The Computer Between the Ears

[NAM001]

Recreational dive algorithms are not just based on the fastest compartment. If you go to any of them with dive logs that show the compartments leading the dives, and you will see that they change as the diver progresses through the dive.

I would agree as far as repeditive dives go but i find it hard to swallow the idea that on a first dive any compartment will fill before teh fastest one or commpartment #1. In no expert but i can see it happening when you go shallow and you off gas the fast compartment and the slower ones lag the process adn end up with more than the fastest. Such is the conditiion at the end of the SI. Grossly speeking the fastest is again empty and the slower ones can still have gas to shed. I have watched models of compartments in simulated dives and they support that every time. You being a technical instructor guy I will assume that we are not looking at things exactly the same. And surely my views is purely in the rec arena (in line with the OP's area of question) and not in the tech where some slower compartments get saturated. Certainly slower compartments can be the controlling ones but IMO not in the rec diving constraints of the rec dive tables.

 

[NAM001]

Recreational dive algorithms are not just based on the fastest compartment. If you go to any of them with dive logs that show the compartments leading the dives, and you will see that they change as the diver progresses through the dive.

Im not able to dispute that cause im not an in depth data base for that. For what i have seen for dives that remain with in the time duration of the rec dive table the fastest compartment fills first and the slower ones are sequentially filled. My shearwater also indicates that position..

 

[boulderjohn]

I would agree as far as repeditive dives go but i find it hard to swallow the idea that on a first dive any compartment will fill before teh fastest one or commpartment #1. In no expert but i can see it happening when you go shallow and you off gas the fast compartment and the slower ones lag the process adn end up with more than the fastest. Such is the conditiion at the end of the SI. Grossly speeking the fastest is again empty and the slower ones can still have gas to shed. I have watched models of compartments in simulated dives and they support that every time. You being a technical instructor guy I will assume that we are not looking at things exactly the same. And surely my views is purely in the rec arena (in line with the OP's area of question) and not in the tech where some slower compartments get saturated. Certainly slower compartments can be the controlling ones but IMO not in the rec diving constraints of the rec dive tables.

You seem to think that NDL is reached when the fastest compartment is equalized to its depth. That isn't how it works.

A 4 minute compartment is equalized (saturated) when it has gone through 6 halftimes--24 minutes--at that depth (or a combination of that depth and the depths earlier in the dive). If a diver goes to 10 feet and stays there 24 minutes, that compartment is saturated. Is it your opinion that the NDL at 10 feet is 24 minutes?

Here is a video of the tissue bar graph for the Shearwater computer. Watch how the different tissues react to changing depths.

 

[boulderjohn]

Im not able to dispute that cause im not an in depth data base for that. For what i have seen for dives that remain with in the time duration of the rec dive table the fastest compartment fills first and the slower ones are sequentially filled. My shearwater also indicates that position..

They do fill first--but that first compartment alone is not what determines NDL.

 

[kelemvor]

"Why would you want to use a dive computer when you have a perfectly good computer between your ears?"

The above statement makes an assumption that is clearly incorrect: the human brain is incredible, and highly unreliable...

Well, the incorrect assumption was made by a human brain after all. The erroneous statement just serves to highlight the problem.

 

[boulderjohn]

Im not able to dispute that cause im not an in depth data base for that. For what i have seen for dives that remain with in the time duration of the rec dive table the fastest compartment fills first and the slower ones are sequentially filled. My shearwater also indicates that position..

Perhaps I am misreading your posts, but I get the sense that you are misunderstanding the part that is in bold above. If I am wrong, I am sorry, but in that case, what follows will perhaps help others who might have a misconception. The word "fill" is commonly used to describe tissue saturation, but that is not a good word to use because it suggests something that is not true. It suggests each compartment is like a measuring cup with a fill line, and once the fill line is reached, it can hold no more. That is not what is happening.

What happens is that as we inhale and exhale, nitrogen enters and leaves our bodies. Some enters the body with each breath and travels around the body through blood flow and simply following through the tissues themselves. While that is happening, some nitrogen is leaving the body through the reverse process. Unless you have been doing something very unusual at this time of the morning, right now roughly as many nitrogen molecules are entering the body as are leaving it. You are at equilibrium. Your tissues are saturated. All of your compartments are full.

So you strap on some scuba gear, drop into the water, and descend to 99 FSW. You are now inhaling 4 times as many nitrogen molecules as you were at the surface. That means you have times as many molecules entering your tissues as are leaving it. You are on-gassing in all tissues, and some are on-gassing much more rapidly than others, due to the nature of the tissues and the amount of blood flow in them. After 4 minutes at 99 FSW, the 4 minute compartment will have taken on so much nitrogen that they there is only twice as much nitrogen entering the tissues as leaving it. This slows down the speed of on-gassing dramatically--the tissue has reached its half time. After another 4 minutes, the rate of exchange has slowed even more, with 3 nitrogen molecules leaving the tissues for every 4 entering. The on-gassing gets slower and slower until, once again, there are as many molecules leaving the tissue as entering it--the tissue is saturated. It is at equilibrium. It is "full."

As the diver stays at that depth, the other, slower tissues are also on-gassing to various degrees. More and more of them will become saturated. The diver begins to ascend. Now the faster tissues are supersaturated--they have more nitrogen in them than is in the inspired air. They will now begin to lose more nitrogen than they are gaining. They are off-gassing. But not all of them are doing that. The slower tissues are lagging behind, meaning that they still have less nitrogen than the inspired air, and they are still on-gassing. As the diver ascends, more and more compartments begin to off-gas, but some will still be on-gassing all the way to the surface.

The ascending diver must be sure that those tissue pressures are not too much greater than ambient pressure, the ascent must not be too fast. This will prevent or limit bubble creation and growth. Different tissues can tolerate different amounts of supersaturation, and the fastest tissues will tolerate the most. (I once read that if our entire body were composed of tissues with half-times like blood itself, we could ascend directly from 3,000 feet with no harm.) If you look at the Shearwater graph to which I linked above, you will see that the fastest tissues begin to drop rapidly in relation to their significance for decompression as the diver ascends, leaving slower tissues as the ones that are most critical in avoiding decompression sickness.

Once all the tissues are within a safe range in relation to ambient pressure, the diver can safely go to the surface. At that point, all tissues are now supersaturated and will off-gas.

I gave the example at 99 FSW. What if the diver had only gone to 10 FSW? In that case, the 4-minute compartment would be saturated in 24 minutes, just as it was saturated at 24 minutes at 99 FSW--but being saturated at 10 feet is not the same as being saturated at 99 feet. At 10 FSW, the tissues will become saturated one by one, just as they did at 99 FSW. The difference is that NONE of them will ever reach the point that they have too much nitrogen in relation to ambient pressure, so the diver can always go safely to the surface without fear of bubbling.

 

[txgoose]

I gave the example at 99 FSW. What if the diver had only gone to 10 FSW?

As a newbie, I just wanted to thank you. This was a dang nice write up. My concept and mental image of what was going on has been greatly improved.

 

[doctormike]

As a newbie, I just wanted to thank you. This was a dang nice write up. My concept and mental image of what was going on has been greatly improved.

I want to take a moment to make a meta-comment, and to thank John for taking the time to do that writeup.

Too often we get absorbed with the social media aspects of this place, and figure that our main effort is to convince our "opponent" that we are right and to argue our point. We figure that it all ends when the other side capitulates or we get bored. More often, we just say something like "it's your dive, do whatever feels best for you, there is no scuba police".

But this is really a library as much as it is a forum. New divers search and read these threads, often years later, and that's why many experienced divers take the time to set the record straight. If someone promotes an unsafe diving practice on the board, we may not be able to convince them to change their thinking. But if we don't challenge it - respectfully but thoroughly - then those unsafe practices stand alone for any new diver who finds the thread in the future.

 

[NAM001]

You seem to think that NDL is reached when the fastest compartment is equalized to its depth. That isn't how it works.

A 4 minute compartment is equalized (saturated) when it has gone through 6 halftimes--24 minutes--at that depth (or a combination of that depth and the depths earlier in the dive). If a diver goes to 10 feet and stays there 24 minutes, that compartment is saturated. Is it your opinion that the NDL at 10 feet is 24 minutes?

Here is a video of the tissue bar graph for the Shearwater computer. Watch how the different tissues react to changing depths.

not that is not my opinion the 6 half times is the exponential rate of the saturation process to equlize 2 diffferent pressures.

 

[NAM001]

Perhaps I am misreading your posts, but I get the sense that you are misunderstanding the part that is in bold above. If I am wrong, I am sorry, but in that case, what follows will perhaps help others who might have a misconception. The word "fill" is commonly used to describe tissue saturation, but that is not a good word to use because it suggests something that is not true. It suggests each compartment is like a measuring cup with a fill line, and once the fill line is reached, it can hold no more. That is not what is happening.

What happens is that as we inhale and exhale, nitrogen enters and leaves our bodies. Some enters the body with each breath and travels around the body through blood flow and simply following through the tissues themselves. While that is happening, some nitrogen is leaving the body through the reverse process. Unless you have been doing something very unusual at this time of the morning, right now roughly as many nitrogen molecules are entering the body as are leaving it. You are at equilibrium. Your tissues are saturated. All of your compartments are full.

So you strap on some scuba gear, drop into the water, and descend to 99 FSW. You are now inhaling 4 times as many nitrogen molecules as you were at the surface. That means you have times as many molecules entering your tissues as are leaving it. You are on-gassing in all tissues, and some are on-gassing much more rapidly than others, due to the nature of the tissues and the amount of blood flow in them. After 4 minutes at 99 FSW, the 4 minute compartment will have taken on so much nitrogen that they there is only twice as much nitrogen entering the tissues as leaving it. This slows down the speed of on-gassing dramatically--the tissue has reached its half time. After another 4 minutes, the rate of exchange has slowed even more, with 3 nitrogen molecules leaving the tissues for every 4 entering. The on-gassing gets slower and slower until, once again, there are as many molecules leaving the tissue as entering it--the tissue is saturated. It is at equilibrium. It is "full."

As the diver stays at that depth, the other, slower tissues are also on-gassing to various degrees. More and more of them will become saturated. The diver begins to ascend. Now the faster tissues are supersaturated--they have more nitrogen in them than is in the inspired air. They will now begin to lose more nitrogen than they are gaining. They are off-gassing. But not all of them are doing that. The slower tissues are lagging behind, meaning that they still have less nitrogen than the inspired air, and they are still on-gassing. As the diver ascends, more and more compartments begin to off-gas, but some will still be on-gassing all the way to the surface.

The ascending diver must be sure that those tissue pressures are not too much greater than ambient pressure, the ascent must not be too fast. This will prevent or limit bubble creation and growth. Different tissues can tolerate different amounts of supersaturation, and the fastest tissues will tolerate the most. (I once read that if our entire body were composed of tissues with half-times like blood itself, we could ascend directly from 3,000 feet with no harm.) If you look at the Shearwater graph to which I linked above, you will see that the fastest tissues begin to drop rapidly in relation to their significance for decompression as the diver ascends, leaving slower tissues as the ones that are most critical in avoiding decompression sickness.

Once all the tissues are within a safe range in relation to ambient pressure, the diver can safely go to the surface. At that point, all tissues are now supersaturated and will off-gas.

I gave the example at 99 FSW. What if the diver had only gone to 10 FSW? In that case, the 4-minute compartment would be saturated in 24 minutes, just as it was saturated at 24 minutes at 99 FSW--but being saturated at 10 feet is not the same as being saturated at 99 feet. At 10 FSW, the tissues will become saturated one by one, just as they did at 99 FSW. The difference is that NONE of them will ever reach the point that they have too much nitrogen in relation to ambient pressure, so the diver can always go safely to the surface without fear of bubbling.

i agree, and your discription of the amount of N2 and bubble formation is excellenlt. I think we are both on the same page but our wording differs because of our perspective differences. Your wording is more technical, and my wording is more in the laymans terms. Most people have little clue regarding equalized vs saturated vs super saturated in the laboratory use of the words.