no need for the GF low number at all on such a dive.

There is a gray area in between. The post currently in question is in that gray area.

 

[boulderjohn]

And all I can do is sit here and wait for the epic battle between SM and RH to start,,,, again

No, RH is banned, so you will not see his fervid and angry response in this thread. That's my job. He will be taking it out on me about my last post via email, probably by noon.

 

[BoltSnap]

No, RH is banned, so you will not see his fervid and angry response in this thread. That's my job. He will be taking it out on me about my last post via email, probably by noon.

Who are SM and RH?

 

[Subcooled]

Deep stops vs. shallow stops. Too much theory, too little personal experience.

Sure, there's the bubble formation. Some people do prefer deeper stops to avoid that in the first place.
Some people allow some, and spend more time at shallow depths.
Depending on the dive profile, either one could be better.
Correct me if I am wrong.

The optimal solution might thus be to have moderate deep stops and to extend the shallow stops. This makes the dive longer, which is great!
The Subsurface dive log has a GREAT tissue loading heat map. You should study it.

Many people wrongfully believe that it is great to get out of the water ASAP. This might be true for arctic waters or where wave action is significant, but probably nowhere else.

Now. I am NOT a decompression expert. So, if you follow my recommendations and die, then what can I say? You did not ask the doctor in the medical forum? You trusted me instead? Well, your choice.

 

[boulderjohn]

Who are SM and RH?

SM = Simon Mitchell.
RH = Ross Hemingway

Simon Mitchell is one of the world's foremost authorities on decompression theory. He chaired the workshop on Deep Stops years ago. Ross Hemingway is the software engineer who created V-Planner and Multi-deco.

Years ago there were endless discussions about the deep stops controversy on several different diving social media platforms. I believe the first was on Rebreather World, but eventually it reached ScubaBoard, too. All the discussions starred Simon talking about the research studies that indicated that deeps stops were not the great idea everyone (including Simon) had once thought them to be. The opposing point of view was always pretty much Ross Hemingway arguing against that research. Ross seemed to have a particular affinity for VPM.

In I believe all the platforms that featured these debates, Ross was eventually banned. That includes ScubaBoard.

 

[boulderjohn]

For a summary of the changing thinking on deep stops, check out this article. Simon helped me with it tremendously.

 

[Underwater Tourist]

@BoltSnap I am not at my laptop to send the links right now, but search Simon Mitchell on youtube and there is a ton of info on the topic, including the evolution of thinking about how deep your first stop should be etc.

 

[Subcooled]

SM = Simon Mitchell.
RH = Ross Hemingway

Simon Mitchell is one of the world's foremost authorities on decompression theory. He chaired the workshop on Deep Stops years ago. Ross Hemingway is the software engineer who created V-Planner and Multi-deco.

Years ago there were endless discussions about the deep stops controversy on several different diving social media platforms. I believe the first was on Rebreather World, but eventually it reached ScubaBoard, too. All the discussions starred Simon talking about the research studies that indicated that deeps stops were not the great idea everyone (including Simon) had once thought them to be. The opposing point of view was always pretty much Ross Hemingway arguing against that research. Ross seemed to have a particular affinity for VPM.

In I believe all the platforms that featured these debates, Ross was eventually banned. That includes ScubaBoard.

A battle between a scholar and a software engineer is sad, as they should co-operate.
A software engineer should concentrate on what (s)he does better than the scholar - software development and numerical approximations.
We cannot live without either. We need both knowledge and coding skill.

I am relying on the software designed by RH to stay alive. But, in the more significant questions I must rely on SM.
Now, it really pisses me off if RH challenges SM. It is all about my ******* life.

 

[L13]

Deep stops vs. shallow stops. Too much theory, too little personal experience.

Sure, there's the bubble formation. Some people do prefer deeper stops to avoid that in the first place.
Some people allow some, and spend more time at shallow depths.
Depending on the dive profile, either one could be better.
Correct me if I am wrong.

The optimal solution might thus be to have moderate deep stops and to extend the shallow stops. This makes the dive longer, which is great!

The Subsurface dive log has a GREAT tissue loading heat map. You should study it.

It is an absolutely marvelous feature that lets you visualize the decompression stress for various dive profiles.

Many people wrongfully believe that it is great to get out of the water ASAP. This might be true for arctic waters or where wave action is significant, but probably nowhere else.

Now. I am NOT a decompression expert. So, if you follow my recommendations and die, then what can I say? You did not ask the doctor in the medical forum? You trusted me instead? Well, your choice.

In the end, deco stops are a trading resources (time and gas) for reduced DCS risk. For any given dive profile (within reason) you can trade more resources for less DCS risk. Each deco algorithm establishes a procedure for allocating the resources to reduce the risk. Each algorithm is based on theories about what causes DCS, and what increases/decreases DCS risk. VPM and other bubble models emphasize micro-bubbles as a risk driver (and tend to produce deeper first stops). Buhlmann and other Haldane models emphasize tissue dissolved gas tension as a risk driver.

Gradient Factors are a modification of Buhlmann introduced by Baker to make the Buhlmann algorithm produce profiles more like the bubble models. It does this by reducing the allowed dissolved gas tension at deeper stops. Gradient Factors turned out to be a great way to parameterize Buhlmann and adjust Conservativeness, and has been almost universally adopted by Buhlmann users.

The preponderance of recent research suggests that for a given dive profile and a given resource allocation, the Buhlmann model with GF's closer to equal (shallower stop solutions) reduces DCS risk more than VPM or a realatively low GF Low (deeper stop solutions). Conversely, to reduce DCS risk the same amount for a given dive, the deep stop solutions consume more resources (time and gas) than the shallower stop solutions.

Integrating instantaneous GF over time during the deco and surface interval seems to correlate well with DCS risk. If this hypothesis is correct, then a lower GF High should be used for long deco's vs. short deco's for the same DCS risk, since the elevated surface GF's will persist for longer as the GF is driven more by slower tissues. I subscribe to this hypothesis, but have not found the research to prove it.