Recreational Ascent Rate in the last 15 feet

[chillyinCanada]

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

View attachment 544656


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):

View attachment 544644




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.

Thank you @Jay

 

[Jay]

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.

Nope, you can't add oranges and apples and compare that with pumpkins even if the volumes happen to add

Do you dive a SW or a Mares Genius? If so, please try out what your suggesting with GF99 (Mares GFNow). There's a reason why SW created SurfGF and Mares 'borrowed' it. Or perhaps see if you can borrow / rent one with both GF99 and SurfGF displayed ... you'll have fun!

 

[EFX]

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.

Sorry, I screwed up. The results above were for fresh water not salt water. Here are the results for salt water for the same profiles (all profile NDL = 1 minute):

1. GF = 90
2. GF = 87, with ss, GF = 85
3. 60 ft at 56 min, GF = 97
4. 60 ft at 56 min, GF = 96,with ss, GF = 94

 

[EFX]

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?

With a 3 fpm ascent rate from the 15 ft ss I got an ending GF of 76 for profile 2 and 86 for profile 4.

 

[rsingler]

Well, that's encouraging!
Shifting to an ultra slow final ascent moves you from GF85 to 76 on the 100' dive, and
GF94 to 86 on the longer 60' dive.

I think we have some solid possible recommendations here (neither of which are that surprising in retrospect):
1) longer safety stops make a small but significant difference in surfacing GF;
2) ultra slow final ascents make a small but significant difference in surfacing GF.
3) doing both...well?
At average gas consumption rates, this requires only an extra 5 cu ft of gas to add both to your profile. Or, as I dive it, at a depth of five feet with the boat in sight, surface with 300 psi instead of 500. No need to cut your bottom time short for your new technique...

Like I said, not surprising in retrospect. But thanks to @Shearwater and guys like @EFX and @Jay for pushing the envelope of our theoretical knowledge! GFSurf is a critical new piece of data, for those that take the time to explore it.

Perhaps this will begin to quiet the nay-sayers who suggest that cramming 25 dives into a week-long trip is inherently dangerous. My answer is, "No, if you understand the physiology, you can get the very most out of your $2,000 vacation in paradise."

 

[FreeFlyFreak]

IMO the slow or staged ascent from the safety stop is not primarily to reduce the GFsurf as that effect is relatively small.

The slow or staged ascent is more akin to slowly screwing the top off a soda bottle vs taking the top off quickly and risking an overflow.

In the soda bottle you are slowly releasing the pressure above the soda. The air pressure above the soda is keeping the gas in solution in the soda.

In the dive example you are slowly reducing the water pressure on body by slowing down the ascent from the SS. The water pressure that is keeping the gas in your tissues.

 

[dmaziuk]

The slow or staged ascent is more akin to slowly screwing the top off a soda bottle vs taking the top off quickly and risking an overflow.

This is the bubble model argument, as I alluded upthread. Now we know the bubble model's all wrong in the deeper portions of the dives, therefore it must be right near the surface, is that the logic?

Point being, in the simplistic dissolved gas model more supersaturation equals faster off-gassing, and faster off-gassing remains "statistically safe" all the way up to surfacing M-aximum-allowed-overpressure value. Here we are assuming that reducing the overpressure gradient will make us "safer". (At the expense of slower off-gassing and therefore longer ascent times and surface intervals afterwards, obviously.)

Is the assumption warranted? As I understand it, studies to date say it doesn't look like it.

 

[FreeFlyFreak]

This is the bubble model argument, as I alluded upthread. Now we know the bubble model's all wrong in the deeper portions of the dives, therefore it must be right near the surface, is that the logic?

Point being, in the simplistic dissolved gas model more supersaturation equals faster off-gassing, and faster off-gassing remains "statistically safe" all the way up to surfacing M-aximum-allowed-overpressure value. Here we are assuming that reducing the overpressure gradient will make us "safer". (At the expense of slower off-gassing and therefore longer ascent times and surface intervals afterwards, obviously.)

Is the assumption warranted? As I understand it, studies to date say it doesn't look like it.

I dont know what is warranted.

I do however know the difference in results of releasing the pressure above the soda quickly vs slowly.
I also know what my GF numbers look like from 15-20ft to the surface.
I also know how I feel after various dive profiles although I do have a very small sample size.

 

[NAM001]

I agree totally. this could more good stugff that could reside in an AOW course assuming that an OW gets a bunch of dives prior to taking it. I think the whole crux of it again resides in. Upon completion of OW if you use these few global rules of thumb you will be safe. rules of thumb made to be super conservative and can be done with a absolute min of skills or understanding. Understanding is a quality of growth

I dont know really but things like this could become a type skills bridge from rec to tech.

 

[NAM001]

IMO the slow or staged ascent from the safety stop is not primarily to reduce the GFsurf as that effect is relatively small.

The slow or staged ascent is more akin to slowly screwing the top off a soda bottle vs taking the top off quickly and risking an overflow.

In the soda bottle you are slowly releasing the pressure above the soda. The air pressure above the soda is keeping the gas in solution in the soda.

In the dive example you are slowly reducing the water pressure on body by slowing down the ascent from the SS. The water pressure that is keeping the gas in your tissues.

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. You can do faster If youahve the ratio room to do it. SS permits a faster ascent to the surface with out exceeding the 2:1 ratio.. Is this something thta new OW's should be trainied in. I would say no but in a AOW class there should be some sort of deco covered and what things abourt the rec dive allows you to either do or not to do based on your prifile. GF99 is in my thinking a indicator to what is available to do.

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.