Retroactive gradient factor/tissue saturation calculation

[rjack321]

I did only have 1 deco gas. Not worth the effort to implement any of those to demonstrate the point: If you consider super-saturation (GF) significant then deep stop profiles will either have longer deco times, or higher GFs, or both.

Droping to a 30min max deco wont change that fact. changing deco gases wont change that fact.

But here you go. 34 min dive to 130ft on 21/35 with 100% deco gas:

• VPM-B deco time 30 min, surface GF 90% max stop GF 73%.
• 
  
• B-16C GF70/75 deco time 30 min, surface GF 74%, max stop GF 67%.
• 
  
• To match surfacing GF, VPM-B+4 deco time 44 min, surface GF 73%.

Of course if you are more concerned about Micro-Bubble formation, none of this matters. But in that case, why bother with any kind of science at all if you are going to ignore recent research anyway.

This still isn't remotely similar to ratio deco or what they call a pragmatic ascent. And a T1 diver wouldnt use 100% for this dive either. Nor 70/75. The starting GFs for RD are 30/85. You're using different starting assumptions and naturally going to get different answers.

The T1 'pragmatic' RD version would be 40min BT at 130ft on 21/35 & 50% = 30mins deco spread like this:
130 - 40 (includes 3-4min descent)
70-1
60-1
50-1
40-3
30-5
20-19 & in theory "clear"
from 20 up to surface - 6
Total runtime 76min

With GF 30/85, 21/35 and 50% this 40min BT at 130ft would be:
130 - 40 (includes 3-4min descent)
70-0:30
60-1
50-1
40-3
30-5
20-3
10-22
Total runtime = 78mins

The "shift deeper" that VPM does is completely unrelated to the differences between a pragmatic RD ascent and a Buhlmann ascent. In this case there's a couple mins difference between 40 and 30ft stops. A modest difference at the 20ft stop which is mostly driven by the tendency to do "all" the deco at 20ft in RD. And lastly the somewhat uncounted "slow ascent" in RD which Buhlmann doesnt really have since there's a 22min 10ft stop.

It's also right on the edge of a T1 one deco gas dive

 

[L13]

This still isn't remotely similar to ratio deco or what they call a pragmatic ascent. And a T1 diver wouldnt use 100% for this dive either. Nor 70/75.

I looked up GUE standard gases for this dive.

The starting GFs for RD are 30/85. You're using different starting assumptions and naturally going to get different answers.

Those are deep stop GF's. I was choosing modern non-deep stop GF's that produced the same deco time.

The T1 'pragmatic' RD version would be 40min BT at 130ft on 21/35 & 50% = 30mins deco spread like this:
130 - 40 (includes 3-4min descent)
70-1
60-1
50-1
40-3
30-5
20-19 & in theory "clear"
from 20 up to surface - 6
Total runtime 76min

What is the surfacing GF for that dive?

With GF 30/85, 21/35 and 50% this 40min BT at 130ft would be:
130 - 40 (includes 3-4min descent)
70-0:30
60-1
50-1
40-3
30-5
20-3
10-22
Total runtime = 78mins

Ir-relevant, the claim was that if the time was the same, the depths didn't mater. Which is why I chose to highlight the very different GF results between deep and non-deep 30 min deco plans.

The "shift deeper" that VPM does is completely unrelated to the differences between a pragmatic RD ascent and a Buhlmann ascent. In this case there's a couple mins difference between 40 and 30ft stops. A modest difference at the 20ft stop which is mostly driven by the tendency to do "all" the deco at 20ft in RD. And lastly the somewhat uncounted "slow ascent" in RD which Buhlmann doesnt really have since there's a 22min 10ft stop.

It's also right on the edge of a T1 one deco gas dive

The topic that I was responding to was the claim that:

A sentiment that I've heard a lot locally is that total deco time is the important thing, and the ratio/pragmatic tendency to have a higher proportion of deco time being deeper doesn't make much of a real difference.

To evaluate this, you need to compare a deeper stop 30 min deco to a shallower stop 30 min deco and see if it makes a difference. I picked an easy to put into Subsurface deep stop dive that produced a 30 min deco, and compared it to a modern shallow stop algorithm producing the same 30 min deco for the dive.

 

[ChrisMBC]

Playing around with it a little in Subsurface/Decoplanner, it looks like 39m/40 mins

This still isn't remotely similar to ratio deco or what they call a pragmatic ascent. And a T1 diver wouldnt use 100% for this dive either. Nor 70/75. The starting GFs for RD are 30/85. You're using different starting assumptions and naturally going to get different answers.

The T1 'pragmatic' RD version would be 40min BT at 130ft on 21/35 & 50% = 30mins deco spread like this:
130 - 40 (includes 3-4min descent)
70-1
60-1
50-1
40-3
30-5
20-19 & in theory "clear"
from 20 up to surface - 6
Total runtime 76min

With GF 30/85, 21/35 and 50% this 40min BT at 130ft would be:
130 - 40 (includes 3-4min descent)
70-0:30
60-1
50-1
40-3
30-5
20-3
10-22
Total runtime = 78mins

The "shift deeper" that VPM does is completely unrelated to the differences between a pragmatic RD ascent and a Buhlmann ascent. In this case there's a couple mins difference between 40 and 30ft stops. A modest difference at the 20ft stop which is mostly driven by the tendency to do "all" the deco at 20ft in RD. And lastly the somewhat uncounted "slow ascent" in RD which Buhlmann doesnt really have since there's a 22min 10ft stop.

It's also right on the edge of a T1 one deco gas dive

Isn't the pragmatized ascent more like:

130 - 40
70 - 3
60 - 3
50 - 3
40 - 3
30 - 3
20 - 15

Not including the 'slow up' from 20, that gives me a SurfGF of 101 at the end of the 20 foot stop.

Apologies if I'm not understanding correctly, but this is basically what I'm trying to understand - at the edges of the T1 RD envelope (and I think 130 feet/40 mins is the edge of the envelope!), how does a RD profile compare for decompression stress to a 30/85 Buhlmann ascent (pragmatized and non-pragmatized). That's it - not trying to argue whatsoever about the merits of different gradient factors or models or prove that ratio deco is in some way broken.

 

[L13]

This still isn't remotely similar to ratio deco or what they call a pragmatic ascent.

The point I was making had nothing specific to do with either of those algorithms as such.

The fact is, the distribution of stop depths for a 30 min deco DOES make a significant difference in the surfacing GF!

If two algorithms produce close to the same depth/time stop profile, they will have close to the same surfacing GF. If the have different depth profiles for the same total time, they will have different GFs.

 

[ChrisMBC]

Ir-relevant, the claim was that if the time was the same, the depths didn't mater. Which is why I chose to highlight the very different GF results between deep and non-deep 30 min deco plans.

Just to be very clear, I am not claiming that 'depth doesn't matter' for deco stops, that would be somewhat absurd. I am saying that an opinion I frequently hear is that within the realm of T1 dives (<30 min deco, 50% or 100% deco gas, max depth 51m), the difference between a ratio deco profile (with 50% of the decompression time happening below 6m), a 'pragmatized' Buhlmann plan, and a pure Buhlmann plan, is negligible. I was curious about assessing that claim using SurfGF as an evaluative tool for my own personal information, that's all!

 

[L13]

Playing around with it a little in Subsurface/Decoplanner, it looks like 39m/40 mins

Isn't the pragmatized ascent more like:

130 - 40
70 - 3
60 - 3
50 - 3
40 - 3
30 - 3
20 - 15

Not including the 'slow up' from 20, that gives me a SurfGF of 101 at the end of the 20 foot stop.

Apologies if I'm not understanding correctly, but this is basically what I'm trying to understand - at the edges of the T1 RD envelope (and I think 130 feet/40 mins is the edge of the envelope!), how does a RD profile compare for decompression stress to a 30/85 Buhlmann ascent (pragmatized and non-pragmatized). That's it - not trying to argue whatsoever about the merits of different gradient factors or models or prove that ratio deco is in some way broken.

It really doesn't mater. If you thing SurfGF matters, then you shouldn't use deep stop algorithms of any sort. And if you think Micro-Bubble prevention with deep stops is right, you should ignore SurfGF. You are conflating two radically different ideas about what should limit deco ascent.

 

[L13]

Just to be very clear, I am not claiming that 'depth doesn't matter' for deco stops, that would be somewhat absurd. I am saying that an opinion I frequently hear is that within the realm of T1 dives (<30 min deco, 50% or 100% deco gas, max depth 51m), the difference between a ratio deco profile (with 50% of the decompression time happening below 6m), a 'pragmatized' Buhlmann plan, and a pure Buhlmann plan, is negligible.

And I just gave you an example of a "within the realm of T1 dives (<30 min deco, 50% or 100% deco gas, max depth 51m)" dive with very different results in SurfGF, proving them wrong.

 

[L13]

If you do a deco dive, and you do 30 min of deco, the algorithm you use to allocate that 30 min of deco maters.

Micro-bubble limited:

• make early stops deeper, let SurfGF go up.

Super-saturation limited (GF):

• make initial stops shalower to keep GF lower across all stops and SurfGF

 

[OrcasC205]

Playing around with it a little in Subsurface/Decoplanner, it looks like 39m/40 mins

Isn't the pragmatized ascent more like:

130 - 40
70 - 3
60 - 3
50 - 3
40 - 3
30 - 3
20 - 15

Not including the 'slow up' from 20, that gives me a SurfGF of 101 at the end of the 20 foot stop.

I built this profile in Subsurface planner and had a slightly different result of SurfGF of 100 at the end of 15 min @ 20ft (I'm not sure where you and I differed, but it is minor.) With a 6 minute slow ascent from 20 to the surface, final SurfGF was 86.

 

[Matan]

I took T1 late last year. I'm not confused about how to plan dives to SOPs (either Decoplanner + pragmatize, or ratio deco) - the part I'm particularly curious about is how much difference we'd see in surfGF for a pragmatic/ratio profile vs a pure Decoplanner profile, given the increase in time spent deeper.

A sentiment that I've heard a lot locally is that total deco time is the important thing, and the ratio/pragmatic tendency to have a higher proportion of deco time being deeper doesn't make much of a real difference. I absolutely don't deny that for T1 dives to T1 depths, it's probably not making a huge difference - I am just academically interested in understanding the effectiveness of those deco strategies, as evaluated by the algorithm (B-16C) that GUE uses. If, on the margins, a pragmatic or ratio deco strategy resulted in a surfacing GF of 105 rather than 85%, I'd be interested to know that. If it results in a surfGF of 87, even better.


I feel like an idiot....I'm not a frequent Subsurface user, and I didn't realize that you can see the GF/SurfGF for a planned dive. This is exactly what I was looking for. Thanks!

subsurface solves your quiz visually - enjoy
your T1 instructor will definitely be more helpful than me, but:

SOP is quite clear - plan your dives using Decoplanner - Than pragmatize (which as you know has a set of benefits when is comes to gas sharing scenarios / plan adjustments / etc.)

I'm not sure pragmatic deco makes a longer deco time - 30' deco after a 25' dive to 150ft is 10' in the intermediate stops + 20' @ 20ft - I don't see a higher proportion of deco time being deeper. maybe a minute or two compared to pure exponential curve - but a minute/meter/3ft changes nothing - if it does - the plan is bad.
Similar examples for shallower+longer stuff available.

total deco time matters + how we distribute it matters too - When we leave the bottom everyone should be fully aware to both. this is far more important than +/- minute here and there or +/- 10 to GF hi/lo (Most of us usually ascend slower than 9m/30ft per minute - so our GF low is already "lower" ^^ - but we try).


Matan.