Deeper 2nd dive - why not?

[Charlie99]

I'm no expert, but common sense tells me that when you make a reverse profile dive, once you surpass the previous dive depth, you will no longer off gas, but will retain the remaining nitrogen left in the tissues.

Wrong.

Thus while keeping that nitrogen content and adding to it with continuing depth, tables are not setup to accomidate all this nitrogen.

Wrong. The table will track it.

With a normal profile dive, you will be off gassing the whole dive until the loaded tissues reach an equality with the pressure experienced at the current depth.

Wrong, for at least some, if not most compartments.

 

[Charlie99]

Your understanding and mine regarding manual tables are the same. The one's we commonly use are limited representations of a complete algorithm. Therefore they cannot accomodate the necessary calculations to safely accomplish a deeper second dive.

That is not my understanding of the tables. Although most agencies do not recommend reverse profiles (2nd dive deeper than first), there is nothing in the tables that keep them from tracking such a dive correctly.

Simply put, tables such as the PADI RDP, USN tables, and the many derivatives of USN tables (YMCA, non-RGBM NAUI tables, SSI, etc.) track only 1 compartment with the pressure groups or repetitive groups. This tracking works for either normal or reverse profile repetitive dives. Faster compartments are not directly tracked, but show up in the tables as a limiting time at each depth (that's why one can reach the highest pressure groups only at shallow depths. Beyond 40' or so, a faster compartment has reached it's maximum allowable value before the one tracked for repetitive groups has reached its limits).

If you apply some logic to the situation, you can see that these faster compartments will offgas more rapidly than the controlling compartment, so that, if one keeps the controlling compartment within the reduced limit of the table for that depth,then the faster compartment is also within its limits.

There may or may not be valid reasons to avoid reverse profiles. Fear that the tables don't work for reverse profiles is not a valid reason.

 

[Jason Ooi]

HiCharlie99,
Good advice.

I always make repetitives dives to shallower than previous dive,this might be allow offgas nitrogen on progressively shallower dives and prevent you from carry progressively larger amount if RN on deeper repetitives dives...

That is not my understanding of the tables. Although most agencies do not recommend reverse profiles (2nd dive deeper than first), there is nothing in the tables that keep them from tracking such a dive correctly.

Simply put, tables such as the PADI RDP, USN tables, and the many derivatives of USN tables (YMCA, non-RGBM NAUI tables, SSI, etc.) track only 1 compartment with the pressure groups or repetitive groups. This tracking works for either normal or reverse profile repetitive dives. Faster compartments are not directly tracked, but show up in the tables as a limiting time at each depth (that's why one can reach the highest pressure groups only at shallow depths. Beyond 40' or so, a faster compartment has reached it's maximum allowable value before the one tracked for repetitive groups has reached its limits).

If you apply some logic to the situation, you can see that these faster compartments will offgas more rapidly than the controlling compartment, so that, if one keeps the controlling compartment within the reduced limit of the table for that depth,then the faster compartment is also within its limits.

There may or may not be valid reasons to avoid reverse profiles. Fear that the tables don't work for reverse profiles is not a valid reason.

 

[ArcticDiver]

That is not my understanding of the tables. Although most agencies do not recommend reverse profiles (2nd dive deeper than first), there is nothing in the tables that keep them from tracking such a dive correctly.

Simply put, tables such as the PADI RDP, USN tables, and the many derivatives of USN tables (YMCA, non-RGBM NAUI tables, SSI, etc.) track only 1 compartment with the pressure groups or repetitive groups. This tracking works for either normal or reverse profile repetitive dives. Faster compartments are not directly tracked, but show up in the tables as a limiting time at each depth (that's why one can reach the highest pressure groups only at shallow depths. Beyond 40' or so, a faster compartment has reached it's maximum allowable value before the one tracked for repetitive groups has reached its limits).

If you apply some logic to the situation, you can see that these faster compartments will offgas more rapidly than the controlling compartment, so that, if one keeps the controlling compartment within the reduced limit of the table for that depth,then the faster compartment is also within its limits.

There may or may not be valid reasons to avoid reverse profiles. Fear that the tables don't work for reverse profiles is not a valid reason.

OK. On reflection you are correct in that one can work the manual tables either way.

Let us say that a diver doing the deep dive last ends up with more nitrogen in their body than if they did the shallow dive last. As a matter of fact that will probably be the case. So what? The world doesn't end at the conclusion of the dive day. It merely means that the diver off gases on the beach instead of on the dive boat or at the dive site. Frankly, there are some real advantages to that. If a boat dive there is an economic advantage to the boat operator that the diver is off gassing on the diver's time instead of the boat's. For the boat diver it means off gassing on the shore doing something else productive instead of hanging out on the boat. In any case it shortens the dedicated dive time since people can do other things while off gassing.

 

[Slipstrm]

http://www.baue.org/library/irvine_baue_talk.html#Bounce_diving

Good answers to the question...
George: The lungs are and incredible filter of all kinds of garbage. Fat globules, tissue damage that breaks away, all kinds of other things get caught here. Bubbles get stopped in the lungs. The capillary beds of the lungs get so small it stops them. It's one area where if you do block the flow of blood, since it's still being directly exposed to oxygen it doesn't damage the lung tissue. If you block the flow of blood some place else that doesn't have a supply of oxygen — it dies. If you treat somebody with oxygen right away — especially at an elevated partial pressure where oxygen is dissolved in the fluids of the blood and in the tissues and you can surround an area with oxygen — you can keep it alive. But effectively this area right here in the lungs is always exposed. It filters out the bubbles and allows you to off-gas in bubble form. In fact, when you get out of the water you are generally off-gassing violently at your last step, say, 10ft to the surface. That last decompression step is huge. Because that's how you get the last of the gas out, by relieving that pressure. You're bubbling into the bloodstream and the lungs are catching it.

If the lungs don't catch the bubbles or they get around the lungs you get central nervous system hits, cerebral hits and spinal hits. Bubbles will not come out of tissue into arteries. Arteries are a working mechanism — tubes. They'll end in capillaries. The capillary areas are where the arteries turn to veins and get into bigger and bigger and bigger vessels going back to the heart and lungs. Whereas the arteries coming from the heart are the bigger vessels going into smaller and smaller ones. So if you send a bubble down through the arteries it's going to go until it catches something and then it's going to block the surrounding tissue. Generally the first place blood goes right out of the heart is right up here in the brain and to the spine. The brain, spine, and heart. The spine and brain are where that bubble catches usually. So you get these spinal hits and brain hits and they're instantaneous if bubbles get by. Bubbles can get by if you had larger vessels some place in your lung matrix. Then it would just go right by. People with that defect can't dive at all — they get hit [snaps fingers]. They look at water and they're bent. There's no way around it. You can't give them anything that will stop it. The other defect is the valve across the atria of your heart, simply because it's open before you're born. This is because in the womb (A) you're not breathing, and (B) you're getting all your oxygen from the placenta. Once you're born the lungs are used and the left side pressure increases and pushes the valve shut. Then the blood goes from the right side to the lungs and back through the left side. If that hole doesn't heal up — in roughly 30% of the population it's either somewhat open or opens or can open or there's a defect. If it's just an ongoing hole you won't be able to dive because you'll get bent every time. If it's just a mild one you may never know about it until 2000 dives later and then you get wheelchair bent.

It's a good idea to get tested for it. The other thing is to treat yourself as if you have a PFO all the time. In other words, don't do anything to press the bet. In other words you don't get out of the water from 50ft. You don't ascend straight-line to the surface. You ascend slowly over the last little bit so that you give a chance for that gas coming into the blood stream enters in a nonviolent fashion while there's still pressure on you and you still compressed somewhat. Get that gas out of the tissues with some pressure on you. You don't just let it fly. Because if something gets by, it's going to be bad. So you just follow some general cautionary things. You don't exert right away. It's best just to lay around on the surface of the water. You don't want to be bending over or coughing or doing anything that would press against the heart in any way. Picture the heart with a vertical wall between the atria. You do not want to cause pressure to be put on that wall or along its length either, since this will flex the wall and allow any unsealed flap to open. You don't want to tempt fate. People that get central nervous system hit bent are getting it that way.

The other way to get it to do a dive and then to another quick dive down and come back up again. In other words, you've done a dive, you're getting out of the water and you are bubbling. Now you remember, "Oh, I left my oxygen bottle at 20ft." So you jump down with your mask on, grab the oxygen bottle and come right back up. Well, as you go down you compress the bubbles that are coming on the venous side enough to get by the heart, by the lungs. It just takes you a couple seconds to go down, a few seconds to go back up. Now they're expanding on the arterial side, and lodging in tissue. That's how we bent a bunch of support divers. That's how we found out about it. These guys didn't even do a dive, they'd be in a chamber. All he'd done is get oxygen bottles and be bent like a pretzel from that bouncing. Free diving after a dive, that's a classic one. Bent free diving. Central nervous system hit free diving.

 

[airsix]

Awesome post, Slipstream.

-Ben M.

ps - found great info on wikipedia regarding the atrial valve defect. Even has a paragraph about DCS risk under 'Associated Conditions'. Here's the link.

 

[daniel f aleman]

George aught to know - he's been bent more thatn most. Still, I dive the deeper dive first; and on my second dive, I off-gas a 10' for as long as possible.

 

[Jason Ooi]

Awesome......

http://www.baue.org/library/irvine_baue_talk.html#Bounce_diving

Good answers to the question...
George: The lungs are and incredible filter of all kinds of garbage. Fat globules, tissue damage that breaks away, all kinds of other things get caught here. Bubbles get stopped in the lungs. The capillary beds of the lungs get so small it stops them. It's one area where if you do block the flow of blood, since it's still being directly exposed to oxygen it doesn't damage the lung tissue. If you block the flow of blood some place else that doesn't have a supply of oxygen — it dies. If you treat somebody with oxygen right away — especially at an elevated partial pressure where oxygen is dissolved in the fluids of the blood and in the tissues and you can surround an area with oxygen — you can keep it alive. But effectively this area right here in the lungs is always exposed. It filters out the bubbles and allows you to off-gas in bubble form. In fact, when you get out of the water you are generally off-gassing violently at your last step, say, 10ft to the surface. That last decompression step is huge. Because that's how you get the last of the gas out, by relieving that pressure. You're bubbling into the bloodstream and the lungs are catching it.

If the lungs don't catch the bubbles or they get around the lungs you get central nervous system hits, cerebral hits and spinal hits. Bubbles will not come out of tissue into arteries. Arteries are a working mechanism — tubes. They'll end in capillaries. The capillary areas are where the arteries turn to veins and get into bigger and bigger and bigger vessels going back to the heart and lungs. Whereas the arteries coming from the heart are the bigger vessels going into smaller and smaller ones. So if you send a bubble down through the arteries it's going to go until it catches something and then it's going to block the surrounding tissue. Generally the first place blood goes right out of the heart is right up here in the brain and to the spine. The brain, spine, and heart. The spine and brain are where that bubble catches usually. So you get these spinal hits and brain hits and they're instantaneous if bubbles get by. Bubbles can get by if you had larger vessels some place in your lung matrix. Then it would just go right by. People with that defect can't dive at all — they get hit [snaps fingers]. They look at water and they're bent. There's no way around it. You can't give them anything that will stop it. The other defect is the valve across the atria of your heart, simply because it's open before you're born. This is because in the womb (A) you're not breathing, and (B) you're getting all your oxygen from the placenta. Once you're born the lungs are used and the left side pressure increases and pushes the valve shut. Then the blood goes from the right side to the lungs and back through the left side. If that hole doesn't heal up — in roughly 30% of the population it's either somewhat open or opens or can open or there's a defect. If it's just an ongoing hole you won't be able to dive because you'll get bent every time. If it's just a mild one you may never know about it until 2000 dives later and then you get wheelchair bent.

It's a good idea to get tested for it. The other thing is to treat yourself as if you have a PFO all the time. In other words, don't do anything to press the bet. In other words you don't get out of the water from 50ft. You don't ascend straight-line to the surface. You ascend slowly over the last little bit so that you give a chance for that gas coming into the blood stream enters in a nonviolent fashion while there's still pressure on you and you still compressed somewhat. Get that gas out of the tissues with some pressure on you. You don't just let it fly. Because if something gets by, it's going to be bad. So you just follow some general cautionary things. You don't exert right away. It's best just to lay around on the surface of the water. You don't want to be bending over or coughing or doing anything that would press against the heart in any way. Picture the heart with a vertical wall between the atria. You do not want to cause pressure to be put on that wall or along its length either, since this will flex the wall and allow any unsealed flap to open. You don't want to tempt fate. People that get central nervous system hit bent are getting it that way.

The other way to get it to do a dive and then to another quick dive down and come back up again. In other words, you've done a dive, you're getting out of the water and you are bubbling. Now you remember, "Oh, I left my oxygen bottle at 20ft." So you jump down with your mask on, grab the oxygen bottle and come right back up. Well, as you go down you compress the bubbles that are coming on the venous side enough to get by the heart, by the lungs. It just takes you a couple seconds to go down, a few seconds to go back up. Now they're expanding on the arterial side, and lodging in tissue. That's how we bent a bunch of support divers. That's how we found out about it. These guys didn't even do a dive, they'd be in a chamber. All he'd done is get oxygen bottles and be bent like a pretzel from that bouncing. Free diving after a dive, that's a classic one. Bent free diving. Central nervous system hit free diving.

 

[boulderjohn]

I read an interesting article about four years ago--I think--on this topic, and I just failed to find it. However, a Google search uncovered a lot of other articles, and just about every one said that all research indicates that there is no reason that a second dive cannot be deeper than the first.

So where did the concept come from originally?

The article I can't find, as I recall it, said that a group of people studied the literature to find the first mention of reverse profiles. The first one they could find was in a PADI manual, and I believe it was a 1972 publication. PADI cooperated in the study, apparently, and they could find nothing in their records to indicate who put that concept into the manual, or why.

In other words, no one knows who first said that we should always do the deeper dives first, and no one knows why that person believed it.

 

[ArcticDiver]

So, in sort of an Interim Summary:
-Nitrogen loading is one factor contributing to DCS.
-The wise diver will monitor nitrogen loading and plan dives to keep the loading within accepted limits.
-It really doesn't matter in what sequence dives are done; deep-shallow, shallow-deep, deep-deep, shallow-shallow, as long as nitrogen loading is accounted for in dive planning.
-Dive Planning doesn't begin and end with the dive sequence. It begins when the diver begins preparation for diving and ends when all effects of any dives have ceased to affect the diver.
-The diver should not forget other factors that affect the diver's health and wellbeing.