Recreational Ascent Rate in the last 15 feet

[chillyinCanada]

I've been doing slowest ascent from SS that I'm able to manage for a number of years now. I also do them horizontal whilst turning in circles looking around and up for danger. Sometimes my fins break the surface first.

I'll often do a longer SS, conditions providing for all and I remain in the water with anyone that has somehow managed to accrue a longer stop for themselves or just prefers to do a longer stop.

 

[NAM001]

I dont think there is a real answer because there may be too many variables.

Perhaps what would be useful would be to use the GF99 reading to make that decision. The real issue would be to not exceed 99 as that would be the 2:1 pressure ratio. If micro bubbles are the concern then use far less than the 99 as your GF setting limit.
As limits go The last I have read (which is not that much) says there is no negative associated issues with micro bubbles and an extended safety stop is the remedy for ththose bubbles,,,, as the majority of micros (90% or so) are gone in a 3-5 min safety stop period. The remainder is gone n the first hour of the SI.
Next if I am not in error and I dont really understand the relationship on this,,,,,, If I have a GF high setting of 60 that would( as i would like to understand it) mean a max allowed pressure ratio of 1.6:1. If such a setting was used. would it make a difference on the final ascent rate as compared to if you used a GF high of 90. the setting has a double whammy. NDL and max GF ratio allowed.
With out knowing more about the subject It seems that this is one of the variables that would determine whether one should actually need to do a safety stop or not. When looking at the recommended (I HATE THAT WORD RECOMMENDED) protocol of optional safety stop if less than a 60 ft dive greater than then you do the 3 minute safety stop and if close to ndl then the safety stop should be extended to 5 min. Again with the NDL driven by GF high the final ascent rate from the safety stop to surface may again be a moot issue if 50 or 60 setting was used as opposed to 90. This leaves things as before where the one rule fits all when it comes to safety stops. If you did an ascent with a say 60 GF high any micros that happened to prematurely form would be gone in the first couple of minutes of a safety stop. after that one could probably go to the surface at 100 ft per minute for the last 15 ft with out exceeding the 2:1 ratio. If a new diver with no real depth control skills, can be trusted to incorporate the global rule of always doing a 3 min safety stop while complying with their RECOMMENDED max depth of 60 ft ,, It wold say to me that the final rate is not an issue at least for dives less than 60 ft. for the advanced rec diver the GF99 should be the best dynamic guide to use in that decision making process.

 

[Jay]

I dont think there is a real answer because there may be too many variables.

Perhaps what would be useful would be to use the GF99 reading to make that decision. The real issue would be to not exceed 99 as that would be the 2:1 pressure ratio. If micro bubbles are the concern then use far less than the 99 as your GF setting limit.
As limits go The last I have read (which is not that much) says there is no negative associated issues with micro bubbles and an extended safety stop is the remedy for ththose bubbles,,,, as the majority of micros (90% or so) are gone in a 3-5 min safety stop period. The remainder is gone n the first hour of the SI.
Next if I am not in error and I dont really understand the relationship on this,,,,,, If I have a GF high setting of 60 that would( as i would like to understand it) mean a max allowed pressure ratio of 1.6:1. If such a setting was used. would it make a difference on the final ascent rate as compared to if you used a GF high of 90. the setting has a double whammy. NDL and max GF ratio allowed.
With out knowing more about the subject It seems that this is one of the variables that would determine whether one should actually need to do a safety stop or not. When looking at the recommended (I HATE THAT WORD RECOMMENDED) protocol of optional safety stop if less than a 60 ft dive greater than then you do the 3 minute safety stop and if close to ndl then the safety stop should be extended to 5 min. Again with the NDL driven by GF high the final ascent rate from the safety stop to surface may again be a moot issue if 50 or 60 setting was used as opposed to 90. This leaves things as before where the one rule fits all when it comes to safety stops. If you did an ascent with a say 60 GF high any micros that happened to prematurely form would be gone in the first couple of minutes of a safety stop. after that one could probably go to the surface at 100 ft per minute for the last 15 ft with out exceeding the 2:1 ratio. If a new diver with no real depth control skills, can be trusted to incorporate the global rule of always doing a 3 min safety stop while complying with their RECOMMENDED max depth of 60 ft ,, It wold say to me that the final rate is not an issue at least for dives less than 60 ft. for the advanced rec diver the GF99 should be the best dynamic guide to use in that decision making process.

A GFHi of 60 means if your NDL reaches zero when you're at depth and if you surface then, you'll be at 60% of the (unmodified) m_value (for your leading tissue). Ditto for 90%. 30/60=50% difference. Neither affect the ascent rate, but the recommended ascent rate is used in the computation.

Cutting short your bottom time / using a lower GFHi merely so you can ascend faster and surface with the same theo leading tissue pressure at the surface doesn't seem too wise and seems like you've jettisoned to depth for a quick how's ya father and not much else.

Attempting to watch or map your ascent vs GF99 (which is really fast moving when shallower) ... hmmmm .... unless one really doesn't care and wants GFHi to be a goal. (It's be an own-goal big time one day ...).

For a given Rec dive (whether it be 40m or 20m etc ...), using a given GFHi, taking it super slow from as soon as GF99 starts to rise (call it from ~10m), and super duper slow from a SS to the surface seems much smarter.

p.s. if GF99>99 then you're in deco.

 

[stepfen]

I saw GF99 mentioned several times in this thread. What exactly is GF99 ??

Edit: Fount it What does the “GF99” display value mean? - Shearwater Research
Never mind!

 

[EFX]

A GFHi of 60 means if your NDL reaches zero when you're at depth and if you surface then, you'll be at 60% of the (unmodified) m_value (for your leading tissue). Ditto for 90%. 30/60=50% difference. Neither affect the ascent rate, but the recommended ascent rate is used in the computation.

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

 

[NAM001]

A GFHi of 60 means if your NDL reaches zero when you're at depth and if you surface then, you'll be at 60% of the (unmodified) m_value (for your leading tissue). Ditto for 90%. 30/60=50% difference. Neither affect the ascent rate, but the recommended ascent rate is used in the computation.

Cutting short your bottom time / using a lower GFHi merely so you can ascend faster and surface with the same theo leading tissue pressure at the surface doesn't seem too wise and seems like you've jettisoned to depth for a quick how's ya father and not much else.

Attempting to watch or map your ascent vs GF99 (which is really fast moving when shallower) ... hmmmm .... unless one really doesn't care and wants GFHi to be a goal. (It's be an own-goal big time one day ...).

For a given Rec dive (whether it be 40m or 20m etc ...), using a given GFHi, taking it super slow from as soon as GF99 starts to rise (call it from ~10m), and super duper slow from a SS to the surface seems much smarter.

p.s. if GF99>99 then you're in deco.

everything you said is true. I still have not got my answer to what the GF is Ill ask it differently. Is a gf of 60 equate to a modified M value of 60% of 2 being 1.2 : 1 ratio or is it 60% of the range of range between 1 : 1 and 2: 1 or 1.6: 1 ratio. I am thinnking it is the latter that 60 equates to 1.6 :1 ratio.

Aside from that if you cut your dive short and get to 20 ft and find you have a GF99 value of say 40 ,,,, Is a safety stop a logical productive step. The hard rule of doing the stop IMO is there from the days when the current GF level was unknown such as diving with tables and a watch.

In addition I was not suggesting per se changing ascent rates because you cut your bottom time short but whether doing so would remove the consequence of a faster ascent rate because of the adidtional reserve (so to speek) GF available to you. The question is no different than saying yo have a GF hi of 85 and you get a 100 ft NDL of 20 min . doing the full 20 requires ascent to the surface, doing only half of the NDL at 100 leaves time to do at other depths or physically accomodate a faster ascent rate or ellimination of the safety stop all to gether. Much like the advantages of nitrox you get either shorter SI's of exended bottom time with out consequence.

faster ascent rates increase the GF99 longer stays increases the GF 99. If we were talking ratio deco this would be a discussion of the ascent curve where you adjust your stop time to give at this depth and take at another.

Lastly the use of using the GF 99 was for the purpose of deciding to skip do a 3 or a 5 min stop. the GF99 pretty tells your time needs. If you get to the stop depth and have a gf99 of 40 there is no need to do the stop because proceeding to the surface will not drive you past the M value of 2:1

There are so many rules that were established in the day when we did not have the data to make a decision and hence a one rule fits all was made like the 3 SS. It is either not required or recommended OORR it is wise to do it. The GF99 value allows that decision making, one done from being data informed as opposed to the logic of THINK OF THE WORST CASE which does not always exist.

 

[rsingler]

If you get to the stop depth and have a gf99 of 40 there is no need to do the stop because proceeding to the surface will not drive you past the M value of 2:1

This is where I think there is room for debate. As I noted above, my personal experience of the "bloom" in supersaturation showed me graphically that what is happening is not linear, but logarithmic. Just as a 20' depth change at 100 ft is an 18% change (from 4 to 3.3 atm), but a 20' depth change from 20' to the surface is a 60% change (from 1.6 to 1 atm).
Therefore, while we may be staying below Buhlman's 2:1 ratio with our SurfGF, the rate at which offgassing is occurring at the end nonetheless has me concerned. While mathematically, the end result looks "safe", I'm older with probably impaired perfusion to some old scars in my body, and may not offgas like the model.
So intuitively, slowing down during the "bloom" makes sense to me, whether or not I'll end up at a worrisome supersaturation ratio or not.
My question then, is slowing for final ascent just window dressing? Or does impaired offgassing during a period of rapidly rising supersaturation (even if the end M-value is "safe") make slowing down advisable?

 

[rsingler]

To graphically elaborate on my post above about slow final ascents, let's go back to the damned Coke can that we always talk about.
Stipulate that the amount of overpressure in a Coke can is "safe". Stipulate that the bubbles you see in a glass of Coke are the same bubbles that you see on ultrasound for every diver, and DON'T end up in DCS. Stipulate that if you shake the can of Coke, and it spews all over the place, that's like the bends that have been attributed to sudden heavy exercise right after climbing out of the water.
The analogy seems to fit so far, right?

So take a pair of Coke cans that got just a little jostled as you carried them in from the store. They both have a "safe amount of overpressure" (in other words, they're below GF100).

Now, pop one can open quick as a wink. Every once in awhile, that can will fizz over. That can "got bent."
But take the identical "jostled can", and open it reeeeaaallly carefully, so the CO2 slowly hisses out. That can WON'T get bent.

That's basically why I think that even with a dive with a "safe" final GF, or safe bottom time, or safe supersaturation, ascending reeeaaallly slowly in the last 15 feet is safer.

 

[Johnoly]

... open it reeeeaaallly carefully, so the CO2 slowly hisses out. ....

And the bubble sizes will be much smaller & easier to filter ----- which is a whole 'nuther variable to add into the algorithm mix.
I still agree with you, crawl slowly to the surface from the SS.

 

[EFX]

My question then, is slowing for final ascent just window dressing? Or does impaired offgassing during a period of rapidly rising supersaturation (even if the end M-value is "safe") make slowing down advisable?

Using my spreadsheet I created these profiles (air, salt water, GF: 100/100, des rate = 60 fpm, asc rate = 30 fpm, all profiles NDL = 3 min):

1. 80 ft 26 min, 30 fpm all the way to surface, upon surfacing GF = 88
2. 80 ft 26 min, 30 fpm to 15 ft, 15 fpm from 15 ft to surface, upon surfacing GF = 85
3. 60 ft 58 min, 30 fpm all the way to surface, upon surfacing GF = 96
4. 60 ft 58 min, 30 fpm to 15 ft, 15 fpm from 15 ft to surface, upon surfacing GF = 95

There is a safety advantage in limiting the ascent rate for the last 15 ft but not much. If you have impaired offgassing due to past injuries or other factors (age, hydration, temperature, etc.) it may make a greater difference in your overall safety.

Inserting a safety stop for 3 min at 15 ft and then ascending at 15 fpm to the surface gave these results:

profile 2: surfacing GF = 82
profile 4: surfacing GF = 94

Again, very little difference but it could matter for DCS prone divers.