Riding GF99 instead of mandatory/safety stops

[LFMarm]

SurfGF: What GF99 would look like if you were on the surface. >100 means you've exceeded your NDL.

Depending on GF high choice, NDL can be exceeded well below GF99=100

 

[td275]

Maybe think of it from a slightly different viewpoint. Check out the tissue graph on your shearwater. Note there are 16 black lines on there. All in green right now at the surface with over 24 hours since your last dive. Now the top lines are fast tissues, and the lower ones are slow tissues. Fast meaning they on gas fast and they off gas fast. The reason they do this is because they move oxygen, nitrogen etc back and forth (plus blood) quickly though the body. There is not a particular tissue associated with each, but let’s just say the top is your heart. Then down from there are the lungs, kidneys, liver, etc. when you get to the slower tissues (bottom lines), they are tissues that on gas slow and OFF GAS SLOW. we are talking about skin, joints and bone. These don’t need as much gas exchange as the higher ones. The solid black line (up and down is 1 ATM to the surface). So the yellow area is where you will see these lines get longer/shorter depending on what your dive profile is.

As you go down the top line (faster tissues) will get longer faster). And as you come up it will drop faster. But here is an important fact. Depending on how far you went down, and how long, as you come up the top line will off gas (get shorter) but the bottom lines will grow (get longer). Because the fast tissues will be off gassing WHILE the sliw tissues are on gassing. Meaning those joints where you actually get the bends in will have much more gas in them upon reaching the surface (stops or no stops), than the fast gas tissues.

Now for the GF. Gradient factors are just a percentage. If you go back to the graph we are talking about all the way on the right is the red. That is the M value for Buhlmanns tissue model. That M value is the MAX that any one of those tissues can absorb. (Note not all 16 tissues can absorb the same amount, but 100% is the M value for any given tissue).

Okay so what are we doing with the GF? In the 1990s Eric Baker realized if we took a percentage of the M value and made that a limit we could stop gas bubble growth and formation to certain levels. Otherwise if we did a true 99/99 GF we would be within 1% chance of death, bends etc.

So the GF (low), or the first number is the percentage of how big we want those bubbles in our system by the first stop. Okay, so we go with 50, that means our computer will have us stop when one of those lines on our shearwater graph is halfway (or at 50%). Remember it may not be the top line, because it off gasses fast it could be line two or three if we come up slowly.

GF (high) or the second number, is for the percentage we was that gas bubble to be in ANY tissue UPON SURFACING. Now I put that in all caps because as we come up not one of those lines will be very long. But once our body comes above the surface BOOM those lines expand big time. (Shearwater has a really cool video on this in slow motion). Remember they are under pressure, even at 3 ft. But once they are at 1 ATM they have so much pressure they expand greatly.

So less say we went with 50/70 (a typical tech dive setup). That means we are telling the computer we only was a tissue to explanation at 50% underwater. But once i surface I only want it to be 70% or less. Of course our computer cannot monitor our body without some plug in. So all of this is tissue model math based on Bulhmenns M values.

To further enhance other answers here, if you pick a lower number say 30/70 you will have deeper stops. Because as you come up and one of those lines gets to 30%, the computer will give you a stop. Maybe that is at 50 ft, let’s say. But if you choose a higher number like 50/70 it is more likely you won’t have a deep stop but a 30 ft stop.

Which is why most tech divers like 50/70 so they can sip on 50% EAN on the way up to 30ft… stop for a short while. Then switch over to a 100 EAN at the 20 ft stop. The higher O2 helps those tissues off gas faster and the gas bubbles are not so big when you get on the boat.

 

[L13]

Maybe think of it from a slightly different viewpoint. Check out the tissue graph on your shearwater. Note there are 16 black lines on there. All in green right now at the surface with over 24 hours since your last dive. Now the top lines are fast tissues, and the lower ones are slow tissues. Fast meaning they on gas fast and they off gas fast. The reason they do this is because they move oxygen, nitrogen etc back and forth (plus blood) quickly though the body. There is not a particular tissue associated with each, but let’s just say the top is your heart. Then down from there are the lungs, kidneys, liver, etc. when you get to the slower tissues (bottom lines), they are tissues that on gas slow and OFF GAS SLOW. we are talking about skin, joints and bone. These don’t need as much gas exchange as the higher ones. The solid black line (up and down is 1 ATM to the surface). So the yellow area is where you will see these lines get longer/shorter depending on what your dive profile is.

As you go down the top line (faster tissues) will get longer faster). And as you come up it will drop faster. But here is an important fact. Depending on how far you went down, and how long, as you come up the top line will off gas (get shorter) but the bottom lines will grow (get longer). Because the fast tissues will be off gassing WHILE the sliw tissues are on gassing. Meaning those joints where you actually get the bends in will have much more gas in them upon reaching the surface (stops or no stops), than the fast gas tissues.

Now for the GF. Gradient factors are just a percentage. If you go back to the graph we are talking about all the way on the right is the red. That is the M value for Buhlmanns tissue model. That M value is the MAX that any one of those tissues can absorb. (Note not all 16 tissues can absorb the same amount, but 100% is the M value for any given tissue).

Okay so what are we doing with the GF? In the 1990s Eric Baker realized if we took a percentage of the M value and made that a limit we could stop gas bubble growth and formation to certain levels. Otherwise if we did a true 99/99 GF we would be within 1% chance of death, bends etc.

So the GF (low), or the first number is the percentage of how big we want those bubbles in our system by the first stop. Okay, so we go with 50, that means our computer will have us stop when one of those lines on our shearwater graph is halfway (or at 50%). Remember it may not be the top line, because it off gasses fast it could be line two or three if we come up slowly.

GF (high) or the second number, is for the percentage we was that gas bubble to be in ANY tissue UPON SURFACING. Now I put that in all caps because as we come up not one of those lines will be very long. But once our body comes above the surface BOOM those lines expand big time. (Shearwater has a really cool video on this in slow motion). Remember they are under pressure, even at 3 ft. But once they are at 1 ATM they have so much pressure they expand greatly.

So less say we went with 50/70 (a typical tech dive setup). That means we are telling the computer we only was a tissue to explanation at 50% underwater. But once i surface I only want it to be 70% or less. Of course our computer cannot monitor our body without some plug in. So all of this is tissue model math based on Bulhmenns M values.

To further enhance other answers here, if you pick a lower number say 30/70 you will have deeper stops. Because as you come up and one of those lines gets to 30%, the computer will give you a stop. Maybe that is at 50 ft, let’s say. But if you choose a higher number like 50/70 it is more likely you won’t have a deep stop but a 30 ft stop.

Which is why most tech divers like 50/70 so they can sip on 50% EAN on the way up to 30ft… stop for a short while. Then switch over to a 100 EAN at the 20 ft stop. The higher O2 helps those tissues off gas faster and the gas bubbles are not so big when you get on the boat.

Some right, but a lot wrong in this. I don't want to take the time to go through it point by point, but do want to warn readers that much of it is only mostly right.

(ex. GF of 99 is no where near 1% chance of death, GF is not related to size of bubbles but rather to supersaturation, etc.)

 

[td275]

Some right, but a lot wrong in this. I don't want to take the time to go through it point by point, but do want to warn readers that much of it is only mostly right.

(ex. GF of 99 is no where near 1% chance of death, GF is not related to size of bubbles but rather to supersaturation, etc.)

Okay wrong word choices. Got it. Just trying to make a difficult subject a little easier to understand. I will refrain from commenting further.

 

[kaylee_ann]

Okay wrong word choices. Got it. Just trying to make a difficult subject a little easier to understand. I will refrain from commenting further.

Aw, I don’t think you should do that. We learn by saying our thoughts and hearing what’s right and wrong about them. Absolutely nothing wrong with not being completely accurate. Nobody is. Judging by how long your response was, you care a lot about the topic, so ya shouldn’t quit talking. Plus, I’d take being “mostly right” about this very complicated stuff as a win

 

[OctopusLover]

Shake a soda bottle. You have a choice between cracking it open (not allowed to close it) OR repeatedly open then close it (letting things calm down each time). You make the choice.

Well, maybe I'm the odd one out but I legitimately have always tried to just unscrew to cap enough that I hear some hissing without the bubble level raising. I feel like the analogy here is that GF99 is like seeing the bubble level inside a transparent bottle as you unscrew the cap, vs deco stops being an opaque bottle and you just open it for "1 second at a time and leave 10 seconds to rest". If I can see the bubbles, why would I care for "opening protocols"?

This is wrong. GFLow is the GF99 value that will be targeted to calculate the depth of the first deco stop. The remaining deco stops will be linearly interpolated between GFLow and GFHigh.


You could do that, or increase your GF99 as you cross each depth threshold to the next linearly interpolated GF number.

I think that would be optimal (at least from an off gassing perspective), and maybe even practical for the beginning of deco when stops are just a few minutes, but not so much at the end as stops get shallow and long.

Oh that's great information thanks. I had read in some other thread that a GF99 of 1/2 to 1/3 of your GfHigh value is a sweet spot for safe degassing, is that wrong? From what you're saying, your last deco stop would be done at GF99 close to your Gf high value? Thinking about it that makes sense as the last stage of deco is... being at the surface at GfHigh, but then if degassing at GfHigh is acceptable, why not do that at the bottom too?

Depending on GF high choice, NDL can be exceeded well below GF99=100

I'm not sure I understand what you mean. Surely the NDL can be exceeded irrespective of both GF99 readings and GFHigh setting (simplest example I can think of is a slow but continuous descent where offgassing never even begins to occur)

 

[L13]

Well, maybe I'm the odd one out but I legitimately have always tried to just unscrew to cap enough that I hear some hissing without the bubble level raising. I feel like the analogy here is that GF99 is like seeing the bubble level inside a transparent bottle as you unscrew the cap, vs deco stops being an opaque bottle and you just open it for "1 second at a time and leave 10 seconds to rest". If I can see the bubbles, why would I care for "opening protocols"?

Great Analogy! Not sure if it matches the reality of deco, that would require research, but it makes sense.

Oh that's great information thanks. I had read in some other thread that a GF99 of 1/2 to 1/3 of your GfHigh value is a sweet spot for safe degassing, is that wrong?

That was the suggested values back in the era of deep stops. Modern thinking is probably closer to 80% (a lot use 83%).

From what you're saying, your last deco stop would be done at GF99 close to your Gf high value? Thinking about it that makes sense as the last stage of deco is... being at the surface at GfHigh, but then if degassing at GfHigh is acceptable, why not do that at the bottom too?

The idea was that early supersaturation led to early micro-bubbles that seeded bigger bubbles later. By limiting early supersaturation you limited micro-bubbles.

Recent research suggests that this is less impact-full than the additional gas absorbed into slow tissues at the deeper stops.

I'm not sure I understand what you mean. Surely the NDL can be exceeded irrespective of both GF99 readings and GFHigh setting (simplest example I can think of is a slow but continuous descent where offgassing never even begins to occur)

If your GF high is set to 85%, then you will exceed NDL when your GF99 would be >=85%on the surface (SurfGF >=85%) which is well bellow 100%.

 

[L13]

Okay wrong word choices. Got it. Just trying to make a difficult subject a little easier to understand. I will refrain from commenting further.

When trying to write a definitive essay, it is important to try and get the details right, or you will get called on it.

Actually any time you don't get the details right on SB you will get called on it. Half the time, it is just a detail you forgot to include. But it is easier to deal with when it is just one or two key ideas at a time. Point by point analysis of long posts are tedious, so often all you get in those cases are summary critiques.

I know from experience. check my posting history, I've gotten details wrong often enough. And every time I miss a detail I get called on it too. It happens to everybody.

Listen to @kaylee_ann, about this.

 

[kaylee_ann]

When trying to write a definitive essay, it is important to try and get the details right, or you will get called on it.

Actually any time you don't get the details right on SB you will get called on it. Half the time, it is just a detail you forgot to include. But it is easier to deal with when it is just one or two key ideas at a time. Point by point analysis of long posts are tedious, so often all you get in those cases are summary critiques.

I know from experience. check my posting history, I've gotten details wrong often enough. And every time I miss a detail I get called on it too. It happens to everybody.

Listen to @kaylee_ann, about this.

Absolutely. Scubaboard is known for telling it like it is, sometimes really harshly lol. When I first started commenting in the tech forum, if I got a couple details wrong, people were really rough on me. Combine that with the fact that I’m not tech, just obsessed with theory, and it was pretty hard for a while.

Now I come in here relatively often on topics that interest me, and if I know a little bit. It’s much better now. @td275 you really should stay, there are many many nice and helpful people here. And honestly L13 wasn’t rude about it either, very nice by SB standards
Stick around. You’ll learn a lot. Bounce your ideas off others, hear what they say, it’ll click

 

[OctopusLover]

That was the suggested values back in the era of deep stops. Modern thinking is probably closer to 80% (a lot use 83%).

Why do I keep seeing 40/70 as a commonly used profile for tech diving then? Are you saying modern thinking is 80/80 or something flat like that?

The idea was that early supersaturation led to early micro-bubbles that seeded bigger bubbles later. By limiting early supersaturation you limited micro-bubbles.

Recent research suggests that this is less impact-full than the additional gas absorbed into slow tissues at the deeper stops.

That actually makes a lot of sense, surface tension will keep bubbles together and prevent the gas trapped inside them from participating in the offgasing I assume?


Thanks a lot for all this info, really appreciate your time. If you have any resources you can recommend (articles, posts or papers) please share