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

[Joemdown]

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

 

[tursiops]

at the end of the day, it's all theory

This suggests you are misunderstanding what "theory" means here. In a lot of everyday usage, "theory" is the same as saying something is just a guess and pretty much useless. But the sciences, a theory is an encapsulation of the available knowledge into a coherent, useful package that can be used (for example) to make predictions. The decompression theory you are talking about is NOT a representation of the detailed workings of our physiology, but rater a model based on several (for example, 16) "tissue compartments, each with a time-response to on-gassing and off gassing, and each with an allowable maximum gas overpressure before bubbling occurs (the m-value). The point is, WHICH of those compartments is closest to bubbling at any moment in your dive varies with the dive profile and time, so the computer has to track ALL of them and focus on the "most-likely-to-bubble" compartment -- called the "leading" compartment -- at the moment. So GF99 and SurfGF are talking about the leading compartment, and that keeps changing.

I believe the descent mk3i can add GF99 and SurfGF to the data screens

Only SurfGF. If you want GF99 you need a Shearwater.

b) missing anything super important?

I suggest you do a little more reading, for example:

• ask Google AI "what is the meaning of GF99 and SurfGF?"
• Shearwater Dive Computer Settings: Become A GF Power User!
• Surface GF and Other Teric Musings

 

[inquis]

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

This is perhaps the crux of the matter, and a reasonable rule of thumb most of the time. The main exception is GF99 will increase in the final ascent to equal surfGF, so the "under 50% of GFHigh" guideline can't apply then.

Aim to keep SurfGF at minimum to your GFHi, lower increases the safety margin

At a maximum, actually, but I think you know that. When SurfGF goes above GFHigh, you're in mandatory deco stop territory with an elevated risk of DCS should you surface anyway. Yes, lower SurfGF is safer.

GF99 - tells you how much nitrogen in the body

This is the only thing I'd hedge a bit. I think of surfGF as a proxy for "how much nitrogen in the body" -- it's relative to the tolerable amount should I surface immediately. GF99 is similar, but it's relative to the ambient pressure (frequently changing). As such, I think of it as a proxy for "how fast am I off-gassing."

 

[Joemdown]

This suggests you are misunderstanding what "theory" means here. In a lot of everyday usage, "theory" is the same as saying something is just a guess and pretty much useless. But the sciences, a theory is an encapsulation of the available knowledge into a coherent, useful package that can be used (for example) to make predictions. The decompression theory you are talking about is NOT a representation of the detailed workings of our physiology, but rater a model based on several (for example, 16) "tissue compartments, each with a time-response to on-gassing and off gassing, and each with an allowable maximum gas overpressure before bubbling occurs (the m-value). The point is, WHICH of those compartments is closest to bubbling at any moment in your dive varies with the dive profile and time, so the computer has to track ALL of them and focus on the "most-likely-to-bubble" compartment -- called the "leading" compartment -- at the moment. So GF99 and SurfGF are talking about the leading compartment, and that keeps changing.

Noted, thanks

Only SurfGF. If you want GF99 you need a Shearwater.

GF99 has been added as of one of the later software updates FYI, roughly 6 months ago. Along with other metrics.

I suggest you do a little more reading, for example:

• ask Google AI "what is the meaning of GF99 and SurfGF?"
• Shearwater Dive Computer Settings: Become A GF Power User!
• Surface GF and Other Teric Musings

Thanks, I had already done the AI (Copilot) search and the second link is where I got most of my info.

I was simply trying to put it into my own words to check my understanding was ok.

 

[Joemdown]

This is perhaps the crux of the matter, and a reasonable rule of thumb most of the time. The main exception is GF99 will increase in the final ascent to equal surfGF, so the "under 50% of GFHigh" guideline can't apply then.

In terms of final ascent are you referring to the final slow push to the surface from a safety stop? If so I understand.

If you mean from bottom up to the safety stop, surely if GF99 is >50% then you're ascending too quickly?

At a maximum, actually, but I think you know that. When SurfGF goes above GFHigh, you're in mandatory deco stop territory with an elevated risk of DCS should you surface anyway. Yes, lower SurfGF is safer.

Yes, exactly at a max, thanks for clarifying.

This is the only thing I'd hedge a bit. I think of surfGF as a proxy for "how much nitrogen in the body" -- it's relative to the tolerable amount should I surface immediately. GF99 is similar, but it's relative to the ambient pressure (frequently changing). As such, I think of it as a proxy for "how fast am I off-gassing."

I agree, thanks, more just looking to understand these two metrics in more detail.

I understand following the dive computer should mean I don't run in to any problems, but if my surfGF is indicating quite high to my convservatism levels, if I have gas available and in no rush, I don't see any harm in making the educated discision to sit a little longer at safety stop to bring it down.

Especially if I plan on diving more that day

 

[sea_ledford]

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

Just to clarify, no computer tells you how much nitrogen is in your body or the size of bubbles. They are all mathematical models that have zero to do with your actual physiology. They might approximate what should be happening in your body, but they don't read your body.

I don't actually use the GF99 screen, but my understanding is it is typically used in a "I need to get out of the water as soon as possible" situation, so it tells you how close the current limiting compartment (out of the 16 compartments modeled) is to the M-value (GF100, but 99 requires one less space on the screen). It basically allows you to adjust your GFhi on the fly, based on your willingness to accept higher decompression stress, without having to program your computer.

Again, I don't use it, so I'm happy to be corrected if my understanding is incorrect.

 

[Joemdown]

Just to clarify, no computer tells you how much nitrogen is in your body or the size of bubbles. They are all mathematical models that have zero to do with your actual physiology. They might approximate what should be happening in your body, but they don't read your body.

I don't actually use the GF99 screen, but my understanding is it is typically used in a "I need to get out of the water as soon as possible" situation, so it tells you how close the current limiting compartment (out of the 16 compartments modeled) is to the M-value (GF100, but 99 requires one less space on the screen). It basically allows you to adjust your GFhi on the fly, based on your willingness to accept higher decompression stress, without having to program your computer.

Again, I don't use it, so I'm happy to be corrected if my understanding is incorrect.

Agreed, sorry I was simplyifying. To put it correctly as you have, the computer is mathemtically modelling how much nitrogen it estimates in my body, but that is far better than any other source I have so I can take it as "how much nitrogen is in my body and the estimated size of those bubbles"

I thought surfGF is I need to get out immediately - i.e. if I am on the boat now, this is my GF at 1ATM assuming no stopping on the way up.

Whereas GF99 indicates the pressure differences and therefore the rate at which off-gassing is occuring and therefore potential bubbles forming?

 

[inquis]

In terms of final ascent are you referring to the final slow push to the surface from a safety stop?

Exactly.

if my surfGF is indicating quite high to my convservatism levels, if I have gas available and in no rush, I don't see any harm in making the educated discision to sit a little longer at safety stop to bring it down.

Yep, that's one of the most common uses for it. While not necessarily computer-related, a "surface stop" is a good thing as well, before exerting yourself getting out of the water. Some folks will even use their computer, waiting for surfGF or GF99 to drop enough. (They still work.)

 

[dmaziuk]

I thought surfGF is I need to get out immediately - i.e. if I am on the boat now, this is my GF at 1ATM assuming no stopping on the way up.

Whereas GF99 indicates the pressure differences and therefore the rate at which off-gassing is occuring and therefore potential bubbles forming?

Bubbles are always present and the only thing that's certain is if you have a whole lot of very big ones, you'll have DCS symptoms. There is a clear correlation between "overshooting" the M-values and the size/number of bubbles, but the exact details are rather fuzzy. As is the exact bubble score that will bend a specific diver on a particular day.

99 Surfing Grapefruits, in particular, may or may not be a good proxy as they track the leading tissue compartment: this may work well for isolated "bounce" dives and not so well for dives where significant loading is present in other (slower, typically) tissue compartments. There is also a flip side with the fast compartments: with the 4-5-minutes halftime of the fastest compartment, anything faster than that is covered by "safe ascent rate"; blowing that on the way up may cause excessive bubbles even if you don't overshoot your M-values -- or only do it momentarily.

Of course all that is computed relative to your GF High setting, so exactly how much risk is involved in overshooting your personal "conservatism level" is a bit questionable too: this is all statistically derived and if you haven't run the actual trials on 40/85, you are extrapolating from an already fuzzy dataset.

Enjoy!