Please help me stop!

[Slamfire]

You make good points, however, the fact remains that divers will get bent even when doing everything correctly. That applies to divers running deco profiles as well.

I didn't say nobody would get bent if they do things correctly. I said nobody would get bent if they stay down decompressing long enough. I interpret your meaning of "doing things correctly" to be in compliance of a certain decompression algorithm. What if you where to decompress twice or three times, or even 6x longer than the algorithm calls for? The treatment for DCS is to take you "down to depth" again in a hyperbaric chamber and bring you back up very slowly. Slow down enough in the real dive in the first place and you'll have no need to take a trip to the chamber.

Obviously there are other physical limitations, but I started my post saying that, let's assume for a while these were not present.

 

[boulderjohn]

I didn't say nobody would get bent if they do things correctly. I said nobody would get bent if they stay down decompressing long enough. I interpret your meaning of "doing things correctly" to be in compliance of a certain decompression algorithm. What if you where to decompress twice or three times, or even 6x longer than the algorithm calls for? .

It isn't that simple. Depending upon the decompression profile, it is not such much spending enough time decompressing at a specific depth as it is spending the right time at a specific depth. If you are doing a stop at 70 feet, some of your tissues are off-gassing, but they will stop off-gassing once they reach the equilibrium point at that depth. Staying longer will not help those tissues. At the same time, other tissues are still on-gassing at 70 feet. Prolonging your stop there is making it worse. You have to stay at a specific depth long enough to allow the controlling tissues to offgas to a safe point but not so long that you allow the other tissues to become more dangerously saturated.

And I never even mentioned the need to control existing bubble size.

 

[Slamfire]

It isn't that simple. Depending upon the decompression profile, it is not such much spending enough time decompressing at a specific depth as it is spending the right time at a specific depth. If you are doing a stop at 70 feet, some of your tissues are off-gassing, but they will stop off-gassing once they reach the equilibrium point at that depth. Staying longer will not help those tissues. At the same time, other tissues are still on-gassing at 70 feet. Prolonging your stop there is making it worse. You have to stay at a specific depth long enough to allow the controlling tissues to offgas to a safe point but not so long that you allow the other tissues to become more dangerously saturated.

And I never even mentioned the need to control existing bubble size.

Totally agree. I was trying to keep the conversation within the context of the basic scuba realm. But since you let the cat out of the bag allow me to expand: Do all your deep stops at the correct time and then extend your final stops 2 ATA's and above as much as you can. I know that for deeper/longer than basic scuba times and depths (1hr/100') even this may not give you a guarantee because you could well be already bent say at 60' on your way to the surface. So yes John, you screwed up my whole rationale now... But I think (hope) you guys get my drift. Oversimplification seldom gets the picture totally accurately.

Edit:Ok I feel bad now, like I was caught in my own BS. Here's one more try, simple version: Knowledge = good. Blindly following machines = bad. Get knowledge, not new machine.

On the complicated side of the new attempt. You could then increase conservatism through gradient factors. And furthermore, take bubble mechanics into account to make even more conservative. Isn't that what RGBM tries to do? And it just so happens that is what Suunto uses.

 

[MXGratefulDiver]

You make good points, however, the fact remains that divers will get bent even when doing everything correctly. That applies to divers running deco profiles as well.

There's a basic misconception associated with that statement. Divers don't get bent even when doing everything correctly ... divers get bent because there is no "correct/incorrect" involved in decompression. There's a bunch of variables that interact with each other such that on any given day certain choices might be correct and on a different given day they will be less so. There is no "line" on one side of which you're safe and on the other of which you are not. There's a rather large "gray area" on one side of which your risks increase and on the other side of which your risks decrease ... but there is a finite risk every time you dive.

Decompression algorithms ... whether those used in dive computers or those like V-Planner that are used to manually create decompression schedules ... are nothing more than idealized mathematical models. They know nothing about you, personally ... they don't know the fat content of your body, what you had to drink last night (or on your way to the dive site), how much sleep you had last night, how old you are, whether you're on any medications (or what kind), whether you smoke or not, whether you have a PFO ... or any number of other things about you that may impact your susceptibility to DCS. That's why all dive computer manufacturers build some level of "conservatism" into their dive computers. Some more so than others. Some use a certain type of model that is based on bubble formation, others use a different type that's based on gradients ... most use some combination of the two.

What this means to the diver is that following your computer without understanding why it's producing the numbers it does will most likely keep you from getting bent ... but not necessarily. There will always be a small risk that one or more of your predisposition factors will be present on a given dive that will exceed the parameters built into the computer.

Therefore it doesn't mean that you "did everything correctly" ... it means that you followed the dictates of your computer exactly as the computer said you should ... but in this case, the computer didn't have adequate information to make a correct conclusion.

It's a risk we all take ... particularly when we rely on a piece of technology to make our diving decisions for us.

The risk is very small ... acceptably so given the premise that there are no guarantees in life ... but the fact remains that there is no such thing as an "undeserved hit". If you get DCS it's because you exceeded your body's capacity to deal with the effects of expanding inert gasses in your bloodstream ... you simply didn't understand why ...

... Bob (Grateful Diver)

 

[Randy43068]

Sounds like we're going to discuss what the meaning if "is" is.

By "correctly" ( for brevity ) I meant that we do everything we know to be correct for a particular dive. I'm pretty sure people know decompression isn't a fully understood science and I'd speculate that it will never be fully controlled until such time as we can measure bubble formation in the blood/tissues for an individual during a dive. As we know it, decompression is still an open loop process.

I'm only expanding on the idea mentioned in the previous post that divers still get bent by doing everything they KNOW to be correct at the moment.

There's a basic misconception associated with that statement. Divers don't get bent even when doing everything correctly ... divers get bent because there is no "correct/incorrect" involved in decompression. There's a bunch of variables that interact with each other such that on any given day certain choices might be correct and on a different given day they will be less so. There is no "line" on one side of which you're safe and on the other of which you are not. There's a rather large "gray area" on one side of which your risks increase and on the other side of which your risks decrease ... but there is a finite risk every time you dive.

Decompression algorithms ... whether those used in dive computers or those like V-Planner that are used to manually create decompression schedules ... are nothing more than idealized mathematical models. They know nothing about you, personally ... they don't know the fat content of your body, what you had to drink last night (or on your way to the dive site), how much sleep you had last night, how old you are, whether you're on any medications (or what kind), whether you smoke or not, whether you have a PFO ... or any number of other things about you that may impact your susceptibility to DCS. That's why all dive computer manufacturers build some level of "conservatism" into their dive computers. Some more so than others. Some use a certain type of model that is based on bubble formation, others use a different type that's based on gradients ... most use some combination of the two.

What this means to the diver is that following your computer without understanding why it's producing the numbers it does will most likely keep you from getting bent ... but not necessarily. There will always be a small risk that one or more of your predisposition factors will be present on a given dive that will exceed the parameters built into the computer.

Therefore it doesn't mean that you "did everything correctly" ... it means that you followed the dictates of your computer exactly as the computer said you should ... but in this case, the computer didn't have adequate information to make a correct conclusion.

It's a risk we all take ... particularly when we rely on a piece of technology to make our diving decisions for us.

The risk is very small ... acceptably so given the premise that there are no guarantees in life ... but the fact remains that there is no such thing as an "undeserved hit". If you get DCS it's because you exceeded your body's capacity to deal with the effects of expanding inert gasses in your bloodstream ... you simply didn't understand why ...

... Bob (Grateful Diver)

 

[Slamfire]

So are we now superior divers or what...

I only know that I know nothing.

A teacher of mine told me that you'll go to University and when you think you know everything they'll give you a Bachelors Degree. Then you keep on the studies and find out there's a lot that you really don't know. At that point, they give you a Masters. You keep on studying and realize you know nothing. And then finally when you come to that realization, they'll give you your PhD.

Conservatism is your friend. Diving only within the computer's NDL is not necessarily being more conservative than doing well structured deco dives. You can do very conservative deco dives for the sake of extending times underwater within recreational depths and recreational overall run times.

 

[Randy43068]

Huh?

 

[MXGratefulDiver]

Sounds like we're going to discuss what the meaning if "is" is.

By "correctly" ( for brevity ) I meant that we do everything we know to be correct for a particular dive. I'm pretty sure people know decompression isn't a fully understood science and I'd speculate that it will never be fully controlled until such time as we can measure bubble formation in the blood/tissues for an individual during a dive. As we know it, decompression is still an open loop process.

I'm only expanding on the idea mentioned in the previous post that divers still get bent by doing everything they KNOW to be correct at the moment.

Then we need to discuss what the meaning of KNOW is ... because the post that started this particular branch in the discussion said ...

... and I'll admit I'm very anti-Sunnto, meaning I try to avoid Sunnto divers, nothing personal, it's their business if they want to get half the bottom time I do using my Oceanics/Cochrans (for the same amount of $ spent per dive) but I don't appreciate anyone wasting MY hard earned dive $'s, so they can dive with someone else.

Basing your bottom time ... or even your choice of dive buddies ... on a given brand of computer doesn't give me a lot of confidence that the person making that decision understands what the computer is telling him. And without that knowledge, you aren't making informed choices. Buying a dive computer because it gives you more bottom time isn't an informed choice ... because all that's really saying is that this particular brand or model allows you to push yourself closer to the edge of "safe" ... which is going to increase your risk of exceeding it. Your choice of dive computer isn't going to have any impact on the physics of decompression or how your body will react to it.

It boils down to how many bullets in the chamber are you comfortable with ...

... Bob (Grateful Diver)

 

[Randy43068]

Agreed.

Then we need to discuss what the meaning of KNOW is ... because the post that started this particular branch in the discussion said ...


Basing your bottom time ... or even your choice of dive buddies ... on a given brand of computer doesn't give me a lot of confidence that the person making that decision understands what the computer is telling him. And without that knowledge, you aren't making informed choices. Buying a dive computer because it gives you more bottom time isn't an informed choice ... because all that's really saying is that this particular brand or model allows you to push yourself closer to the edge of "safe" ... which is going to increase your risk of exceeding it. Your choice of dive computer isn't going to have any impact on the physics of decompression or how your body will react to it.

It boils down to how many bullets in the chamber are you comfortable with ...

... Bob (Grateful Diver)

 

[MXGratefulDiver]

So are we now superior divers or what...

I only know that I know nothing.

A teacher of mine told me that you'll go to University and when you think you know everything they'll give you a Bachelors Degree. Then you keep on the studies and find out there's a lot that you really don't know. At that point, they give you a Masters. You keep on studying and realize you know nothing. And then finally when you come to that realization, they'll give you your PhD.

Huh?

To quote a famous article by Richard Pyle ...

If you ask a random, non-diving person on the street to explain what's really going on inside a diver's body that leads to decompression sickness, the answer is likely to be "I don't know".

If you ask the same question of a typical scuba diving instructor, the answer will likely be that nitrogen is absorbed by body under pressure (a result of Henry's Law); and that if a diver ascends too quickly, the excess dissolved nitrogen in the blood will "come out of solution" in the blood to form tiny bubbles; and that these bubbles will block blood flow to certain tissues, wreaking all sorts of havoc.

Pose the question to an experienced hyperbaric medical expert, and you will probably get an explanation of how "microbubbles" already exist in our blood before we even go underwater; and that ratios of gas partial pressures within these bubbles compared with dissolved partial pressures in the surrounding blood (in conjunction with a wide variety of other factors) determine whether or not these microbubbles will grow and by how much they will grow; and that if they grow large enough, they may damage the walls of blood vessels, which in turn invokes a complex cascade of biochemical processes called the "complement system" that leads to blood clotting around the bubbles and at sites of damaged blood vessels; and that this clotting will block blood flow to certain tissues, wreaking all sorts of havoc. You will likely be further lectured that decompression sickness is an unpredictable phenomenon; and that a "perfect model" for calculating decompression schedules will never exist; and that the best way to calculate the best decompression schedules is by examining probabilistic patterns generated from reams of diving statistics.

If, however, you seek out the world's most learned scholars on the subject of decompression and decompression sickness, the top 5 or 6 most knowledgeable and experienced individuals on the subject, the ones who really know what they are talking about; the answer to the question of what causes decompression sickness will invariably be: "I don't know".

As it turns out, the random non-diving person on the street apparently had the best answer all along.

... Bob (Grateful Diver)