Deep Stops Increases DCS

[rossh]

So you are agreeing with RainPilot, that increasing the GF-Lo to a larger value (larger than 30%) is a good idea?

Hi,

No, I don't recommend GF for anything. I'm just highlighting what a messed up and abused thing it has become. Back when Eric Baker created the GF method, it came with an understanding that it was to be used carefully, and not to excess. But now, its so badly distorted and over inflated beyond its origins, it needs to be thrown out and start again. The "recommended" GF settings goes up an down the like temperature on a winters day.

To the implied question - it really doesn't matter what planning tool anyone uses today. They all work, they all come enough margin to do the job. It's just boils down to preference and what you learned with, or what matches your dive computer.

.

 

[PfcAJ]

To the implied question - it really doesn't matter what planning tool anyone uses today. They all work, they all come enough margin to do the job. It's just boils down to preference and what you learned with, or what matches your dive computer.

.

Are you just ignoring the fact that when compared to the suggested gradient factors listed on this thread that vpm produces a shorter deco with a shorter time on oxygen? Additionally that vpm's dcs risk goes up as deco time goes up?

That's not "working". It's broke. Straight up broke. At least gf allows you to get rid of worthless stops and lengthen your shallow stop times to reduce the risk.

Your assertion that GFs like 40/70 produce some unreasonably long deco is downright absurd.

 

[boulderjohn]

---

---

A ScubaBoard Staff Message...

A couple of posts that were needlessly inflammatory and personal were removed. Please keep the focus on the topic and not the person.

---

 

[EFX]

It also seems that the conservancy settings on VPM seem to less about adding a padding factor (like lowering GF High would achieve) and more about undoing the tissue stress from the deeper stops. So why overdo the deeper stops to start with?

Because it's not just about tissue stress if by that you mean limiting the supersaturation (SS) pressure gradient on ascent. The deep stops either limit or crush already existing bubbles so that when you get to the shallow stops slower TC's don't feed those bubbles and enlarge them beyond the critical size. Keep in mind that it takes less SS to enlarge a large bubble than a smaller one, and a larger SS to enlarge a small bubble. This explains in part why RGBM with its deep stops get you out of the water faster than models that don't compensate for bubble size. The deep stops crush or limit bubbles to a small size hence they can tolerate greater levels of SS at the more limited shallow stops.

VPM is more conservative than RGBM, adding time to shallow stops due to compensation for the added gas uptake during the deep stops. Depending on the conservation settings there isn't that much difference between B-GF and VPM-B. So, for our 170 ft, 30 minute air dive we get (ft/stop time, DT = decompression time):

B-GF's:
30/70: 80/2, 70/3, 60/3, 50/7, 40/9, 30/17, 20/31, 10/66. DT = 141 minutes.
40/85: 70/2, 60/3, 50/5, 40/6, 30/12, 20/21, 10/46. DT = 99.
45/85: 70/1, 60/3, 50/5, 40/6, 30/11, 20/21, 10/45. DT = 96.

VPM-B:
0: 90/2, 80/3, 70/3, 60/4, 50/6, 40/8, 30/12, 20/18, 10/32. DT = 91.
+1: 100/1, 90/2, 80/2, 70/3, 60/5, 50/5, 40/9, 30/12, 20/20, 10/34. DT = 96.
+2: 100/1, 90/2, 80/3, 70/3, 60/5, 50/6, 40/9, 30/13, 20/21, 10/36. DT = 102.
+3: 100/1, 90/2, 80/3, 70/3, 60/6, 50/6, 40/10, 30/14, 20/23, 10/41. DT = 112.
+5: 100/2, 90/3, 80/3, 70/4, 60/6, 50/9, 40/11, 30/18, 20/30, 10/60. DT = 149.

The two algorithms for limited conservative settings, 45/85 and VPM-B + 1 have the same DT. Going more conservative the DT is very close between 40/85 and VPM-B +2, and 30/70 and VPM-B +5. With VPM-B there is only a slight decrease in the last two shallow stops over the B-GF profiles.

 

[UWSojourner]

Thanks for the link to the article. It supports what I already knew from previous reading that VPM uses Buhlmann with it's M-values to limit SS on ascent.

No. VPM calculates it's own M-values. I called them "vpM-Values" in the article. They are the maximum supersaturation values allowed by the method, but they aren't Buhlmann's.

If VPM is using the Buhlmann algorithm or something essentially the same won't it do the same SS limiting as a Buhl-GF?

No. Profiles allowed by VPM and a Buhlmann-GF approach will diverge quite sharply as you go deeper and stay longer. The post here points out that VPM's M-values stay relatively constant after wear-off of the critical volume algorithm (i.e. for dives requiring more deco). The post was in response to Diver0001 posting something Mark Ellyatt wrote about 'not enough shallow time'. There won't be enough because VPM's M-values don't adjust as you get longer SS times (i.e. slower compartments become involved). That's why if you go deep and stay long you MUST adjust conservatism, pad shallow stops, or some combo of both. Otherwise you're simply increasing risk. I think most divers are padding it like crazy on bigger dives, or have abandoned it by now.

I want to be clear. I'd happily dive RGBM or VPM or pretty much anything at or near the no-decompression-limits. I wouldn't dive it deep or for decompressions that amounted to much. The NEDU study I think convincingly showed bubble model approaches (at least those now available) to be inefficient. If I have to make constant adjustments to even attempt to keep it in the same risk stratosphere, why not just go with a shallower stop approach that more naturally does that? GF with a low of 40 or 50, or even 60 does that. Keep in mind the NEDU shallow profile was roughly 90/65 or something around there and it had 1/3 the risk of the deep stop profile that looks very similar to a VPM approach (see for yourself. Explore some of the links from this post).

 

[RainPilot]

Because it's not just about tissue stress if by that you mean limiting the supersaturation (SS) pressure gradient on ascent. The deep stops either limit or crush already existing bubbles so that when you get to the shallow stops slower TC's don't feed those bubbles and enlarge them beyond the critical size. Keep in mind that it takes less SS to enlarge a large bubble than a smaller one, and a larger SS to enlarge a small bubble. This explains in part why RGBM with its deep stops get you out of the water faster than models that don't compensate for bubble size. The deep stops crush or limit bubbles to a small size hence they can tolerate greater levels of SS at the more limited shallow stops.

VPM is more conservative than RGBM, adding time to shallow stops due to compensation for the added gas uptake during the deep stops. Depending on the conservation settings there isn't that much difference between B-GF and VPM-B. So, for our 170 ft, 30 minute air dive we get (ft/stop time, DT = decompression time):

B-GF's:
30/70: 80/2, 70/3, 60/3, 50/7, 40/9, 30/17, 20/31, 10/66. DT = 141 minutes.
40/85: 70/2, 60/3, 50/5, 40/6, 30/12, 20/21, 10/46. DT = 99.
45/85: 70/1, 60/3, 50/5, 40/6, 30/11, 20/21, 10/45. DT = 96.

VPM-B:
0: 90/2, 80/3, 70/3, 60/4, 50/6, 40/8, 30/12, 20/18, 10/32. DT = 91.
+1: 100/1, 90/2, 80/2, 70/3, 60/5, 50/5, 40/9, 30/12, 20/20, 10/34. DT = 96.
+2: 100/1, 90/2, 80/3, 70/3, 60/5, 50/6, 40/9, 30/13, 20/21, 10/36. DT = 102.
+3: 100/1, 90/2, 80/3, 70/3, 60/6, 50/6, 40/10, 30/14, 20/23, 10/41. DT = 112.
+5: 100/2, 90/3, 80/3, 70/4, 60/6, 50/9, 40/11, 30/18, 20/30, 10/60. DT = 149.

The two algorithms for limited conservative settings, 45/85 and VPM-B + 1 have the same DT. Going more conservative the DT is very close between 40/85 and VPM-B +2, and 30/70 and VPM-B +5. With VPM-B there is only a slight decrease in the last two shallow stops over the B-GF profiles.

Hey EFX

What I meant is that if the bubbles are so important why is it that the VPM conservancy settings all end up padding shallow time almost exclusively? In your example, the difference between +1 and +5 settings is a DT of 53 min. Of that time 44 min or 83 % is allocated at 40 feet or shallower. If being conservative in a VPM context means spending a higher proportion of your allocated DT shallower, then I think that seems to follow what everyones been saying anyway, or have I gone mad?

 

[UWSojourner]

My comments below are for dives big enough to minimize the impact of the CVA (just to keep on topic with Diver0001's post).

EFX, your statements about VPM reveal a more popular, but flawed, understanding of VPM, it's theory and goals.

The deep stops either limit or crush already existing bubbles so that when you get to the shallow stops slower TC's don't feed those bubbles and enlarge them beyond the critical size.

No. The theoretical "crushing" of the bubbles occurs on maximum descent.

VPM's deep stops have the same goal and use the same theory as the shallow stops. The goal is to prevent a portion (not all) of the theorized distribution of bubbles from initiating growth. That's it. VPM's mechanism provided to the diver to accomplish that is the "critical radius (CR)". When you pick the CR (chosen via then VPM-B+X conservatism setting) you are telling VPM, "Don't allow growth in bubbles this size or smaller". But what about bubbles larger than the CR? They are not formally considered. They are allowed to grow.

Your statement's connection between the fast and slow TCs is misguided. VPM attempts to limit growth in both identically -- via the CR which is a constant chosen for all 16 compartments. Since it applies to all compartments, it also ignores in all compartments bubbles with a size greater than the CR . Consideration of those "growing bubbles" is not a part of the algorithm. That's not bad necessarily because as I've said I think the logic is something like, "Well, if those larger bubbles do start causing problems, then we can just increase the CR and limit growth in those too."

But you now should see the issue I tried to bring out here. If you simply allow part of the bubble distribution to grow, and if you don't adjust the supersaturation levels when slower and slower compartments come into play (i.e. when your exposure to supersaturation gets longer and longer), what happens? Yeah. That part of the bubble distribution just grows more and more. So, as pointed out in that post and a few after, you MUST either continually increase conservatism, pad shallow stops, or some combo of both as you increase depth and time. Otherwise you are simply increasing risk. There's not another option.

*Remember, all the comments above assume VPM's theory and approach is correct. That is, even if VPM were exactly true the issues discussed above exist. That's granting quite a bit in the discussion since I think current science clearly says deep stop approaches are not doing what they theoretically aim to -- they aren't being particularly effective at limiting bubble growth.*

 

[Dr Simon Mitchell]

Because it's not just about tissue stress if by that you mean limiting the supersaturation (SS) pressure gradient on ascent. The deep stops either limit or crush already existing bubbles so that when you get to the shallow stops slower TC's don't feed those bubbles and enlarge them beyond the critical size. Keep in mind that it takes less SS to enlarge a large bubble than a smaller one, and a larger SS to enlarge a small bubble. This explains in part why RGBM with its deep stops get you out of the water faster than models that don't compensate for bubble size. The deep stops crush or limit bubbles to a small size hence they can tolerate greater levels of SS at the more limited shallow stops.

Hello,

This is a restatement of aspects of the attractive theory underpinning the bubble models. As uwsojouner has pointed out, there is no "crush" advantage in deep stops; after all, you have already been much deeper at your bottom depth no matter what decompression approach you subsequently use to ascend. There remains, however, the hypothetical "limiting" advantage of deeper stops that you mention, because you reduce supersaturation in the faster tissues early in the ascent. The goal (as you obviously know) is that there will be a smaller population of (assumed) bubble micronuclei that will be excited into growth by this smaller degree of supersaturation.

However, you have (probably unintentionally) portrayed this theory as fact, and there is little or no evidence that supports such a position. It is really important to understand that. I think that many divers, having been taught bubble model theory by their instructors, simply assume there must be supporting evidence. There isn't and never was. It was just an attractive theory. Indeed, the reason this debate is taking place is that the NEDU study (the first human study with a very hard outcome) showed that protection of the fast tissues early in the ascent did not result in better outcomes. Ross claims that there were no "proper deep stops" in the NEDU study. Maybe not according to his definition (whatever that is), but the NEDU deep stops did protect the fast tissues from supersaturation compared to the NEDU "shallow stop" profile, and this did not result in a better outcome (the opposite in fact). This is clearly illustrated and discussed at the following link:

Deep stops debate (split from ascent rate thread) - Page 108

Other studies have shown that bubble models actually produce more bubbles than decompression approaches that do not emphasise protection of fast tissues using deep stops to the same extent, which is further corroboration of the suspicion that the theory does not play out in practice.

Simon M

 

[Dr Simon Mitchell]

Ross,

I want to keep this one in front of you because I think you need to be accountable for some of your statements:

Please see this post:Deep Stops Increases DCS

Can you provide evidence that my ideas (presumably you mean the ones I am articulating on this thread) were rejected by my "peers" in 2008?

 

[EFX]

Hey EFX.......What I meant is that if the bubbles are so important why is it that the VPM conservancy settings all end up padding shallow time almost exclusively? In your example, the difference between +1 and +5 settings is a DT of 53 min. Of that time 44 min or 83 % is allocated at 40 feet or shallower. If being conservative in a VPM context means spending a higher proportion of your allocated DT shallower, then I think that seems to follow what everyones been saying anyway, or have I gone mad?

No you're not going mad but if you keep reading this thread you will. I completely agree that VPM will pad (biased) the shallow stops with more time. It does this because it is adding time at deep stops. But so what? Let's change the terminology from VPM style deep stops to very brief deep stays of a multilevel dive. Would you not expect a B-GF algorithm to pad, oops sorry, schedule longer stops at the shallower end of the dive when your staying briefly at deeper depths? Hell yes. Well , according to UWSojouner because VPM uses the same algorithm as Buhlmann this is exactly what VPM does exercising its dissolved gas phase portion of its algorithm.

My point was, VPM and bubble models in general, are not just looking at dissolved gas rates like its bubble blind cousin B-GF, but is looking out for that bubble that's going to whack you when you're not looking!