Deco thoughts and theory - GI3 - Repetative diving

*Clare Gledhill* · May 15th, 2006, 11:48 PM

Reproduced with permission from DIR UK

Repetitive diving - George Irvine

This area of the discussion needs to be prefaced by a couple of important reminders. One, keep in mind what I said about the level of offgassing in bubble form when the move is made to 1 ATA at the surface - this can be a real shower of bubbles if you do not follow the ascent recommendations that
I made.

Be that as it may, the other important point is the bubbles tend to GROW post dive as they take like gas in from around them. They peak in size and then diminish. This is all relative. If you look at decompression folklore, like what IANTD teaches, you will see references to bubbles peaking in size and frequency up to four hours after a dive. First of all, this applies more to people who should not be diving in the first place, and secondly it applies to air diving where there is no adequate way to decompress.

In my case, I totally clear of any signs of bubbles from the most horrendous dives in 30 minutes or less. That is what you should shoot for , but more importantly, shoot for no bubbles to start with by following my final ascent guidelines. Bubble growth is fact life. Showering bubbles post dive is a screwup.

As far as "residual nitrogen" or helium or whatever, this is total bullshit. It is inherently more true for nitrogen since we are full of it anyway, but the fact is the only consideration you have to give to repets is in the concept of ascent once you have taken a surface interval and may be still bubbling - you have to be far more careful about your ascent rate on a second dive due to this effect.

Otherwise, repetitive diving is a good thing, and you should do your shallower dive first and then your deeper one. The stupidity taught in that regard is beyond the pale.

First, what is the real risk? It is not DCS, it is CNS toxicity. The risk of pulmonary toxicity is also an issue more so than DCS. Repetitive diving needs to be done with this in mind. You do not want to run high ppo2s over and over, and yo u certainly do not want to do multiday diving on high ppo2s. So the first thing we need to do is back off the working ppo2, and plan the decompressions such that we are not accumulating an excess exposure.

If you do you decompression the way I outlined it in the other posts, including the way I ascend to the surface, you will greatly reduce the heavy bubble -form offgasing that generally occurs post-dive. If you are basically clean, you can dive again without penalty. If you are using the correct gas, the "residual" effect is greatly reduced. This effect is more designed to explain accumulation of gases in tissues which are not well perfused and as such tend to trap gas which becomes a battery for supplying gas to formed bubbles later on, so repetitive diving with a gut, or battery which holds gas, could contribute to making any bubbles on the next dive worse and contribute to them growing well after the dive. If you do the decompression for the subsequent dives correctly, there is no reason to belabor the issue. From a logistical standpoint in the ocean, it is far safer to do a couple of back to backs than one long dive which requires a long mandatory decompression.

From a decompression point of view, we have seen that repetitive diving makes no difference, so we ignore the first dive in calculating the second. The only trick is that the second dive should be deeper than or equal to the first, and you can not bounce dive after a dive of any kind. We have done back to back 300's with 60 minute bottom times with no change of deco schedule . In the WKPP we have discontinued that practice due to the oxygen exposure risk, however.

Effects of nitrogen

High partial pressures of nitrogen cause the red cells to become rigid and get hung up in capillaries or damage them. The response of the body to this and the immune response tend to close off the area . Anyone (but a dive instructor) can see what that means for decompression.

In addition, it is so funny to me that these half wits think they are "ok" on air since studies show impairment relative to the norm at even 40 feet on air. The chess experiments are the funniest, where the guy outside the chamber keeps beating the guy inside the chamber at 40 feet, no matter who is in which position.

We found in the WKPP that mistakes were being made on nitrox and air at relatively shallow depths, so we outlawed the use of nitrox as a diving gas or air for any reason. We also found inadequate decompression results from air and nitrox at deeper depths. There is no decompression that works with air. We use helium in most mixes. If you look at cave diving death statistics among "trained" cave divers, you will see the preponderance of 100+ foot dives where nobody seems to know what went wrong.

Narcotic level is easy - any increase is more narcotic than no increase. Impairment starts immediately, and just gets worse. As for rigidity due to nitrogen tension, it appears to be relieved by the presence of helium in the same fashion - the more helium , the less tension. We have found that <30%
is not going to do a whole lot . 25% is the minimum useful range for deco gas, 30 for diving, and then we are talking <120 feet. For all else we go to at least 35 and prefer higher.

After thousands, as in the tens of thousands, of man dive hours in the WKPP doing extreme exposure mixed gas dives over the course of 16 years , and intensely over the last 9 years, these have been our findings.

Decompression gas(es)

I personally think of dives as 100 ft, 150, 200, 250 or 300, and I use gas for 1,2 or 3. The deco gas choice is dependent on the bottom time. While the gases are standard, whether you use them all or not is operational and time dependent. Below 100 is a special case, and beyond 350 is a special case. I use less oxygen because there is no advantage to higher and it 1) limits range ( which you mentioned yourself) , and I don't want a high ppo2 deep. The only reason that deco is shorter if you do the deeper part correctly. If you do not, then you need every bit of a longer shallow hang, and more.v First of all, there is no "comparison" of a 125 air dive to anything from a decompression standpoint since there is no deco that works for that. Forget "air tables" - they are nonsense, as are nitrox "air equivalent" tables. Drop those concepts from your mind. Secondly, use 30% helium or greater - the lower percentages don't do the job. Next, leave the oxygen out of the first dive and take it on the second

Here is what I would do for deco. I am not doing anything different because of the cold - you took care of that by keeping the bottom time reasonable. I am assuming that anyone who dives in the 40's uses C-4 , a proper WKPP type hood and gloves , and argon.

23 bottom time
count all time as if at 240
Your gas or anything close to that deck to
150 - 2 min
140-80 5 min
70 - 3
60 - 3
50 - 3
40 - 5
30 - 8
20 - 15
5 minutes going to the surface from 20

Same for the next dive This schedule will work up to about 25 minutes at the
bottom. On the last two days of diving, switch to the 50% at 60' instead of 0' and do your oxygen at 18 feet or above, maybe splitting it into 18,15,12,9 or something like that.

Your risk in this is exposure and depletion of brain chemistry. Don't party with the mutants and take some vitamin E each day. If you go on the Seeker, take some pictures so we can all have a good laugh. Breaks have to be at less than 20 minutes, and you do not change the total time for them.

About shorter bounce dives

On these real short dives you are not hitting it too hard anyway, but the correct answer is that the break regimen is taken for granted in all of my deco posts since they speed deco and make the process work, as well as make it safer and prevent damage, so I do not specifically mention them in the general schedules. As the dives get longer or more days of diving are done back to back, then you have to start making a point of it. Notice what I said to Ted Green on his second four Doria dives about starting the 50% gas ten feet shallower and starting the oxygen 2 feet shallower and splitting it into decreasing depth increments above 18 feet.

The toggling also takes place by virtue of the reducing ppo2 of each gas in its range of stops. You can see that for dives that require longer deco, the time at the higher ppo2s of each set of gases is greater, so a real break is needed, and then the reduced ppo2 is badly needed to make it all the way through successfully, as well as get all of the effects that I described. This is why when some moron tries to tell you that a constant ppo2 rebreather will "get you out faster" , you can laugh in his face. Funny that Bill Stone's team of wretches and mutants who told me they could do it better and faster ended up doing 24 hour decos to my 6 or 7 hours for half the dive I did and still got burnt lungs, slobittis, bent, injured, and sick.

Post dive practises

We do take a ten minute clean up break to backgas before getting back into the water, and we go to something other than oxygen to move up at one foot per minute to 20, where you can do the rest of the one foot per minute ascent on oxygen or anything else you can breathe safely.

The idea is to get the last of the gas out by gradient gradually. You rapidly diminish the amount of gas being removed by oxygen at one depth as time passes. The rest of the tricks need to be employed as discussed in the earlier.

May 15th, 2006, 11:48 PM	#1 (permalink)
Clare Gledhill(Offline) GUE Instructor Site Admin Pacman Champion! Join Date: Nov 2005 Location: UK Posts: 1,540	Deco thoughts and theory - GI3 - Repetative diving *Reproduced with permission from DIR UK* *Repetitive diving - George Irvine* This area of the discussion needs to be prefaced by a couple of important reminders. One, keep in mind what I said about the level of offgassing in bubble form when the move is made to 1 ATA at the surface - this can be a real shower of bubbles if you do not follow the ascent recommendations that I made. Be that as it may, the other important point is the bubbles tend to GROW post dive as they take like gas in from around them. They peak in size and then diminish. This is all relative. If you look at decompression folklore, like what IANTD teaches, you will see references to bubbles peaking in size and frequency up to four hours after a dive. First of all, this applies more to people who should not be diving in the first place, and secondly it applies to air diving where there is no adequate way to decompress. In my case, I totally clear of any signs of bubbles from the most horrendous dives in 30 minutes or less. That is what you should shoot for , but more importantly, shoot for no bubbles to start with by following my final ascent guidelines. Bubble growth is fact life. Showering bubbles post dive is a screwup. As far as "residual nitrogen" or helium or whatever, this is total bullshit. It is inherently more true for nitrogen since we are full of it anyway, but the fact is the only consideration you have to give to repets is in the concept of ascent once you have taken a surface interval and may be still bubbling - you have to be far more careful about your ascent rate on a second dive due to this effect. Otherwise, repetitive diving is a good thing, and you should do your shallower dive first and then your deeper one. The stupidity taught in that regard is beyond the pale. First, what is the real risk? It is not DCS, it is CNS toxicity. The risk of pulmonary toxicity is also an issue more so than DCS. Repetitive diving needs to be done with this in mind. You do not want to run high ppo2s over and over, and yo u certainly do not want to do multiday diving on high ppo2s. So the first thing we need to do is back off the working ppo2, and plan the decompressions such that we are not accumulating an excess exposure. If you do you decompression the way I outlined it in the other posts, including the way I ascend to the surface, you will greatly reduce the heavy bubble -form offgasing that generally occurs post-dive. If you are basically clean, you can dive again without penalty. If you are using the correct gas, the "residual" effect is greatly reduced. This effect is more designed to explain accumulation of gases in tissues which are not well perfused and as such tend to trap gas which becomes a battery for supplying gas to formed bubbles later on, so repetitive diving with a gut, or battery which holds gas, could contribute to making any bubbles on the next dive worse and contribute to them growing well after the dive. If you do the decompression for the subsequent dives correctly, there is no reason to belabor the issue. From a logistical standpoint in the ocean, it is far safer to do a couple of back to backs than one long dive which requires a long mandatory decompression. From a decompression point of view, we have seen that repetitive diving makes no difference, so we ignore the first dive in calculating the second. The only trick is that the second dive should be deeper than or equal to the first, and you can not bounce dive after a dive of any kind. We have done back to back 300's with 60 minute bottom times with no change of deco schedule . In the WKPP we have discontinued that practice due to the oxygen exposure risk, however. *Effects of nitrogen* High partial pressures of nitrogen cause the red cells to become rigid and get hung up in capillaries or damage them. The response of the body to this and the immune response tend to close off the area . Anyone (but a dive instructor) can see what that means for decompression. In addition, it is so funny to me that these half wits think they are "ok" on air since studies show impairment relative to the norm at even 40 feet on air. The chess experiments are the funniest, where the guy outside the chamber keeps beating the guy inside the chamber at 40 feet, no matter who is in which position. We found in the WKPP that mistakes were being made on nitrox and air at relatively shallow depths, so we outlawed the use of nitrox as a diving gas or air for any reason. We also found inadequate decompression results from air and nitrox at deeper depths. There is no decompression that works with air. We use helium in most mixes. If you look at cave diving death statistics among "trained" cave divers, you will see the preponderance of 100+ foot dives where nobody seems to know what went wrong. Narcotic level is easy - any increase is more narcotic than no increase. Impairment starts immediately, and just gets worse. As for rigidity due to nitrogen tension, it appears to be relieved by the presence of helium in the same fashion - the more helium , the less tension. We have found that <30% is not going to do a whole lot . 25% is the minimum useful range for deco gas, 30 for diving, and then we are talking <120 feet. For all else we go to at least 35 and prefer higher. After thousands, as in the tens of thousands, of man dive hours in the WKPP doing extreme exposure mixed gas dives over the course of 16 years , and intensely over the last 9 years, these have been our findings. *Decompression gas(es)* I personally think of dives as 100 ft, 150, 200, 250 or 300, and I use gas for 1,2 or 3. The deco gas choice is dependent on the bottom time. While the gases are standard, whether you use them all or not is operational and time dependent. Below 100 is a special case, and beyond 350 is a special case. I use less oxygen because there is no advantage to higher and it 1) limits range ( which you mentioned yourself) , and I don't want a high ppo2 deep. The only reason that deco is shorter if you do the deeper part correctly. If you do not, then you need every bit of a longer shallow hang, and more.v First of all, there is no "comparison" of a 125 air dive to anything from a decompression standpoint since there is no deco that works for that. Forget "air tables" - they are nonsense, as are nitrox "air equivalent" tables. Drop those concepts from your mind. Secondly, use 30% helium or greater - the lower percentages don't do the job. Next, leave the oxygen out of the first dive and take it on the second Here is what I would do for deco. I am not doing anything different because of the cold - you took care of that by keeping the bottom time reasonable. I am assuming that anyone who dives in the 40's uses C-4 , a proper WKPP type hood and gloves , and argon. 23 bottom time count all time as if at 240 Your gas or anything close to that deck to 150 - 2 min 140-80 5 min 70 - 3 60 - 3 50 - 3 40 - 5 30 - 8 20 - 15 5 minutes going to the surface from 20 Same for the next dive This schedule will work up to about 25 minutes at the bottom. On the last two days of diving, switch to the 50% at 60' instead of 0' and do your oxygen at 18 feet or above, maybe splitting it into 18,15,12,9 or something like that. Your risk in this is exposure and depletion of brain chemistry. Don't party with the mutants and take some vitamin E each day. If you go on the Seeker, take some pictures so we can all have a good laugh. Breaks have to be at less than 20 minutes, and you do not change the total time for them. *About shorter bounce dives* On these real short dives you are not hitting it too hard anyway, but the correct answer is that the break regimen is taken for granted in all of my deco posts since they speed deco and make the process work, as well as make it safer and prevent damage, so I do not specifically mention them in the general schedules. As the dives get longer or more days of diving are done back to back, then you have to start making a point of it. Notice what I said to Ted Green on his second four Doria dives about starting the 50% gas ten feet shallower and starting the oxygen 2 feet shallower and splitting it into decreasing depth increments above 18 feet. The toggling also takes place by virtue of the reducing ppo2 of each gas in its range of stops. You can see that for dives that require longer deco, the time at the higher ppo2s of each set of gases is greater, so a real break is needed, and then the reduced ppo2 is badly needed to make it all the way through successfully, as well as get all of the effects that I described. This is why when some moron tries to tell you that a constant ppo2 rebreather will "get you out faster" , you can laugh in his face. Funny that Bill Stone's team of wretches and mutants who told me they could do it better and faster ended up doing 24 hour decos to my 6 or 7 hours for half the dive I did and still got burnt lungs, slobittis, bent, injured, and sick. *Post dive practises* We do take a ten minute clean up break to backgas before getting back into the water, and we go to something other than oxygen to move up at one foot per minute to 20, where you can do the rest of the one foot per minute ascent on oxygen or anything else you can breathe safely. The idea is to get the last of the gas out by gradient gradually. You rapidly diminish the amount of gas being removed by oxygen at one depth as time passes. The rest of the tricks need to be employed as discussed in the earlier. __________________ Clare . "Keep away from people who try to belittle your ambitions....Small people always do that, but the really great make you feel that you too can become great." Interested in DIR dive training? Always happy to chat/answer questions so get in touch via PM or visit www.dirdiver.co.uk Last edited by Clare Gledhill; May 15th, 2006 at 11:57 PM.