Recreational Ascent Rate in the last 15 feet

[FreeFlyFreak]

IMO right and wrong. the slow ascent is to compensate for the exponential rapid increase of change in differential pressure that is only felt in the shallows. for the sake of argument 200' tp 100 ft is the same ratio change as 33' to the surface is . 30 fpm at 100 should be somewhat equal to 3 fpm at 20 ft.QUOTE]

Exactly

 

[EFX]

From "Deco for Divers" (published 2008), Mark Powell writes that a study was done by DAN studying the effect of safety stops and ascent rates using the BSI (bubble score index). The study found that a ss at 6 m produced a lower BSI than no stop, and an ascent rate of 10 m/min was better than either an 18 or 3 m/min rate. The 3 m/min rate produced the highest BSI. I don't understand the reason and Powell didn't explain it either, but this indicates there's more to it than lowering the surface GF.

 

[rsingler]

From "Deco for Divers" (published 2008), Mark Powell writes that a study was done by DAN studying the effect of safety stops and ascent rates using the BSI (bubble score index). The study found that a ss at 6 m produced a lower BSI than no stop, and an ascent rate of 10 m/min was better than either an 18 or 3 m/min rate. The 3 m/min rate produced the highest BSI. I don't understand the reason and Powell didn't explain it either, but this indicates there's more to it than lowering the surface GF.

If I recall correctly, the ascent rates started much deeper than we are contemplating here, so I always presumed the higher bubble index in the 3 mpm group was due to added ongassing at depth.
The slow ascent here (15' up) is starting above no-stop depth, so it's all offgassing. I wouldn't expect a higher bubble index with this technique.
That's how I see it, anyway.

 

[chillyinCanada]

If I recall correctly, the ascent rates started much deeper than we are contemplating here, so I always presumed the higher bubble index in the 3 mpm group was due to added ongassing at depth.
The slow ascent here (15' up) is starting above no-stop depth, so it's all offgassing. I wouldn't expect a higher bubble index with this technique.
That's how I see it, anyway.

That's how I see it too. Hoping not to be proven wrong, lol

 

[Jay]

If my thinking is right then it would be nice to have all dive computers have a SS screen with the data needed to make the decision. replace NDL TTS and other things with GF99 and SURF gff info . perhaps another mode in the computer rec rec adv and tech.

Interesting. You know what I think re GF99 already, and agreed that TTS is useless for Rec divers / divers who don't do deco (compared to Tec where it's vital), but replacing NDL with SurfGF when shallow (and esp when NDL is meaningless - i.e. it displays 99 or infinity etc ...) is a smart idea. I can't think of any reason why not. At depth, you'd still want both displayed as you can't easily conceptualise how many minutes a GF at that depth is.

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@EFX I've merged your text and think I have it right:

1. 80 ft 26 min, 30 fpm all the way to surface, upon surfacing GF = 90
2. 80 ft 26 min, 30 fpm to 15 ft, 15 fpm from 15 ft to surface, upon surfacing GF = 87, with ss, GF = 85
3. 60 ft at 56 min, 30 fpm all the way to surface, upon surfacing , GF = 97
4. 60 ft at 56 min, 30 fpm to 15 ft, 15 fpm from 15 ft to surface, , GF = 96, with ss, GF = 94

A question, in post 67 above I quoted from another post. Do you numbers match theirs? Their differences seem larger and more depth dependent.

Would you also mind running it for 40m and seeing how it goes?

A second question I've got , is how much 'bang for your buck' (for the above) so you get vs simply heading to the surface and spending an extra 3 min (for 3 min SS) etc there (i.e. the SI duration is a fixed time as they tend to be in the real world). We know the first results (the numerator), but I don't know the surface aspect. I can guesstimate the surface aspect as I have an end-of-dive-delay on my computer, and it looks like 1-2 min GF/min (on the surface, for 20-40 max depth) but I could be wrong ...

just a curious mind

 

[EFX]

If I recall correctly, the ascent rates started much deeper than we are contemplating here, so I always presumed the higher bubble index in the 3 mpm group was due to added ongassing at depth.
The slow ascent here (15' up) is starting above no-stop depth, so it's all offgassing. I wouldn't expect a higher bubble index with this technique. That's how I see it, anyway.

I went back to "Deco for Divers" and you are correct, they were testing ascents that started deeper than the ss.

 

[EFX]

@EFX I've merged your text and think I have it right:

1. 80 ft 26 min, 30 fpm all the way to surface, upon surfacing GF = 90
2. 80 ft 26 min, 30 fpm to 15 ft, 15 fpm from 15 ft to surface, upon surfacing GF = 87, with ss, GF = 85
3. 60 ft at 56 min, 30 fpm all the way to surface, upon surfacing , GF = 97
4. 60 ft at 56 min, 30 fpm to 15 ft, 15 fpm from 15 ft to surface, , GF = 96, with ss, GF = 94

A question, in post 67 above I quoted from another post. Do you numbers match theirs? Their differences seem larger and more depth dependent.

Can you be more specific? Which numbers are you referring to?

Would you also mind running it for 40m and seeing how it goes?

Sure. 4 profiles to 40 m for 8 min, salt water, GF: 100/100, des: 18.3 mpm, asc: 9.1 mpm, air, no other gases, NDL = 1 min, SI = 10080 min between profiles.

1. ascent to surface GF = -70, still ongassing, TC3 is leading
2. ascent to 4.6 m at 9.1 mpm, then switch to 4.6 mpm to surface, GF = 67, offgassing, TC6 leading
3. ascent to 4.6 m, ss for 3 min, ascend to surface at 9.1 mpm, GF = 68, offgassing, TC6 leading
4. ascent to 4.6 m, ss for 3 min, ascend to surface at 4.6 mpm, GF = 67, offgassing, TC6 leading

A second question I've got , is how much 'bang for your buck' (for the above) so you get vs simply heading to the surface and spending an extra 3 min (for 3 min SS) etc there (i.e. the SI duration is a fixed time as they tend to be in the real world). We know the first results (the numerator), but I don't know the surface aspect. I can guesstimate the surface aspect as I have an end-of-dive-delay on my computer, and it looks like 1-2 min GF/min (on the surface, for 20-40 max depth) but I could be wrong ...just a curious mind

The above GF's are for the end of the ascent to surface segment. Also, I've selected median depths for the descents and ascents. The Schreiner equation needs an inert gas inspired pressure computed at a specific depth. In the ss you can choose the starting depth, median depth, or final depth. Using final depth I get offgassing at a GF of 60 for the first profile. I'm not sure what you're getting at by the surface aspect. Can you elaborate?

 

[Jay]

GFHi doesn't appear to work that way. That is, if you have a GFHi of 60 that you'll reach the surface with the leading TC at or under 60% of the surfacing m-value (Mo in the literature). The reason I say this is Baker's sample deco program (see his paper "Decolessons") for a GF of 30/75 has the diver surfacing at an m-value of 92% of Mo. In my spreadsheet under the column %AoM which presents the same data as Shearwater's SurfGF, but not in real time, the ascent to surface segment most of the time shows a value greater than GFHi. Common sense tells you that GFHi should work the way you describe it above. In the algorithm in Baker's program and in my spreadsheet GFHi does not modify Mo directly; It modifies the a and b coefficients in the equation that determines the decompression ceiling or NDL time (the same equation is used).

Just for reference here are the equations for calculating a and b:

a = Mo - s(Ps)
b = 1/s

Where s = the slope, Ps = the absolute pressure at the surface at sea level

I'm still intrigued by this. When I play with your SS and enter a near NDL limit dive it seems the %AoM (SurfGF) is near GFHi.
Can you replicate the GFHi of 75 versus a 92% SurfGF in your SS?
FWIW I played a bit with SubSurface and those results are also near GFHi (for a dive just out of deco). <--EDIT/ADD and it should be obvious what I mean: when the model/computer says a 'deco' value of zero / does not display and 'deco' value.

Can you be more specific? Which numbers are you referring to?

Apologies for the delay in replying re this. I was referring to posts 60 & 73 above ... I was merely collating your two pieces of info.

The above GF's are for the end of the ascent to surface segment. Also, I've selected median depths for the descents and ascents. The Schreiner equation needs an inert gas inspired pressure computed at a specific depth. In the ss you can choose the starting depth, median depth, or final depth. Using final depth I get offgassing at a GF of 60 for the first profile. I'm not sure what you're getting at by the surface aspect. Can you elaborate?

I was more or less having a ground-hog moment ... you've already answered s similar question in another post. (not too relevant, but it was post 150 in this thread: Where Is Your GF?).

 

[happy-diver]

every dive is a decompression dive

 

[DeepSeaExplorer]

every dive is a decompression dive

WRONG!