Deep stop question

[ross9]

I'm currently reading Mark Powell's Deco for Divers and I'm very much at the beginning of my learning about the finer details of decompression models. I also just finished reading the excellent thread in the technical forum re: bubble models v gradient factors.

When divers talk about the drawbacks of 'deep stops' - what do they mean by 'deep' i.e. how deep is 'deep'? Is it only those deep stops suggested by bubble models or is it any stop inferred below a certain GFLow?

 

[tbone1004]

I don't know if that has ever been defined...

When I think of deep stops, I consider that to be any stop deeper than half of your absolute pressure. I.e. if you have a 40ft stop on a 100ft dive, or a 130ft stop on a 300ft dive.
Depending on how long you are diving and big of a deco obligation you have, you may well have a couple stops deeper than half your absolute pressure. Let's go to 300ft for 30mins on 12/70 with deco gases of 35/25, EAN50, and O2. 10ft final stop and GF's of 60/70.
Total of 161mins of deco, but only 6 of them are below 130ft and start at 160ft. 99 of them are at 30/20/10. 3.7% below half pressure, and 61.4% in the final atmosphere.
If we do that dive on VPM 0 which gives about the same amount of total deco at 163mins.
Stops start at 220ft and 17 of those minutes are below 130ft. In contrast the 30/20/10ft stops only have a total of 65 minutes. 10.6% what I would consider deep and only 40.4% in the final atmosphere.
A GF of 10/95 gives a comparable ascent for this dive profile however that does not necessarily mean that 10/95 is comparable to VPM0. Since they are different models the GF equivalents shift depending on the specific dive profile. Point of that though is to show that 60/70 has essentially an identical TTS as 10/95 but the stops were distributed differently.

The ascent curve with VPM is much flatter as the distribution of stops is more even. Play around with planning software so you can see what they do on profiles more similar to what you are doing, above was out of Deco Planner for reference.

 

[tursiops]

I'm currently reading Mark Powell's Deco for Divers and I'm very much at the beginning of my learning about the finer details of decompression models. I also just finished reading the excellent thread in the technical forum re: bubble models v gradient factors.

When divers talk about the drawbacks of 'deep stops' - what do they mean by 'deep' i.e. how deep is 'deep'? Is it only those deep stops suggested by bubble models or is it any stop inferred below a certain GFLow?

Rec diving (no deco stops) or tech diving (deco stops)?
For no-deco-stop dives, any stop deeper than a 15ft safety stop is both deep and unnecessary.
For deco dives, many use half the max depth (or pressure) as meaning a deep stop, which is generally a good bit deeper than the first deco stop unless you are using VPM or really small GF-low, both of which are (today) a bad idea.
Mark Powell's book is NOT up-to-date.

 

[boulderjohn]

As Tursiops suggests, there is a big difference between decompression diving and NDL diving. My response will only cover decompression diving.

There have been several attempts by people who know what they are talking about to define deep stops. Those attempts have failed, so what follows will absolutely have detractors.

The problem is that there are two very different algorithms, and the phrase "deep stops" applies differently to each.

1. Long established algorithms based on dissolved gas theory. These algorithms have gone through numerous trials over many decades. If you decide to add stops deeper than those algorithms call for, you are adding deep stops.
2. More recent (still more than 20 years old) algorithms, mostly associated with what has been called bubble theory in various forms, which believe in ascents that start deeper than the traditional dissolved gas algorithms. If you follow those algorithms precisely, you are essentially using comparatively deep stops.

Thus, if someone says that a deep stop is an additional stop added deeper than the algorithm calls for, that definition will get you started very deep if you are using an algorithm that is already starting you deeper than others.

This article may help you.

 

[Manatee Diver]

This article may help you.

So that is the article that you used those Suunto dive plans for.

 

[Dominik_E]

There is no definition that is universal. One always needs to see the context. To me, "deep" in the context of this discussion means that the total deco time stayed fixed, and was (re)distributed from shallow to deeper.

 

[Stefan Liden]

From the horse's mouth, so to speak:

 

[ross9]

Thanks all - some interesting follow up reading/watching.

It's interesting reading about the debate as someone new to the subject. Reading threads here and on other forums, plus various articles you get the impression it's black or white - 'deep stops bad' or 'deep stops good'. But there does appear to be a lot of grey.

This article may help you.

I thought this section of your article summed it up well:

Despite this, deep stops are still commonly used. While some of this is a lack of awareness, a closer look shows that much of it is semantic, for it has evolved beyond a simple “deep stops good/deep stops bad” debate. Most of those opposed to deep stops of the sort typically prescribed by bubble models do not advocate using, for example, an unmodified Bühlmann model. Many are themselves using Bühlmann, but with the low GF value of 40 or 50 instead of 20. Using the previous 60-metre/200-foot example, the first stop would be in 27-30 metre/ 90-100 feet range – still deeper than the pure Bühlmann 18-metre/60-foot first stop.

So is it therefore fair to say that the majority of the diving community agree that bubble models are 'too deep' and pure Buhlmann is 'too shallow'. And if we use those two models as the bookends of the scale, divers themselves are therefore choosing where to place themselves on the scale in selecting their GF Low? And this is largely because there isn't a great deal of research to tell us the optimal point on that scale?

FWIW I'm a rec diver with ~50 dives so not at the stage of putting any of this into practice myself. But I do find the subject fascinating.

 

[tursiops]

Totally agree with your conclusion, but fear the "research" will be minimal. Who would fund it?
The point is, dwelling on bubble models versus pure Buhlmann is a false dichotomy. No reason to go to either extreme, although the BM end is still advocated by a few. Buhlmann plus GFs work well, and there is increasing coalescence on reasonable values. The GF need to be tuned to the dive and the diver. To repeat, for NDL dives this is not really relevant.

 

[boulderjohn]

So is it therefore fair to say that the majority of the diving community agree that bubble models are 'too deep' and pure Buhlmann is 'too shallow'. And if we use those two models as the bookends of the scale, divers themselves are therefore choosing where to place themselves on the scale in selecting their GF Low? And this is largely because there isn't a great deal of research to tell us the optimal point on that scale?

That's pretty accurate.

My current thinking is that the "optimal point" on the scale is more like a fuzzy range than a point, with the true optimal point depending upon the individual diver.