Have I understood the basics of decompression theory, GF99 and SurfGF?

[halocline]

Hi all,

Firstly, apologies in advance as I am sure these topics are discussed regularly and heavily.

I have actually spent a fair amount of time reading through this forum amongst others in order to write what I am going to below.

I appreciate there are many theories, and at the end of the day, it's all theory, but just wanted to write what I have accumulated / compiled from various other sources and check with those more knowledgable, I'm not a) dangerously wrong; or b) missing anything super important? I plan to incorporate some monitoring of GF99 and SurfGF when I get a new dive computer.

As context (hopefully you can see from my profile anyway), I'm only at PADI AOW, looking to progress onto rescue diver as I want to have the tools available to be more self reliant should I need to (even though I always dive with a DM/guide and buddies, I think it's generally good practice in anything to be able to do it with my own skillset). I am looking into getting a new dive computer, and toying with garmin mk3i descent (as may add AI later but not straight away), or the fenix 8 (as I currently only dive max 40m, oxygen/nitrox, and no plans for tec yet anyway, not sure if AI is a dealbreaker yet), and this led me into the deep topic of GF's, conversatism and what it all means. Thanks for suggestions on dive computers, I've read endless reviews/articles and the shearwater/garmin pros/cons. This thread isn't for discussing any of the dive computers please.

I plan to dive on the garmin presets low/med/high, and don't plan to change to my own GF's (just yet - until I understand moreso), I believe the descent mk3i can add GF99 and SurfGF to the data screens and understand these too be helpful tools in limiting risk of DCS.

Anyway.... that's enough context, so, is the below correct, or have I messed something up?

....

DCS doesn't occur because of nitrogen in blood, it occurs when nitrogen makes large bubbles in the blood, the small bubbles just causes fatigue post dive. A greater pressure difference (ascending too rapidly) causes a greater pressure difference and thus larger bubbles are at risk of forming. However the greater pressure difference, the faster nitrogen off-gasses.

GFLo - depth of first stop (higher number better as means shallower - no longer deemed best practice to deep stop)
GFHi - overall conservatism (100% is equal to the M value which is the maximum safe, lower number means less bottom time but safer)

Normally preset on computers are:
Low - 45/95
Medium - 40/85
High - 35/75

GF99 - tells you how much nitrogen in the body and the size of bubbles in the body

0% - normal nitrogen in body
1-49% less and smaller bubbles
50-100% more and large bubbles
>100% emergency

GF99 therefore lets you see: If you are on or off-gassng and how efficiently

Higher GF99 means faster nitrogen release, it will rise as you ascend as the pressure difference increases and off-gassing speed increases. Put simply, a lower GF99 means slower gas release, so smaller bubbles, less small bubbles but longer time. A higher GF99 (up to 100%) means quicker gas release, so bigger bubbles, more small bubbles (so more fatigue) but less time.

Aim to keep GF99 when off-gassing between 33-50% of your GFHi, i.e. at medium GFHi of 85, GF99 should be 28-42%. Lower than this means it takes too long to off-gas and not worthwhile, higher than this risks large bubble formation

SurfGF - tells you what the pressure difference is if you surfaced immediately

Same percentages as GF99. Effectively your dive computer will tell you to ascend when your SurfGF matches your GFHi setting. It will be at the highest when you come from the bottom of the dive and as you ascend and off-gas nitrogen it will reduce. Aim to keep SurfGF at minimum to your GFHi, lower increases the safety margin

Thanks in advance
Joe

After reading this thread, I get the impression that there are two issues you’re interested in, one is a greater understanding of basic decompression theory and terminology, and the other is to increase your safety on OW, NDL dives. I think you should separate these issues in your mind; enjoy the intellectual exercise of learning more, but in terms of your own diving as a new diver, keep things very simple.

For the first, there are two resources I think you might enjoy reading. One is a chapter in the old PADI encyclopedia of recreational diving; I’m not even sure if that book is available anymore. I got it when I did my DM training many years ago. The chapter on deco theory is pretty basic and does a good job at simplifying the concepts of tissue compartments and I believe there’s even some summarized history in there. The other is a very good book called ‘Deco for Divers’ by Mark Powell. Anyhow, once you’ve read those, you’ll have a good grasp of the basics and you can enjoy (if you’re a real deco nerd) more current writing and conversations about more esoteric concepts.

For the second, dive safety, at a new diver level it really is pretty simple. Watch your profile, make sure you have enough gas, generally dive conservatively, and extend your safety stop. In fact, you can go a little further and do a brief stop, say 1-2 minutes, at 20 feet, then descend slowly to 10 feet and do a longer stop. On 2nd dives on air in places like Cozumel I have often extended this stop to 7-10 minutes. Almost all recreational computers have some sort of N2 loading graph, often there’s a red section which means you are under mandatory deco, a yellow caution section, and a green section. Years before I understood anything about gradient factors, I found I felt better after a day of diving if I waited until my computer went into the green before surfacing. And when you do surface from your stop, take a full minute or more to go from 10 feet to the surface.

 

[Joemdown]

After reading this thread, I get the impression that there are two issues you’re interested in, one is a greater understanding of basic decompression theory and terminology, and the other is to increase your safety on OW, NDL dives. I think you should separate these issues in your mind; enjoy the intellectual exercise of learning more, but in terms of your own diving as a new diver, keep things very simple.

For the first, there are two resources I think you might enjoy reading. One is a chapter in the old PADI encyclopedia of recreational diving; I’m not even sure if that book is available anymore. I got it when I did my DM training many years ago. The chapter on deco theory is pretty basic and does a good job at simplifying the concepts of tissue compartments and I believe there’s even some summarized history in there. The other is a very good book called ‘Deco for Divers’ by Mark Powell. Anyhow, once you’ve read those, you’ll have a good grasp of the basics and you can enjoy (if you’re a real deco nerd) more current writing and conversations about more esoteric concepts.

I’ll have a search for those two, thanks

For the second, dive safety, at a new diver level it really is pretty simple. Watch your profile, make sure you have enough gas, generally dive conservatively, and extend your safety stop. In fact, you can go a little further and do a brief stop, say 1-2 minutes, at 20 feet, then descend slowly to 10 feet and do a longer stop. On 2nd dives on air in places like Cozumel I have often extended this stop to 7-10 minutes. Almost all recreational computers have some sort of N2 loading graph, often there’s a red section which means you are under mandatory deco, a yellow caution section, and a green section. Years before I understood anything about gradient factors, I found I felt better after a day of diving if I waited until my computer went into the green before surfacing. And when you do surface from your stop, take a full minute or more to go from 10 feet to the surface.

Makes sense, this alongside the other post on keeping GF99 in the 20-40 range if possible otherwise it’s a waste of time and surfGF at X before surfacing were ultimately what I was looking for answer wise.

I understand everyone has their own risk appetite but good to get a steer on some people’s views.

 

[JMarc]

Don't forget that there is more than one compartment. With GF99 about 20, although you are off-gazing the more saturated one, you are probably still on-gazing some which will be relevant later on the dive. There is a trade-off between being gentle with the more saturated compartments and not saturating those which aren't yet. And don't forget that the compartments slow to on-gaz are also slow to off-gaz.

 

[halocline]

I’ll have a search for those two, thanks

Makes sense, this alongside the other post on keeping GF99 in the 20-40 range if possible otherwise it’s a waste of time and surfGF at X before surfacing were ultimately what I was looking for answer wise.

I understand everyone has their own risk appetite but good to get a steer on some people’s views.

If you are a new diver, diving within recreational NDL limits, and your goal is to dive safely, I don’t think that the GF concepts (both GF99 and surfGF) are as relevant as you are focusing on. Those things matter much more in decompression diving.

 

[LFMarm]

Normally preset on computers are:
Low - 45/95
Medium - 40/85
High - 35/75

This is not the latest thinking. Decompression experts recommend GFLow = 0.83xGFHigh

GF99 - tells you how much nitrogen in the body and the size of bubbles in the body

0% - normal nitrogen in body
1-49% less and smaller bubbles
50-100% more and large bubbles
>100% emergency

Not sure what’s the source for this. When you set your GFHigh/GFLow, the stops are calculated such that GF99 never exceeds that line. Eg if you pick 50/50, you will never exceed 50. I monitor GF99 when I need to deviate from the calculated stops to make sure I stay within what I consider safe for that dive profile / PrT.

 

[dmaziuk]

Don't forget that there is more than one compartment. With GF99 about 20, although you are off-gazing the more saturated one, you are probably still on-gazing some which will be relevant later on the dive. There is a trade-off between being gentle with the more saturated compartments and not saturating those which aren't yet. And don't forget that the compartments slow to on-gaz are also slow to off-gaz.

This isn't relevant to no-stop diving: all this "stretching" of GF99 is typically done in the no-limit zone where you can not on-gas above any surfacing M-values anyway.

And I doubt anyone cares much about watching GF99 "on the fly" on planned deco dives where that on-gassing is a concern: they have a plan, they don't do "on the fly". Edit: unless stuff goes seriously sideways, anyway.

 

[lowwall]

This is not the latest thinking. Decompression experts recommend GFLow = 0.83xGFHigh

Not sure what’s the source for this. When you set your GFHigh/GFLow, the stops are calculated such that GF99 never exceeds that line. Eg if you pick 50/50, you will never exceed 50. I monitor GF99 when I need to deviate from the calculated stops to make sure I stay within what I consider safe for that dive profile / PrT.

.83x is a strangely specific number for something on which there is only a very rough consensus.

But OP was asking about NDL dives so GFLow never comes into play. It doesn't determine stops because there aren't any. For the same reason, even though it wouldn't be a great idea, you could run GF99 all the way up to your GFhi (maybe higher?) and stay within no deco limits.

 

[Joemdown]

This is not the latest thinking. Decompression experts recommend GFLow = 0.83xGFHigh

Thanks, I’ve read this in terms of deep stop no longer though as best practice, but I was just quoting what the dive comps have as pre-made conservatism factors.

Not sure what’s the source for this. When you set your GFHigh/GFLow, the stops are calculated such that GF99 never exceeds that line. Eg if you pick 50/50, you will never exceed 50. I monitor GF99 when I need to deviate from the calculated stops to make sure I stay within what I consider safe for that dive profile / PrT.

Understood, thanks

 

[dmaziuk]

.83x is a strangely specific number for something on which there is only a very rough consensus.

The explanation is about half-way down the page, under "what to do about gradient factors" heading: Gradient Factors in a Post-Deep Stops World

 

[therookie]

Anyway, after dive 2 the computer he has given me to wear entered gauge mode, he said it'd be fine and we dived again, I'd never seen this so wasn't sure what it meant.

I'm a new AOW diver as well, but reading this and the dive stats you posted along with it to me indicates "missed required deco stop on the ascent". The fact that two computers from different manufacturers (Aqualung and Cressi) almost assuredly running different algorithms (PZ+ in the case of the AL, most likely RGBM in the case of the Cressi) went into gauge mode on that second dive makes it seem that something was very odd about your dive profiles. Maybe it was something else, but are you absolutely certain you didn't miss a deco stop on the ascent of that second dive?

If you did miss a required deco stop on the ascent of the second dive, your computer (and that of the other diver with the Cressi) should have been frantically trying to get your attention visually and audibly (unless the audible alarms were turned off, which isn't always possible with some computers). Clearly the instructor you were diving with wasn't concerned about it - from my own simulations of your dives if I set the conservatism really, really low I can with some effort get a dive plan that sort of matches your description of the second dive either without going into NDL or else clearing it on the way up without a required deco stop, so perhaps his own computer was set with similarly low conservatism even though yours weren't. And perhaps from his experience it has indeed been "fine" and no cause to scrap the third dive. Personally that second dive would have been it for me for that day and the next two, no matter what my plans had been before (or what the instructor said) - but I'm a lot older than you and have a very low risk tolerance...

As mentioned above, knowledge of deco theory is a good thing (and intrigues me too, at least partly because I have a technical background). Just don't get so focused on the nitty gritty details of SurfGF/GF99/etc that you miss the fact that your computer is flashing, vibrating and beeping all at once to tell you that according to its calculations you need to stop your ascent at what it determines to be the "best" place for you to off-gas a bit before ascending further... or even if it's just saying "Hey man you've got 5 minutes of NDL left, might be time to start thinking about shallowing up..."