Understanding Decompression Sickness

[RobPNW]

I think a lot has to do with the stability of the gas coming out of liquid solution, be that in your blood or tissue fluids. Think of a pot of boiling water as an analogy. You could have just enough off gassing from the water to form tiny bubbles that gently come to the surface and escape the pot. If you took that same pot of water to a much lower ambient pressure, it would become a violent rolling boil and overflow the pot.

 

[boulderjohn]

When using the old US Navy tables, all dives are deco dives (or, at least, this is what was taught me back in 1975).
Let's make the same example with a shorter bottom time, 25 min at 30m, which was within NDL with those tables.
If during ascent you take an excess time up to 5 min, you will follow the table for 30 min, 30m, doing a deco stop of just 3min at 3m.
II your ascent is even slower, so the extra ascent time is, say, 7 minutes, then you fall back to a bottom time of 40 min, which gives you a deco time of 15 min at 3m.
So the fact that the planned dive was within NDL, or outside it, does not make any difference to the logic.
Furthermore, as originally the US Navy tables were coinceived for an ascent rate of 18 m/min, instead of 10, they were already considered "not safe enough" at the time for sport divers.
So the method recommended was very simple: instead of computing the dive time from the beginning of descent to the beginning of ascent, we were taught to play it safe, counting the dive time from the beginning of the dive to the moment of reaching the deepest deco stop (9m for recreational diving).
This automatically acvounts fot any excess time during ascent.
At that point you look at your timer, and that is the total dive time. You look at the depth meter, and the red arrow shows you the max depth reached.
With these two numbers you look at dive tables, choosing the first depth equal or just larger than your max depth and then the dive time equal or slightly larger than the total dive time.
In most cases this would create enough excess deco stops for being safe, despite the US Navy tables, if followed strictly, where not 100% DCS safe.
Of course computers made this approach entirely obsolete...

Got it.

So let's summarize what I wrote and what you wrote in reply.

I wrote that PADI's (and others) research found that when doing an NDL dive, it is possible to ascend more slowly than the specified ascent rate and still stay within NDLs. A good example would be multi-level dives, which can be far, far slower than the specified ascent rate. I further said that the problem with tables is that you can't tell when your ascent has been so slow that you have gone from an NDL dive to one requiring decompression stops.

Your reply was that it is easy to tell with the tables--all you do is add that extra time and make it a dive requiring decompression.

Let me go back to my example. In the real multilevel dive example I gave, the dive was NDL. (It could have been even longer, but we were low on gas.) Your system would have had us violating NDLs by 60 minutes, meaning a huge required decompression stop.

As I look at it, those two do not look all that similar.

 

[lowwall]

What about this? Let's say you go down to some depth. At some depth your body absorbs enough nitrogen to the point where the pressure in the tissues equals ambient pressure. OK. Then you descend deeper and your body absorbs more nitrogen until it equals ambient pressure. If you keep doing this, wouldn't you reach a point where your tissues just can't absorb anymore nitrogen?

If so, if you reach such a point, and you keep breathing, where does the nitrogen go? It has to go somewhere right?

A given number of molecules of gas at a given pressure takes up a certain amount of space. Double the pressure and you have to double the number of molecules to fill that same space. As the pressure increases additional molecules keep packing into the same space.

Eventually you do reach the pressure at which the gas turns into a liquid. At least if your temperature is above what is known as the "critical temperature" of that molecule. Since the critical temperature of Nitrogen is -146.9°C, you won't get liquid nitrogen in your tissues no matter how deep you go. Unless you are making your first (and last) dive in one of the methane lakes of Titan. If I understand it correctly, even molecules above the critical temperature do reach a packing limit (or at least start exhibiting weird properties) at what is called the "critical pressure".

The critical pressure for N2 is 33.5 atmospheres or around 1,050 feet so it's not particularly relevant for scuba divers. A handful of humans have gone to that depth on scuba, but the fatality rate is around 50%. And you have to replace most or all of the nitrogen (and much of the oxygen) with Helium at that depth anyway or it becomes 100% fatal, so you end up dealing with a very different decompression question.

 

[Angelo Farina]

Got it.

So let's summarize what I wrote and what you wrote in reply.

I wrote that PADI's (and others) research found that when doing an NDL dive, it is possible to ascend more slowly than the specified ascent rate and still stay within NDLs. A good example would be multi-level dives, which can be far, far slower than the specified ascent rate. I further said that the problem with tables is that you can't tell when your ascent has been so slow that you have gone from an NDL dive to one requiring decompression stops.

Your reply was that it is easy to tell with the tables--all you do is add that extra time and make it a dive requiring decompression.

Let me go back to my example. In the real multilevel dive example I gave, the dive was NDL. (It could have been even longer, but we were low on gas.) Your system would have had us violating NDLs by 60 minutes, meaning a huge required decompression stop.

As I look at it, those two do not look all that similar.

The difference between US Navy and Padi tables is that the first always assume a square profile, given by max depth and total time. Padi made an extensive study on multi-level dives, developing an advanced table system known as RDP or simply "the wheel".
I was trained with the US Navy tables, which results in deco dives absolutely conservative and safe. As we were trained to deco procedures since the first OW course, this is not a big deal, it just takes more time for deco stops (and more air, of course).
I know only a small number of divers (3, actually, all are instructors) who understood the RDP and can use the Padi wheel for performing multi-level dives "riding the NDL".
So yes, it is possible using the Padi method for multi-level dives, but it is not easy, and I am not qualified nor trained for attempting this approach.
For all other divers, they use the computer.

 

[boulderjohn]

The difference between US Navy and Padi tables is that the first always assume a square profile, given by max depth and total time. Padi made an extensive study on multi-level dives, developing an advanced table system known as RDP or simply "the wheel".
I was trained with the US Navy tables, which results in deco dives absolutely conservative and safe. As we were trained to deco procedures since the first OW course, this is not a big deal, it just takes more time for deco stops (and more air, of course).
I know only a small number of divers (3, actually, all are instructors) who understood the RDP and can use the Padi wheel for performing multi-level dives "riding the NDL".
So yes, it is possible using the Padi tables, but is not easy, and I am not qualified nor trained for attempting this approach.
For all other divers, they use the computer.

The PADI tables and the wheel--on which I was thoroughly trained when I became an instructor--are not the same thing. You do not "ride the NDL" with the wheel--you plan the multi-level dive ahead of time. The PADI tables also use a square profile. I used them for my example because they were sitting on my desk.

You just wrote:

I was trained with the US Navy tables, which results in deco dives absolutely conservative and safe. As we were trained to deco procedures since the first OW course, this is not a big deal, it just takes more time for deco stops (and more air, of course).

Again, let's look at my comparison, which you (again) imply using the tables is about the same: "this is not a big deal, it just takes more time for deco stops (and more air, of course)."

Let me repeat that the dive I described, using a computer, was an NDL dive--no required deco stops. Doing the same dive with a square table would put me 60 minutes over the NDL. How much deco time would I need to do and at what depths would I need to do them to make up for 60 minutes of deco? How much more air would I need for that?

You may think going 60 minutes over NDL on a dive and needing more air for the required stops is "no big deal," but I think a lot of people would agree that it is a pretty big deal.

 

[Pipehorse]

I think a lot has to do with the stability of the gas coming out of liquid solution, be that in your blood or tissue fluids. Think of a pot of boiling water as an analogy. You could have just enough off gassing from the water to form tiny bubbles that gently come to the surface and escape the pot. If you took that same pot of water to a much lower ambient pressure, it would become a violent rolling boil and overflow the pot.

Not sure this is a great analogy. Bubbles coming from boiling water(steam) are the result of a change of state, not of gas coming out of solution, i.e. the bubbles(steam) were never dissolved inside the water. The water is being changed from a liquid to a gas, hence the bubbles always forming at the bottom of the pot. If you lower the ambient pressure, without lowering the amount of heat then more bubbles (steam) are released because of the heat, not because the water is less stable at lower pressures.

 

[RobPNW]

Not sure this is a great analogy. Bubbles coming from boiling water(steam) are the result of a change of state, not of gas coming out of solution, i.e. the bubbles(steam) were never dissolved inside the water. The water is being changed from a liquid to a gas, hence the bubbles always forming at the bottom of the pot. If you lower the ambient pressure, without lowering the amount of heat then more bubbles (steam) are released because of the heat, not because the water is less stable at lower pressures.

Yeah, you're right. Better analogy would be a bottle of Coke, shake it up, and quickly fizzes out all over the place. Whereas, shake it up, and very slowly crack the top allowing the pressure to change very slowly, and the fizz stays under control.

 

[lowwall]

Not sure this is a great analogy. Bubbles coming from boiling water(steam) are the result of a change of state, not of gas coming out of solution, i.e. the bubbles(steam) were never dissolved inside the water. The water is being changed from a liquid to a gas, hence the bubbles always forming at the bottom of the pot. If you lower the ambient pressure, without lowering the amount of heat then more bubbles (steam) are released because of the heat, not because the water is less stable at lower pressures.

You know those little tiny bubbles that show up on the bottom of the pot before the the boil really gets going? Those are actually the dissolved gasses that make up air coming out of solution. Eventually you get enough energy in there to force water to change state. By the time you get to a rolling boil, it's all water vapor.

Let's see if I can find support for this...

Know the Chemical Composition of Bubbles in Boiling Water

Learn about the chemical composition of the bubbles in boiling water and how they form. Also, learn how to boil water without bubbles!

www.thoughtco.com

What Are the Bubbles in Boiling Water?

Learn what the bubbles in boiling water are and how their composition changes. Also learn how to boil a liquid without having any bubbles.

sciencenotes.org

And as to why cold water can hold more dissolved gas than warmer water? It's because the higher temperature/energy means faster movement of the gas molecules giving them enough zing to escape their liquid prison They will mostly end up stuck to rough surfaces on the inside of the container (formally known as "nucleation sites") until enough of them gather in one spot to form a bubble large enough to break free.

 

[Angelo Farina]

The PADI tables and the wheel--on which I was thoroughly trained when I became an instructor--are not the same thing. You do not "ride the NDL" with the wheel--you plan the multi-level dive ahead of time. The PADI tables also use a square profile. I used them for my example because they were sitting on my desk.

You just wrote:

Again, let's look at my comparison, which you (again) imply using the tables is about the same: "this is not a big deal, it just takes more time for deco stops (and more air, of course)."

Let me repeat that the dive I described, using a computer, was an NDL dive--no required deco stops. Doing the same dive with a square table would put me 60 minutes over the NDL. How much deco time would I need to do and at what depths would I need to do them to make up for 60 minutes of deco? How much more air would I need for that?

You may think going 60 minutes over NDL on a dive and needing more air for the required stops is "no big deal," but I think a lot of people would agree that it is a pretty big deal.

It was an NDL dive according to your computer.
It was a deco dive according to the US Navy table.
As said, I am not trained for evaluating if that dive was within NDL or not using the complex PADI "wheel" tables...
If you use the US Navy tables, you must be prepared and equipped to long deco stops.
There is no workaround, the US Navy tables can only be used safely in one way, taking the total time and considering it spent at max depth.
Which is very simple and quite safe.
The only viable alternative is to use a computer.
I never considered the PADI multi-level method viable for recreational divers, albeit, from a scientific point of view, I admire the research effort undertaken for developing it.
But I admit that I did not study the Padi multi-level method, and, as said, I know very few people who can manage it, and they do not use it in practice. They use a computer, as 99.99% of recreational divers.

 

[pauldw]

Yeah, you're right. Better analogy would be a bottle of Coke, shake it up, and quickly fizzes out all over the place. Whereas, shake it up, and very slowly crack the top allowing the pressure to change very slowly, and the fizz stays under control.

That's how my OW instructor taught us. One lesson I'll always remember.

Gotta go tinker with my PFC system now.