Recreational Ascent Rate in the last 15 feet

[rsingler]

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

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

Thank you for running this!!
I was just about to try it on your spreadsheet, but it would have taken me six times as long.

Hmmmm...
Doesn't sound like as much benefit as I expected.
Sigh.
Now that you have it set up, what about 3 fpm from the SS up? That adds another several minutes.
Let me guess: profile 2 - 78; profile 4 - 91
Again, not so great. True or false?

 

[Imla]

GUE MinDeco Ascent, but modified with 5 up. IE, 1m/min from 5m up.

 

[NAM001]

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?

I think it is a good question. As I has asked about Using the GF99 value as a guide to the conduct of the safety stop. it is a fair question. I visualize it this way. I look at the GF99 and say I can not go to eh surface unless it is below XXX value. I think that is done with the SS. The meat of the question is how much of he safety stop is really necessary if at all prior to hitting the surface. If we treat the gass as a whole and not as a PP component. we know (i think) that going form 20 ft to the surface. That is a .6 atm, change. so that if we call the surface as 1 and 33 ft as 2 then so long as at 20 ft the GF99 is below 40 or 1.4 to 1 ratio then you can add an extra .6 atm with out exceeding the 99 which would put the ratio at 2 to 1. Im sure that there may be some things about this that are not perfect but the concept seems rational.

If you go from 100 ft to 33 ft you get a 2 to 1 ratio. so a safety stop would be necessary. if your tissues were at 65 and you go to 20 ft then the ratio would be 2 to 1.6 or less than 2 to 1. so you sit at 20 ft watching the gf99 counter decrease to below say a 40 and head up. if you can monitor a clock time to measure your time at the SS,,,, why not use the GF to determine when enough is enough. Sounds like a skill for an AOW level diver that has a puter that can display the GF99 data.

 

[NAM001]

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.

I agree with your assessment. look again at your example and ignore the rate of your final ascent to the surface and instead look at it form the view of when to START that ascent to the surface, no matter what speed you want to do it at. The longer of the SS the lower the GF99 the nore forgivness there is for a fast ascent whether controlled or not. If I find myself a bit on the light side I will extend the SS in case i get too light and pop up from surface suction and effectivly go form 20 to surface in very few seconds.

We always hear dont do trust me dives. Using the GF99 can be a tool to determine when to start the ascent form 20 ft it is better than saying or implying that so long as you do 3 minutes you will be ok. 3 minutes is a catch all rule, and IMO was made because we , at the time the rule was made, had no way to measure a true safe final ascent time. Some one doing a 100 ft dive and someone doing a 50 ft dive will have different SS needs if any at all. If the both do 3 min than they are probably both OK or maybe the deep guy really needs 4 minutes minimum and the shallow guy need 1/2 minute minimum. The more technology we have the less reliance we should place on rules of thumb except as a backup method. Im going to start logging my SS GF99 values at 20 and the surface. Discussing this reminds me of hte posts regarding use of the 10 ft stop in regards to using deco50 or deco100.

 

[NAM001]

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.

If you were to do the same dives only to 40 feet... what would he 20 ft and surface GF's be? If the above all resulted in safe surfaces and you do the same dives at 40 vise 80 ft and the GF's are say 20 or so less at 20 ft then should not that imply MAYBE that the SS is not NEEDED to do the surface and stay below a 99 level? If 20 to surface only results in a 30 point jump than waiting until the gf99 gets below 70 should be a good indicator that you have met the MINIMUM time NEEDED prior to heading up the final 20 ft.

 

[rsingler]

@KWS , after looking at @EFX 's spreadsheet results, your approach seems to have merit. Since EFX showed that the ascent rate change didn't help final saturation much, your thought of timing your Safety Stop duration to a certain target GF99 makes sense. That means much less worry about ascent rate in the last 15 feet.

Okay, I'll concede that that is a necessary addition to the profile: not surfacing with a GF99 greater than "x". Which is related to what @Jay was trying to elicit with his thread What's your SurfGF and how does it compare to your (Rec) GFHi?

Lots of good ideas here!
Meat for my next dive trip in April.
I can see my wife now..."Honey, get your damned head out of that device, and just look at all the pretty stuff we paid $2,000 to see, willya?"

 

[Jay]

For divers that don't use SW (or Mares as the Genius has SurfGF) or are not familiar with the definitions:

And for some GF questions here's a nice explanation: Gradient Factors | Dive Rite

Here's what the GF99/GF 'pop' looks like for a couple of dives. (SW Cloud can't yet display SurfGF):

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

That's a large difference. It's a very interesting topic which definitely warrants its own thread. I don't think I've seen it addressed specifically before.

 

[Jay]

Some of the questions above seem conflated ...

Is a SS necessary on every dive? If the dive is a shallow & a long way away from NDL dive then perhaps not. GF99 is not the best metric to look at because GF99 is profile dependent, and you don't know what it's going to be until you're on the surface (the place where it's relevant). What you know at a shallower depth when GF99 starts moving is that a/ it will be higher than now, and b/ it will be less than GFHi providing you've not gone into deco. So I maintain GF99 is quite useless to look at in this regard. SurfGF tells you what you want to know with no guess work.

Is a super-slow ascent near to & from the SS better? Yes, for sure (unless you're gas short ...)

What benefit does the super slow ascent and the SS respectively give? EFX has answered that above, I would also like to see the MultiDeco and SubSurface answers for that too. In my head I sense some disconnect between seeing high SurfGFs and the rate of their decrease underwater vs a longer SI.

To add ... from this thread: Simple surfacing GF question there was this quote in post 15:

The question posed at the start of the thread does not have a unique answer. The effect of 3min of deco on surfacing GF will depend greatly on the dive profile.

The Bühlmann ZHL-16 model (all its subtypes included) is based upon theoretical compartments of varied half-times. The controlling compartment on a given dive will vary with depth and time. Thus, the half-time of the leading compartment may be 4min or over 600min, with 3 additional minutes of deco changing from considerably impactful to absolutely irrelevant.

Two simple examples. First example, a dive to 40m for 9min. Ascending directly to the surface, you will arrive there at 95% of M value on leading compartment (GF high of 95). 3min at 3m will reduce the load on the leading compartment all the way to 65%. Second example, a dive to 15m for 83min. Again, surfacing directly will put you at about 95% in the leading compartment. If you spend 3min at 3m, it should reduce the surfacing GF to about 90.

In both examples, air is used both as back gas and deco gas.

 

[rsingler]

So I maintain GF99 is quite useless to look at in this regard. SurfGF tells you what you want to know with no guess work.

Yep. You're right. My response to @KWS should be modified to "hold your safety stop to a target SurfGF." GF99 really takes a jump from 15 ft to the surface, as your plots clearly showed.

 

[NAM001]

I found this via google on SB

Teric new SurfGF and old GF99

here is the comment with it.... The Teric's SurfGF is showing 62% of the unmodified M-Value. This is the surfacing GF if I surfaced immediately. The Petrel's GF99 is showing 4%. This is the current supersaturation GF as a % of M-Value at the current depth.

the teric as i understand the post it what was sampled at time of surface 62% at teh surface and the petrel is showing 4% just prior to surfacing at 20 ft. 20 ft is about .6 atmosphrees add the .,6 to the 4 % at 20 ft it pretty well matches the pterics surface value.

Sopunds like it may be feasable to monitor your GF99 untill you heave less than 40% prior to doing the final surface from 20 ft or less than 70 if at 10 ft prior to surfacing.