Where Is Your GF?

What are your typical (approximate) settings for GF lo and GF hi?

  • 5/95

    Votes: 0 0.0%
  • 30/85

    Votes: 31 21.4%
  • 50/85

    Votes: 48 33.1%
  • 70/85

    Votes: 6 4.1%
  • 90/85

    Votes: 0 0.0%
  • 30/70

    Votes: 31 21.4%
  • 50/70

    Votes: 22 15.2%
  • 70/70

    Votes: 6 4.1%
  • 90/70

    Votes: 1 0.7%

  • Total voters
    145

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To ME, I might choose more aggressive GF settings for NDL dives because I know that the slower tissue compartments don't on-gas as much during a dive that short. So, there is less chance of them bubbling at the end. On a deco dive, those slower compartments are closer to supersaturation at the end and they let it go more slowly. So, I am inclined to be more conservative in my ascent.
 
I answered 50/70 even though that's not really what I use.

An NDL dive is basically 100/100 in buhlmann terms, although surfacing GFs might be a tiny bit lower after a safety stop. I pretty much don't do those anymore. If I run right up against the NDL, my minimum safety stop is about 5mins and if its a repetitive dive it gets longer. I don't really do NDL dives actually, when I dive OC nowadays they tend to be pretty shallow leisure dives on 32% in <80ft and far from NDL limits.

On single deco dives in the 140-210ft range I do ok on 40/80GFs. I could do GF low of 50 but we rarely ascend that fast, especially in a cave you often just can't. If its a repetitive dive, I feel awful doing those GFs and usually back off to 40/70 (which is basically 8 to 10mins extra shallow time). Buhlmann says I "should" be fine on that repetitive dive with 40/80 but I'm not. 85high is too short for me, I feel it.

Long shallow dives in the 75-100ft range on 32% diluent (like at Ginnie or JB) I usually use 40/70 and my fatigue level varies. Not sure if its deco related or just the fact that those are 4 or 5 hour dives. I would back off more but 40/65 ends up with longer and longer 20ft time and it gets pretty boring after 70-90 mins. But getting rid of N2 is slow and my CNS is already maxed out so I don't really want to be spiking to 1.6 for long. And ppO2 is not really a big factor at that point anyway.
 
To ME, I might choose more aggressive GF settings for NDL dives because I know that the slower tissue compartments don't on-gas as much during a dive that short. So, there is less chance of them bubbling at the end. On a deco dive, those slower compartments are closer to supersaturation at the end and they let it go more slowly. So, I am inclined to be more conservative in my ascent.

This is where I get confused. Doesn't the algorithm base NDLs (and stops in the case of a "deco" dive) on the leading compartment? To put it another way, doesn't the algorithm already take into account for you what compartment is doing what? Why would one adjust GFs based on their own estimation of what one compartment is doing relative to another if the algorithm is doing that for you? Or: If the purpose of the GF is to keep ANY compartment away from the M-value line by some safety margin, in the context of you selecting a GF what does it matter which compartment that is?
 
This is where I get confused. Doesn't the algorithm base NDLs (and stops in the case of a "deco" dive) on the leading compartment? To put it another way, doesn't the algorithm already take into account for you what compartment is doing what? Why would one adjust GFs based on their own estimation of what one compartment is doing relative to another if the algorithm is doing that for you? Or: If the purpose of the GF is to keep ANY compartment away from the M-value line by some safety margin, in the context of you selecting a GF what does it matter which compartment that is?
IDK about Stuart...

In my case its because slow tissues are not offgassing on the SI like the algorithm would predict. If they were, I could do repetitive 40 or 50/80 dives and I can't. I need more shallow time on the second dive to address slower compartments. Been subclinically (and sometimes clinically) bent after repetitive dives too many times to risk it.

I also need more shallow time on low/no helium dives. N2 is a bigger pain to get rid of than He.
 
This is where I get confused. Doesn't the algorithm base NDLs (and stops in the case of a "deco" dive) on the leading compartment? To put it another way, doesn't the algorithm already take into account for you what compartment is doing what? Why would one adjust GFs based on their own estimation of what one compartment is doing relative to another if the algorithm is doing that for you? Or: If the purpose of the GF is to keep ANY compartment away from the M-value line by some safety margin, in the context of you selecting a GF what does it matter which compartment that is?

Yes, it is based on the leading compartment.

So, if you use GF50/80, and you follow it exactly, you get out with your leading compartment being 80% of the way to its M value. But, let's pick a slow compartment that is not the leading compartment. On an NDL dive, that compartment might be at 20% of the way to ITS M value when you surface. On a long deco dive, that same compartment might still be NOT the leading compartment, but you could get out with it being at 50% of its M value, instead of 20%.

IOW, on an NDL dive, getting out with a surfacing GF of 80% means you have a certain amount of total gas dissolved in you. If you do a long deco dive and get out with GF of 80%, you potentially have a lot more total gas dissolved in you becase the leading compartment is still at 80% but the non-leading compartments are all closer to 80% then after an NDL dive.

See wut I meen? :)
 
So if I'm understanding correctly, you guys choose different GF settings for different kinds of dives based mostly on how you have felt after the dives and factors like water temperature and exertion.

Effectively, yes. However, I don't actually "adjust" the GF settings on my computer for those dives with short deco, I just adjust my ascent schedule manually (i.e. my computer says it's cleared but I stay a few more minutes).

I've come up with my ascent strategies based on paying close attention to how I feel after my dives and adjusting. If I felt "unwell" after a dive, I would adjust what I did on a future dive.

To ME, I might choose more aggressive GF settings for NDL dives because I know that the slower tissue compartments don't on-gas as much during a dive that short. So, there is less chance of them bubbling at the end. On a deco dive, those slower compartments are closer to supersaturation at the end and they let it go more slowly. So, I am inclined to be more conservative in my ascent.

Sort of, but you are missing a few things. Anecdotally, based on my own personal experiences, once I've flipped that switch and gone into deco, I need to be a little conservative. However, I have found that once the deco becomes "extreme" (the US Navy has a definition for extreme exposure diving, I'm using their definition here) I can be more aggressive on the ascent schedule.

Please note: I am not listing the GF numbers I actually run, but I am going to throw some numbers down below to make you think about ascent schedules.

So, if I'm doing NDL diving, I may choose to run a GF equivalent of 100/100.

If I'm doing short deco dives (<15 minutes total stop time), I may choose to run a conservative ascent schedule (think something like 40/60).

If I'm doing "extreme" deco dives, I may choose to run a more aggressive ascent schedule (think something like 50/90).

I need to be clear, those are not the GF's I use, I am just trying to describe the ascent patterns I have found success with on the ~700+ deco dives I've done since having my PFO closed in 2014.

I also need more shallow time on low/no helium dives. N2 is a bigger pain to get rid of than He.

My experience jives with that. It's my belief the helium "penalty" is wrong and there probably should be a nitrogen penalty, especially on longer dives.
 
My experience jives with that. It's my belief the helium "penalty" is wrong and there probably should be a nitrogen penalty, especially on longer dives.

Yup quite a few years ago I concluded that the longer "helium penalty" deco was about what I needed - whether I actually had helium in the mix or not.

I think it was about 4 years ago now, but I once did a whole week of 32% cave diving with @lv2dive and felt like poo every night. On the last day I padded the O2 stop like crazy because I had to fly the next day. Felt soooooo much better that night and realized I had spent the whole week prior sub-clinically bent :/ ooof
 
My experience jives with that. It's my belief the helium "penalty" is wrong and there probably should be a nitrogen penalty, especially on longer dives.

Thank you for the whole post.

On this last point, isn't that pretty much the same thing that that article Shearwater put on their website a few months ago (about "the helium penalty") was saying? I.e. that there is no helium penalty, but that having one built into our algorithm(s) is what has been making it safe, and that what we really need is to adjust our plan for nitrogen to match what the helium "penalty" has been dictating?

Eliminating The Helium Penalty - Shearwater Research
 
. . .

IOW, on an NDL dive, getting out with a surfacing GF of 80% means you have a certain amount of total gas dissolved in you. If you do a long deco dive and get out with GF of 80%, you potentially have a lot more total gas dissolved in you becase the leading compartment is still at 80% but the non-leading compartments are all closer to 80% then after an NDL dive.

See wut I meen? :)

Now that's new to me. Granted, the sum total of my deco knowledge comes from SB, Deco for Divers, and a smattering of articles pointed out on SB. But I'm trying to learn more. I don't recall having seen "total dissolved gas" mentioned as a factor, only the concept of the leading compartment. So, for example, if you surface with a faster compartment that is 80% of the way to its M-value, then that is the (only?) one to be concerned about causing DCS, not another compartment that is 50% of the way to its M-value. Or so that has been my understanding. However, if this concept of total dissolved gas in the body is believed to be a factor in DCS, then I learned something.
 
... Or is it that total dissolved gas hasn't been definitively linked to DCS but is believed by a whole lot of experienced divers to be related to the "I feel tired" sub-clinical kind of complaints?
 

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