Why do people add a few minutes to their last deco stop?

[crofrog]

If you want to prove it is insignificant, provide cites or numbers to back up your argument.

Can you cite any research or numbers that show that it isn't significant? See, two can play that "give me cites or exact answers, or you are wrong" game.

Tell you what, even if you don't have the cites or numbers, present a physical explanation consistent with known physics, chemistry and physiology, that could even slightly possibly suggest different, and I will take you seriously enough to go back and re-find the research and give you cites.

Otherwise, go sit in the corner with the other magicians that say that since someone doesn't have a cite memorized, they must be wrong. Keep believing the world is magical and random and we know nothing about it, just because we don't know everything about it.

You think decompression is way more deterministic than it is. You pretend to be an expert but all the experts I’ve seen talk are way less certain than you about the mechanics and probabilities.

if you’re going to assert that extending shallow stops significantly reduces DCS risks, you must provide the numbers to substantiate your claim. This isn’t a game of ‘who can spout the most convincing rhetoric.’ Now, if you can’t provide that, perhaps it’s you who should join the magicians in the corner, not I.

I say I don’t know how much lengthening a stop helps. It probably helps. if you want to do it. Do it.

Here’s another question for you. How long can you stay at 20ft before you have a pDCS of 1%?

 

[L13]

You think decompression is way more deterministic than it is. You pretend to be an expert but all the experts I’ve seen talk are way less certain than you about the mechanics and probabilities.

We are still learning != we know nothing. Go watch those talks again. They don't say "we don't know if these things mater." The say "we don't know exactly how much they matter."

Yes, if you ask those experts "Exactly how much is my pDCS reduced by adding 10% to my last stop time?" They will say "I don't know." But if you ask "Will adding 10% to my last stop time meaningfully reduce my pDCS?" They will all say "Yes", or at least "Probably." Partial knowledge is still useful.

if you’re going to assert that extending shallow stops significantly reduces DCS risks, you must provide the numbers to substantiate your claim. This isn’t a game of ‘who can spout the most convincing rhetoric.’ Now, if you can’t provide that, perhaps it’s you who should join the magicians in the corner, not I.

If you think it doesn't you must provide the numbers to substantiate your claim ...
My statements are consistent with the experts, if you want to claim different you need to provide the evidence.

Here’s another question for you. How long can you stay at 20ft before you have a pDCS of 1%?

Longer than for pDCS of 2%. Do I have a PFO? All the numbers change, but still longer than for pDCS of 2%. Am I a young navy diver? Again the numbers change, and again all evidence since the beginning of dive research says my answer is very likely just as correct.

I could go look at Haldane's research and maybe give you a time. I could go look at Workman's research and maybe give you another time. All the way up to the latest work by NEDU and others. The time from each one would be different. But the fact that there isn't an exact answer doesn't mean we know nothing. The integration of all the research plus what we know about chemistry, physics, physiology and even statistics all together say I am very likely right that the time for pDCS of 1% is longer than for pDCS of 2%. And that (d pDCS/dt ) near pDCS = 2% > (d pDCS/dt) near pDCS = 1%.

If subtracting 5 min from your last stop significantly increases your risk, why would you think that adding 5 min would be insignificant?

If all the other factors we don't understand are aligned on this dive to put you at higher risk, why not shift one in your favor if you can. Just because we don't know exactly how much it helps does't mean we don't know it helps.

 

[crofrog]

If subtracting 5 min from your last stop significantly increases your risk, why would you think that adding 5 min would be insignificant?

Does it? There is that word again. I’m assuming p < 0.05.

If all the other factors we don't understand are aligned on this dive to put you at higher risk, why not shift one in your favor if you can. Just because we don't know exactly how much it helps does't mean we don't know it helps.

I never said you should or shouldn’t. I said you, nor anyone can say how much it helps.

 

[L13]

Does it? There is that word again.

What word?

I’m assuming p < 0.05.

for what?

 

[Germie]

Deco is not absolute science, there is not straight boundery of getting bent or not.
I always tell students, I can learn you how to do save decompression dives, with the knowledge of today, but I cannot avoid you of ever getting bent.
And this is the truth. Why are there still people getting bent on just recreational dives within the NDL? It goes right for all divers on a day, except 1. The why is most times a why and the cause is never found. Even if you have a pfo, it is not said that it is the cause of your dcs, it is only a probability. Why did it goes well on other dives? There is also a form of the day. Some factors can increase the risk of dcs, even if they do more deco. But also fit, young people get it sometimes.

The fact is that the lungfilter is the important way to loose bubbles. And doing shallow stops is bubble growth management. As long as the bubbles are small enough to leave your buddy over the lungfilter, there is no problem. If they grow too big, they got stucked somewhere you don't want it. This is the most easiest way to explain deco, but again, it is more complex, we don't talk here about bubbles that stay in the body and don't go over the lungfilter by shunts.

But, it is not the complete explaination. Some people have more bubbles at the surface than others, even after doing longer deco. Also not all of that bubbles indicate the risk of getting bent. This is why the O'dive is a nice tool, but cannot predict everything. You also must know your 'normal' and that is very difficult to know, as people are human and no robots. So you must do the same dive over and over again in the same circumstances to really know all factors. By just doing 100 dives everywhere, you maybe can find a line, but don't know all factors. Even the food and drink in the day(s) before can give other results. The amount of bubbles found after a dive is a just a gauss curve, on the left the ones with just a few bubbles, in the middle the average people with some bubbles, on the right the ones that have a lot of bubbles. Which one are you?
There is a French cavediver that is doing cavedives over 200m, even to 300m on a regularly base. If I will do such dives, I will probably get bent, he doesn't. Why?

We don't do saturation diving, so if we dive, some tissues maybe get fully saturated, others not. Theoretically, if you do stops, your tissues stay loaded till the ambient pressure. They are saturated. If you go over the ambient pressure line, some tissues get oversaturated. The M-value is the theoretiscally value you can get up safe. This is just a theoretically line, created by Workman/Buhlmann.
To stay between the ambient pressure line and the M-value line, you are offgassing safe (this is the line GF-low/GF-high known in buhlmann algoritms). But again, not further than the new ambient pressure (I don't talk now about taking oxygen, as then you can theoretically get zero nitrogen you your tissues if you stay long enough). So there comes a moment again that even if your Ptissue =Pambient, you still can get bubbles if you go further up.
The only thing is that if you stay longer at a decostop in the shallows, the tissues loose more saturation and go more away from the M-value and go more to the ambient pressure value. So theoretically it adds safety.

See it as the no fly time. If it is 12 hours, your safetystop at surface will be 12 hours. Probably it also goes well if you fly after 10 hours or 8 or even after 4. But the 12 hours is what is ok for almost all divers doing a single dive. But if you make it 1 week, it goes well for all divers. But this is not doable for most people.

Some more information about the m-values Workman/Buhlmann: https://www.shearwater.com/wp-content/uploads/2019/05/understanding_m-values.pdf

And yes, deco can suck, it can be very boring. But remember if you are laying on the ground in a narrow tunnel in a cave looking at the daylight where you can't go yet that the places you came from are worth the waiting now.
Same on an boring anchorline, the wreck you visited was very nice, just wait that 2-4 hours to get safe up and can do it again a next time.
But if you can do deco on a reef, yes, that is the best way to do deco. And it is also not boring to add some more deco by just doing a normal dive at the end of the dive.

 

[L13]

Deco is not absolute science, there is not straight boundery of getting bent or not.

Where does this idea that not a straight boundary = not science come from.

almost as bad as we don't know everything = we don't know anything = not science that others express.

I always tell students, I can learn you how to do save decompression dives, with the knowledge of today, but I cannot avoid you of ever getting bent.

We absolutely know how to prevent DCS 100% of the time, it just isn't very practical. If you always ascend slowly enough that none of your tissues are ever supersaturated, you will not get DCS.

And this is the truth.

Not. Counter example above.

Why are there still people getting bent on just recreational dives within the NDL? It goes right for all divers on a day, except 1. The why is most times a why and the cause is never found. Even if you have a pfo, it is not said that it is the cause of your dcs, it is only a probability. Why did it goes well on other dives? There is also a form of the day. Some factors can increase the risk of dcs, even if they do more deco. But also fit, young people get it sometimes.

The fact is that the lungfilter is the important way to loose bubbles. And doing shallow stops is bubble growth management. As long as the bubbles are small enough to leave your buddy over the lungfilter, there is no problem. If they grow too big, they got stucked somewhere you don't want it. This is the most easiest way to explain deco, but again, it is more complex, we don't talk here about bubbles that stay in the body and don't go over the lungfilter by shunts.

But, it is not the complete explaination. Some people have more bubbles at the surface than others, even after doing longer deco. Also not all of that bubbles indicate the risk of getting bent. This is why the O'dive is a nice tool, but cannot predict everything. You also must know your 'normal' and that is very difficult to know, as people are human and no robots. So you must do the same dive over and over again in the same circumstances to really know all factors. By just doing 100 dives everywhere, you maybe can find a line, but don't know all factors.

Just because we don't know all of the factors doesn't mean we don't know some of the factors. Not knowing all of the factors does not prevent us from increasing safety by controlling the factors we do know.

Even the food and drink in the day(s) before can give other results. The amount of bubbles found after a dive is a just a gauss curve, on the left the ones with just a few bubbles, in the middle the average people with some bubbles, on the right the ones that have a lot of bubbles. Which one are you?
There is a French cavediver that is doing cavedives over 200m, even to 300m on a regularly base. If I will do such dives, I will probably get bent, he doesn't. Why?

Just because we don't know the answer does not mean it doesn't exist. We aren't just rolling the dice and the french guy just happens to have rolled all 6's so far..

We don't do saturation diving, so if we dive, some tissues maybe get fully saturated, others not. Theoretically, if you do stops, your tissues stay loaded till the ambient pressure. They are saturated. If you go over the ambient pressure line, some tissues get oversaturated. The M-value is the theoretiscally value you can get up safe. This is just a theoretically line, created by Workman/Buhlmann.
To stay between the ambient pressure line and the M-value line, you are offgassing safe (this is the line GF-low/GF-high known in buhlmann algoritms). But again, not further than the new ambient pressure (I don't talk now about taking oxygen, as then you can theoretically get zero nitrogen you your tissues if you stay long enough). So there comes a moment again that even if your Ptissue =Pambient, you still can get bubbles if you go further up.
The only thing is that if you stay longer at a decostop in the shallows, the tissues loose more saturation and go more away from the M-value and go more to the ambient pressure value. So theoretically it adds safety.

Actually it is more than just theoretically. There is empirical evidence that it adds safety, and there is zero empirical evidence that it doesn't.

See it as the no fly time. If it is 12 hours, your safetystop at surface will be 12 hours. Probably it also goes well if you fly after 10 hours or 8 or even after 4. But the 12 hours is what is ok for almost all divers doing a single dive. But if you make it 1 week, it goes well for all divers. But this is not doable for most people.

Some more information about the m-values Workman/Buhlmann: https://www.shearwater.com/wp-content/uploads/2019/05/understanding_m-values.pdf

And yes, deco can suck, it can be very boring. But remember if you are laying on the ground in a narrow tunnel in a cave looking at the daylight where you can't go yet that the places you came from are worth the waiting now.
Same on an boring anchorline, the wreck you visited was very nice, just wait that 2-4 hours to get safe up and can do it again a next time.
But if you can do deco on a reef, yes, that is the best way to do deco. And it is also not boring to add some more deco by just doing a normal dive at the end of the dive.

 

[Germie]

We absolutely know how to prevent DCS 100% of the time, it just isn't very practical. If you always ascend slowly enough that none of your tissues are ever supersaturated, you will not get DCS.

There is only one way not to get bent and that is don't dive. With every ascend, how slow it is, there must be some oversaturation, otherwise you cannot loose the gas from your tissues. The tissueloading must get a new equilibrium. If you stay at a certain ambient pressure, you will not loose any gases from the tissues anymore as soon as the equilibrium is reached, the tissues are saturated, like you have at surface the 21% oxygen and 79% nitroxgen, this is in the air you breathe and in your tissues. The equilibrium is hold. If you want to get to the surface, and hold that equiluibrium, there must be a small distortion first and then you get the new equilibrium. This can only be done by ascending and get some oversaturation.
You only can theoretically go very slow, and so slow that you almost hold that equilibrium, theoretically that only 1 breath out over the lungfilter gives the new equilibrium. But this does not work for us divers. A simple dive will take days to finish. As you set not practical.
So waiting on the last stop a little bit longer will not hurt, but there will be still some risk of dcs left.
Yes we dive as a hobby and are all human.

 

[L13]

There is only one way not to get bent and that is don't dive. With every ascend, how slow it is, there must be some oversaturation, otherwise you cannot loose the gas from your tissues.
The tissueloading must get a new equilibrium. If you stay at a certain ambient pressure, you will not loose any gases from the tissues anymore as soon as the equilibrium is reached, the tissues are saturated, like you have at surface the 21% oxygen and 79% nitroxgen, this is in the air you breathe and in your tissues. The equilibrium is hold. If you want to get to the surface, and hold that equiluibrium, there must be a small distortion first and then you get the new equilibrium. This can only be done by ascending and get some oversaturation.

Incorrect. Your tissues are not supersaturated till the partial pressure of inert gas is greater than the total ambient pressure. Breathing air, if you ascend till the partial pressure of N2 in your tissues equals ambient pressure, you will have a 21% partial pressure gradient driving the N2 out of your tissues, without being supersaturated. This is a significant fraction of the gradient you can achieve going to the the M-values of Buhlmann et. al. (typical M-values are ~0.5 ATM above ambient +- a fair amount due to tissue or depth). Adding the effects of water vapor, and the gradient is even greater. Add in Nitrox and it even greater.

You only can theoretically go very slow, and so slow that you almost hold that equilibrium, theoretically that only 1 breath out over the lungfilter gives the new equilibrium. But this does not work for us divers. A simple dive will take days to finish. As you set not practical.

Mostly incorrect. It would not take days. But you are right that it would often be impractical. However, if you do the math you will find that for many shallow NDL dives, you would be actual doing this.

So waiting on the last stop a little bit longer will not hurt, but there will be still some risk of dcs left.
Yes we dive as a hobby and are all human.

We totally agree here.

 

[Germie]

Incorrect. Your tissues are not supersaturated till the partial pressure of inert gas is greater than the total ambient pressure. Breathing air, if you ascend till the partial pressure of N2 in your tissues equals ambient pressure, you will have a 21% partial pressure gradient driving the N2 out of your tissues, without being supersaturated. This is a significant fraction of the gradient you can achieve going to the the M-values of Buhlmann et. al. (typical M-values are ~0.5 ATM above ambient +- a fair amount due to tissue or depth). Adding the effects of water vapor, and the gradient is even greater. Add in Nitrox and it even greater.


Mostly incorrect. It would not take days. But you are right that it would often be impractical. However, if you do the math you will find that for many shallow NDL dives, you would be actual doing this.


We totally agree here.

I think we use another defnition for 'oversaturation or supersaturation'.
If we have the tissues loaded till ambient pressure, we have 'saturation'. If we ascend, we get oversaturation, that is how I would call it. I would speak about supersaturation if you go over the theoretically M-value. But to ascend, your tissues must loose the gas that is there too much. In this we agree.

The time depends on the time you went down, that is true. If you go to 100m depth, it will take more time than if you do a dive to 5m depth. On a dive to 5m depth, you don't have to do a safetystop. So if you want to go up from 100m on the way you said, it will takes a very long time, maybe not days, but for sure hours more than a normal 100 minute dive if you stay 15 minutes at 100m.
Also if you stay 17 minutes at 30m (the NDL time on air) it will take a long time, more than 1 tank will last. So we agree it is not practical. But how long it will really take, I haven't calculated, and I don't see any need to do as I don't do saturation dives. But I think my longest cave dives (5 till 6 hours) will last very long if I had to go up as slow as you mentioned, haha.

 

[inquis]

Your tissues are not supersaturated till the partial pressure of inert gas is greater than the total ambient pressure.

Afraid not. "Saturation" is when tissues equal the ambient inspired inert gas pressure, not the total ambient pressure. "Supersaturation" is when it's greater -- supersaturation is synonymous with "off-gassing". See Powell, Deco for Divers, p. 52-56. He repeatedly uses the "inspired" qualifier, though he does drop it later in the book, saying "ambient pressure" as shorthand for "inspired inert gas pressure" (e.g., in the Gradient Factor section (p. 181).

ETA: Baker does "redefine" that lower line to be total ambient pressure in his "Understanding M-Values" paper. He incorrectly states, "when the inert gas loading in a compartment goes above the ambient pressure line, an overpressure gradient is created." In fact, there is a gradient when the inert tissue loading is above the inspired partial pressure. And yes, GF=0 is on the ambient (total) pressure line.