Where Is Your GF?

[DanielRJones]

50/80

 

[leadduck]

60/90 on a dive below 600' got me a serious spinal DCS hit that I am still recovering from. I will be rethinking this GF for future deep or long dives.

I just read your blog post at products -- very interesting, thanks for sharing.

You maybe think now that GFhigh=90 was too high and caused the DCS, right? But I don't think so. GFhigh kicks in very late in the shallows, so when you got paralyzed around 110', your GF was still quite low, probably around 70, and an ascent with GF60/80 wouldn't have been much different.

I think the key to this story is where you write "The swimming became harder and my breathing rate increased to the point that my ADV would not keep up. This lack of loop volume and my quick descent very soon became uncomfortable. I intentionally have my ADV set so that it is a little difficult to fire and takes extra effort to pull it open. In this case I was having more difficulty firing the ADV so I took an extremely forceful deep breath to make it flow. This caused a mild pain right in my sternum area but added a bit of gas. A second breath was still needed and this hurt again."

Seems like a lot of hard work at depth both swimming and breathing. Exertion loads your tissues with inert gas faster and has a similar effect as extended bottom time, but your computer doesn't notice. Let's assume you virtually overstayed by 3min at 600' ("virtually" in the sense that your work loaded your tissues as if you had stayed 3min longer), then at your first stop at 220' your effective GFlow was not 60 but above 100, and that hurt your fast and medium spinal tissues.

I don't dive so deep myself, so just guessing, but I think changing GF is not the solution, but rather avoid exertion at such depths.

 

[Dsix36]

I just read your blog post at products -- very interesting, thanks for sharing.

You maybe think now that GFhigh=90 was too high and caused the DCS, right? But I don't think so. GFhigh kicks in very late in the shallows, so when you got paralyzed around 110', your GF was still quite low, probably around 70, and an ascent with GF60/80 wouldn't have been much different.

I think the key to this story is where you write "The swimming became harder and my breathing rate increased to the point that my ADV would not keep up. This lack of loop volume and my quick descent very soon became uncomfortable. I intentionally have my ADV set so that it is a little difficult to fire and takes extra effort to pull it open. In this case I was having more difficulty firing the ADV so I took an extremely forceful deep breath to make it flow. This caused a mild pain right in my sternum area but added a bit of gas. A second breath was still needed and this hurt again."

Seems like a lot of hard work at depth both swimming and breathing. Exertion loads your tissues with inert gas faster and has a similar effect as extended bottom time, but your computer doesn't notice. Let's assume you virtually overstayed by 3min at 600' ("virtually" in the sense that your work loaded your tissues as if you had stayed 3min longer), then at your first stop at 220' your effective GFlow was not 60 but above 100, and that hurt your fast and medium spinal tissues.

I don't dive so deep myself, so just guessing, but I think changing GF is not the solution, but rather avoid exertion at such depths.

For me to avoid the work at depth it would have meant smashing into the bottom, but I do agree that this was a big part of the problem. I still think that my first stop was too shallow for this deep of a dive regardless of workload or a couple of extra minutes. I firmly believe that a lower low GF would have worked much better and safer for this dive.

 

[stuartv]

I don't dive so deep myself, so just guessing, but I think changing GF is not the solution, but rather avoid exertion at such depths.

He was diving GF60/90. If he had dived, just for example, GF30/70, his first stop would have been a fair bit deeper (more like 280, instead of 220), and the ascent would have been slower (arrive at 100' at 49:00 instead of 36:00). Total runtime would have been over an hour longer. Even at GF30/90, the first stop would still have been around 280, instead of 220, arriving at 100' at 45:00. Total runtime 30-something minutes longer than with GF60/90.

All, roughly.

We'll never know, but that could have made all the difference.

 

[leadduck]

For me to avoid the work at depth it would have meant smashing into the bottom, but I do agree that this was a big part of the problem. I still think that my first stop was too shallow for this deep of a dive regardless of workload or a couple of extra minutes. I firmly believe that a lower low GF would have worked much better and safer for this dive.

He was diving GF60/90. If he had dived, just for example, GF30/70, his first stop would have been a fair bit deeper (more like 280, instead of 220), and the ascent would have been slower (arrive at 100' at 49:00 instead of 36:00). Total runtime would have been over an hour longer. Even at GF30/90, the first stop would still have been around 280, instead of 220, arriving at 100' at 45:00. Total runtime 30-something minutes longer than with GF60/90.

All, roughly.

We'll never know, but that could have made all the difference.

I think too a lower GFlow could have prevented the spinal DCS because it would have made the first stop deeper. But you also need to lower the GFhigh, because exertion loaded all tissues and you'd run into a type I DCS problem else. So, GF30/90 is just shifting the problem to other tissues, you'd need something like GF30/60. However, this is a weird strategy. You only know that the effect of exertion is huge but not how big it is, your dive computer doesn't consider it at all, and now we're guessing GF. This is only a workaround for a single dive, or would you suggest GF30/60 for every dive because exertion could happen?

Think this way: the diver descends and hits bottom at 500', but he wanted to reach 600'. He swam down the slope very fast in order to meet his 600' goal and stay within planned bottom time, while exertion increased inert gas tissue load a lot in a way that the dive computer ignored. He wanted to stay inside the plan only by the letter of the law but kind of cheated on his computer, with obviously bad results.
Alternative: swim down to 600' in normal speed and rather exceed planned bottom time by 3-4min than exert yourself. The inert gas tissue load will increase too, gas consumption may be even lower, but most important: the dive computer tracks tissue loading and adjusts the ascent schedule automatically. No need to adjust GF.

 

[Dsix36]

I think GF need to be adjusted according the dive plan. Things need to change from a shallow dive, to a tech dive, to a deep deep dive, to a deep and long dive. Deco is not linear and needs to be dealt with in an exponential manner

 

[ajduplessis]

Sorry to hear about your misfortune. Speedy recovery. It would seem shooting for the shallows is not the way to go then?

 

[leadduck]

I think GF need to be adjusted according the dive plan. Things need to change from a shallow dive, to a tech dive, to a deep deep dive, to a deep and long dive. Deco is not linear and needs to be dealt with in an exponential manner

Can you elaborate how you would set GF for these different types of dives? The Bühlmann model itself is already nonlinear and calculates exponential inert gas uptake and elimination. If you pick always the same (say GF30/80) for every dive, ascent profiles will depend on depth and bottom time in a non-linear manner (as you certainly know); this effect is already considered inside the model. Is it so sufficiently? Bühlmann's experiments included wet dives to 1000' and dry chamber dives to 1600'. But other important effects are not part of the model: ambient temperature, exertion, obesity/hypertension/fitness, age, hydration, ... I'd first think about how to adjust planning for these.

 

[Dsix36]

Therein lies part of the issue with GF. Until you look at the saturation levels and then the corresponding supersaturation levels you will not see the problems that happen when going blow 600'. My GF was not conservative enough but I do not wish to elaborate on exactly what changes I would make simply because some dummy would ready this and end up dead by following it.

I was clearly bent like a pretzel during the ascent and early in deco, thus I have to conclude that my first stop was indeed too shallow.

BTW, I consider 30/80 a conservative GF and probably safe for almost every diver at any depth, but I am merely a dive idiot not a deco god.

 

[KenGordon]

Therein lies part of the issue with GF. Until you look at the saturation levels and then the corresponding supersaturation levels you will not see the problems that happen when going blow 600'.

This is a good point.

It seems to me that people think that GF numbers are meaningful in themselves, and that they can assume GF 60 at 30m is equally risky as GF 60 at 10m.

Computer (not just dive computers) users often read too much into the bit of a system which is exposed to them. Users of GF computers obsess over these two numbers because that is all they see.

Behind those two numbers is a interpolation between some poorly defined levels over some limits which are changed in the opposite direction. So while x/(x+ a bit)looks like it allows less oversaturation at depth it may or may not depending on the depths and the compartments.