But remember, freedivers are spending several minutes (10 in this case) at the surface off-gassing between each 1:00 period of bottom time (the other 1:30 or so being spent in transit). So I don't know how useful it is to compare with scuba tables. On the other hand, ascent rates are very fast.
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DCS type 1
- Thread starterMemo
- Start date
Thread Status: Hello
, There was no answer in this thread for more than 60 days.
It can take a long time to get an up-to-date response or contact with relevant users.
It can take a long time to get an up-to-date response or contact with relevant users.
From my experience and calculations, off-gassing during surface intervals (for freedivers) is pretty much negligible. For example, even using a standard scuba diving algorithm, a single freedive to 20m for 2'00" results in a nitrogen load which will take 3 hours to completely off-gas. By the time I am done a recreational diving session, the computer and algorithm tell me that it will take more than 24 hours to completely eliminate the nitrogen. When we are talking about hours and hours to off-gas, a surface interval of 1-8 minutes is negligible.
Since off-gassing is negligible, and ascent rates are drastically faster, I would say that a freediver has an even shorter no-decompression limit than a scuba diver, for a given depth.
The typical scuba NDL for a 40m dive is 5 minutes, and according to my model and experience you only get about 2 long (2:30) freedives dives to 40m before you start to enter a minor danger zone.
Since off-gassing is negligible, and ascent rates are drastically faster, I would say that a freediver has an even shorter no-decompression limit than a scuba diver, for a given depth.
The typical scuba NDL for a 40m dive is 5 minutes, and according to my model and experience you only get about 2 long (2:30) freedives dives to 40m before you start to enter a minor danger zone.
Are you treating whole dives as bottom times? A 2:00 bottom time @20m is pretty extreme.
What do SCUBA tables say about 30m for example - how much bottom time on air? I haven't kept logs but I've probably spent 30-40 minutes at 30m (~1:00 bottom time per dive) per day on a fairly regular basis. Possibly quite a lot more on occasion. Same goes for hundreds or thousands of spearos around the world. If you had that many SCUBA divers doing the same bottom times with no deco, surely there'd be quite a few serious DCS cases. More than we see in freedivers anyway.
What do SCUBA tables say about 30m for example - how much bottom time on air? I haven't kept logs but I've probably spent 30-40 minutes at 30m (~1:00 bottom time per dive) per day on a fairly regular basis. Possibly quite a lot more on occasion. Same goes for hundreds or thousands of spearos around the world. If you had that many SCUBA divers doing the same bottom times with no deco, surely there'd be quite a few serious DCS cases. More than we see in freedivers anyway.
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Well, my buddies are in the town again and we managed to have a deep session again. This time i limited deep dives to 5 and total number of dives around 20 and drank much more water than usual. Although i didn't try an apnea deco the result was really nice! No pain at all! Thanks for all the advices.
I agree that you are taking a significant DCS risk with that kind of diving. In theory you could use a dive computer designed for scuba, assuming it doesn’t lock you out, but the physiology of breath-hold diving is different. The diving response lowers cardiac output, and vasoconstriction should reduce tissue nitrogen loading, but blood flow to the brain increases, as does carbon dioxide level. How this alters DCS risk is not entirely known. It could lower risk of limb bends relative to open circuit dives but increase risk of CNS hits.
DISCLAIMER: What follows are model estimates only and not validated for free diving. Do not rely on these numbers to keep you safe. I accept no liability for anyone getting bent. The DCIEM risk model, the basis of the Canadian dive tables, is based on a moderately conservative and respected algorithm that gives fairly safe No-D times. The nice feature of this kind of probabilistic model is that the output is not just a hard limit telling you when to stop or decompress, but gives a percent risk probability of DCS. Calibration is based on open circuit air chamber dives.
Assume up to 20 free dives of 2.5 minutes each, triangular profiles down and up with no time spent on the bottom, and 5, 10, or 20 minute surface intervals (SI). Here is what percent P(DCS) you get for number of dives running the model. Be aware that apnea physiology could alter these risks in ways unknown. The model could also be under-estimating more subtle signs of DCS.
SI = 20 min
Dives: 1 5 10 15 20
30 m 0.0 0.0 0.0 0.0 0.0
40 m 0.0 0.0 0.1 0.2 0.4
50 m 0.0 0.1 0.5 1.3 2.9
SI = 10 min
Dives: 1 5 10 15 20
30 m 0.0 0.0 0.0 0.1 0.3
40 m 0.0 0.1 0.6 1.4 2.8
50 m 0.0 0.4 1.8 4.4 9.5
SI = 5 min
Dives: 1 5 10 15 20
30 m 0.0 0.1 0.3 0.8 1.5
40 m 0.0 0.2 1.1 2.6 5.2
50 m 0.0 0.6 2.5 6.1 12.2
You might just be lucky so far. You can see the nonlinear effect of increasing risk with both depth and number of dives. Either cut back or take a long surface interval after a few dives. The model in its present form does not factor in oxygen breathing or decompression.
DISCLAIMER: What follows are model estimates only and not validated for free diving. Do not rely on these numbers to keep you safe. I accept no liability for anyone getting bent. The DCIEM risk model, the basis of the Canadian dive tables, is based on a moderately conservative and respected algorithm that gives fairly safe No-D times. The nice feature of this kind of probabilistic model is that the output is not just a hard limit telling you when to stop or decompress, but gives a percent risk probability of DCS. Calibration is based on open circuit air chamber dives.
Assume up to 20 free dives of 2.5 minutes each, triangular profiles down and up with no time spent on the bottom, and 5, 10, or 20 minute surface intervals (SI). Here is what percent P(DCS) you get for number of dives running the model. Be aware that apnea physiology could alter these risks in ways unknown. The model could also be under-estimating more subtle signs of DCS.
SI = 20 min
Dives: 1 5 10 15 20
30 m 0.0 0.0 0.0 0.0 0.0
40 m 0.0 0.0 0.1 0.2 0.4
50 m 0.0 0.1 0.5 1.3 2.9
SI = 10 min
Dives: 1 5 10 15 20
30 m 0.0 0.0 0.0 0.1 0.3
40 m 0.0 0.1 0.6 1.4 2.8
50 m 0.0 0.4 1.8 4.4 9.5
SI = 5 min
Dives: 1 5 10 15 20
30 m 0.0 0.1 0.3 0.8 1.5
40 m 0.0 0.2 1.1 2.6 5.2
50 m 0.0 0.6 2.5 6.1 12.2
You might just be lucky so far. You can see the nonlinear effect of increasing risk with both depth and number of dives. Either cut back or take a long surface interval after a few dives. The model in its present form does not factor in oxygen breathing or decompression.
I would have to strongly object to these tables -- both from my own experience and also the experience of others. Taking Herbert's worst DCS, 9 dives to 35-45m, 2'30"-3'00" each, 6'30" surface intervals, and total paralysis afterwards. My worst DCS was on 8 dives, 30-51m, 8'00" intervals, dive time around 2'10" to 2'30" each. I had another bad DCS on 9 dives, 25-38m, 2'44" average dive time, 6'50" average interval. All of these profiles would be showing up in the 0.5-2% chance of DCS on the above tables, whereas it is probably more like 80-90% for most people.
Using the model I programmed into the Xen computer, on the 2nd table, three dives to 40m with 5 minute intervals and 2'30" dive times and you will be in violation already, but mildly.
It is true that you absorb less nitrogen during freediving due to slower pulse etc., but there are several huge problems:
1. Ascent rate is way faster than scuba causing way more bubble formation
2. Descent rate is way faster than scuba which according to VPM bubble model actually worsens bubble formation compared to a slow descent
3. Multiple ascent/descent cycles cause strange bubble formation, bubble crush mechanics. Many scuba computers add increasing conservatism on repetitive dives (even just 2 or 3 dives), whereas after 30 repetitive dives the added conservatism would be enormous, and for good reason.
4. Most scuba divers do a 2 or 3 minute safety stop at 5m which alone will prevent most cases of DCS, whereas freedivers make no such stop and ascend quickly in the last critical meters.
5. Blood shift means that nitrogen can absorb into certain areas of the body, and then get trapped, since lack of blood flow means off-gassing will be minimal
Putting it all together, any freediving model would need to be drastically more conservative than a scuba model, for the same degree of safety -- even if only for the extreme ascent speed and lack of safety stop. In my case I started with an existing algorithm and tweaked the conservatism until it matched existing DCS cases known from freedivers, and so far it has been working well.
I would comment that on my worst DCS hit (where I ended up in the chamber), I followed an existing freediving 'table', which stated that for the dives I was doing, an 8 minute surface interval was adequate, which turned out to be false.
I've become so interested in this subject that I'm trying to go back 50 years to when I was taught how to make my own tables longhand. Speed seems to be almost a non factor, if you ascend and descend at the same constant rate, at least for the first dive. Then you will also spend ages in the last third coming up. Eric has already covered that when he discussed free dive decomp stops.
Eric,
Have you considered publishing your observations and the resulting model in a scientific journal? That way you can get a peer review and the information would be out there for interested people to refer to. If the study (and the resulting model) is significantly different from the standard scuba model then this must surely be at least of academic interest to mainstream dive medicine.
I'd love to see the instances you refer to plotted against available scuba tables and against findings of studies carried out on the divers in the Tuamotos and others on breathold diving DCS. I'm an engineer at heart and like to see graphs!
Also, I was thinking, if an empirical correction is required to established models (perhaps, as you suggest for the rapid ascent and short surface intervals of freediving) deeper blue would be a good place to find the people to appear in the database.
PS - Not me, I'm not in that league!
Phil C
aucklandfreediving.co.nz
Have you considered publishing your observations and the resulting model in a scientific journal? That way you can get a peer review and the information would be out there for interested people to refer to. If the study (and the resulting model) is significantly different from the standard scuba model then this must surely be at least of academic interest to mainstream dive medicine.
I'd love to see the instances you refer to plotted against available scuba tables and against findings of studies carried out on the divers in the Tuamotos and others on breathold diving DCS. I'm an engineer at heart and like to see graphs!
Also, I was thinking, if an empirical correction is required to established models (perhaps, as you suggest for the rapid ascent and short surface intervals of freediving) deeper blue would be a good place to find the people to appear in the database.
PS - Not me, I'm not in that league!
Phil C
aucklandfreediving.co.nz
I don’t claim that the numbers I presented represent valid free diving limits. This model is merely a benchmark that matches a large database of air chamber dives, most of longer duration. Free dives are much shorter and highly repetitive, presumably loading faster compartments to a higher degree than typical air dives. In industry this is called yo-yo diving. There is very little calibration DCS data for very short deep dives in the regime of free dives. The DCS risk analysis I did for single no-limits dives was based on 445 submarine escape trials with 10 cases of DCS, which are single dives of similar time and depth as NL exposures, but of course not the same type of dives. Although the same algorithm is used, the model parameters I derived for NL diving are different from the air chamber dives. This is because the calibration process for each model is specific to type of diving, in terms of depths and time regimes. There is not yet any single DCS risk model that covers deep short bounce dives right up to saturation dives. These various models can be considered boundary conditions when kept in context.
The bottom line is that data specifically of BH diving incidents is needed. Accurate dive profiles are also essential. Bill’s comment on dive profile is right. 10 triangle dives to 50 m with zero time on the bottom and SI 10 min gives a predicted risk equivalent to 10 square dives of same duration to 27 m with all time spent on the bottom. The shape of the dive profile can make a big difference on effective depth.
Here is a proposal:
I would ask that for each known incident, a small report be written, even if just a paragraph outlining all possible dive and symptom data. This should include details of all preceding dive profiles that day, any prior health concerns that day or recent illnesses, and most importantly time course and description of all symptoms: what was felt (most important), how long it lasted. What kind of therapy was initiated (less important) and response. As much detail as possible, particularly to facilitate reconstructing gas loading and trying to define symptom syndromes. Anecdotes are interesting, but must meet a standard that should be established. There may alternate explanations in some cases. Sorry but I am very fussy on this point. I would recommend we establish a means of reporting that would allow a database of incidents to be built.
Some divers might prefer not to discuss their incidents. I don’t know if is a factor, but strictly speaking it is personal information and there is a right to confidentiality. Some high-profile competitors might worry about their public image. Personally I don’t see it that way, as there is a large random component to DCS, and experiencing something does not necessarily reflect any physiological weakness. Reporting may ultimately benefit a greater good.
In a perfect world we would have dive profiles recorded, any symptoms assessed and documented according to a standardized protocol to include specific. These would be reviewed by some panel with a diversity of expertise, and entered into a database. As a large group, we do have the resources to carry this out. I am certainly interested and willing to help.
For comparison, DAN set out 20 years ago to collect scuba profiles and DCS cases. They have something approaching 200,000 dives on record, a massive effort over decades. But now they can build and validate very good risk models specific for scuba diving.
The other thing DAN did was to train their data collectors how to do standardized physical exams to remove arbitrary descriptions of symptoms. For example to say that a diver had “total paralysis” is just not helpful. In medicine we don’t see people having total paralysis unless they are either in a coma or have some rare locked-in syndrome from a brainstem stroke. It is not a useful description. There is much more to a neurological exam, and clarity and precision is what helps lead to a diagnosis.
As an aside:
The problem with adding correction factors to make models more conservative is that none of this is ever evidence-based. It is completely arbitrary. I remember when deco computers started coming out and the manufacturers were advertising that you could correct for water temperature and workloads. Marketing strategy because they would claim greater safety, but no one would really want to buy a computer that gave the shortest dives. How is a user to know what is an appropriate correction factor when scientists don’t know either.
Rapid descent and ascent rates probably do affect DCS risk, but this is difficult to quantify. In terms of micronuclei, work of Yount and others that led to the VPM concept suggests a protective effect from fast descents. In theory gas nuclei in tissues are compressed on descent causing their elastic surfactant skins to become less permeable to inward gas diffusion, thereby making them more resistant to growing into macroscopic bubbles following decompression. Yount showed a positive linear relation between crushing pressure (ambient minus tissue gas tension) and tolerable supersaturation in gelatin specimens that was later replicated in shrimps, salmon, and rats suggesting generality. Fast ascents probably do increase DCS risk, but does this mean that a fast descent buys you some protection to allow a fast ascent independent of gas load? Do the effects cancel each other? Not known.
The bottom line is that data specifically of BH diving incidents is needed. Accurate dive profiles are also essential. Bill’s comment on dive profile is right. 10 triangle dives to 50 m with zero time on the bottom and SI 10 min gives a predicted risk equivalent to 10 square dives of same duration to 27 m with all time spent on the bottom. The shape of the dive profile can make a big difference on effective depth.
Here is a proposal:
I would ask that for each known incident, a small report be written, even if just a paragraph outlining all possible dive and symptom data. This should include details of all preceding dive profiles that day, any prior health concerns that day or recent illnesses, and most importantly time course and description of all symptoms: what was felt (most important), how long it lasted. What kind of therapy was initiated (less important) and response. As much detail as possible, particularly to facilitate reconstructing gas loading and trying to define symptom syndromes. Anecdotes are interesting, but must meet a standard that should be established. There may alternate explanations in some cases. Sorry but I am very fussy on this point. I would recommend we establish a means of reporting that would allow a database of incidents to be built.
Some divers might prefer not to discuss their incidents. I don’t know if is a factor, but strictly speaking it is personal information and there is a right to confidentiality. Some high-profile competitors might worry about their public image. Personally I don’t see it that way, as there is a large random component to DCS, and experiencing something does not necessarily reflect any physiological weakness. Reporting may ultimately benefit a greater good.
In a perfect world we would have dive profiles recorded, any symptoms assessed and documented according to a standardized protocol to include specific. These would be reviewed by some panel with a diversity of expertise, and entered into a database. As a large group, we do have the resources to carry this out. I am certainly interested and willing to help.
For comparison, DAN set out 20 years ago to collect scuba profiles and DCS cases. They have something approaching 200,000 dives on record, a massive effort over decades. But now they can build and validate very good risk models specific for scuba diving.
The other thing DAN did was to train their data collectors how to do standardized physical exams to remove arbitrary descriptions of symptoms. For example to say that a diver had “total paralysis” is just not helpful. In medicine we don’t see people having total paralysis unless they are either in a coma or have some rare locked-in syndrome from a brainstem stroke. It is not a useful description. There is much more to a neurological exam, and clarity and precision is what helps lead to a diagnosis.
As an aside:
The problem with adding correction factors to make models more conservative is that none of this is ever evidence-based. It is completely arbitrary. I remember when deco computers started coming out and the manufacturers were advertising that you could correct for water temperature and workloads. Marketing strategy because they would claim greater safety, but no one would really want to buy a computer that gave the shortest dives. How is a user to know what is an appropriate correction factor when scientists don’t know either.
Rapid descent and ascent rates probably do affect DCS risk, but this is difficult to quantify. In terms of micronuclei, work of Yount and others that led to the VPM concept suggests a protective effect from fast descents. In theory gas nuclei in tissues are compressed on descent causing their elastic surfactant skins to become less permeable to inward gas diffusion, thereby making them more resistant to growing into macroscopic bubbles following decompression. Yount showed a positive linear relation between crushing pressure (ambient minus tissue gas tension) and tolerable supersaturation in gelatin specimens that was later replicated in shrimps, salmon, and rats suggesting generality. Fast ascents probably do increase DCS risk, but does this mean that a fast descent buys you some protection to allow a fast ascent independent of gas load? Do the effects cancel each other? Not known.
so, after all, can somebody come up with simple-stupid table, based on what is believed to be fairly safe? Some kind of rough guidance.
Another thing I am thinking about - instead of deco-dives after each deep dive, wouldn't it help to simply taper off depth in each consequtive dive? Like for example, move to shallower spot and continue fishing at 10-15 meters of depth and such?
collecting massive database would sure include some odd-balls. And "safest" algorithm from such database would be impractical to most users. I am thinking, how far serious DCS case is from a minor one? Or rather, how safe are minor DCS cases? Can DCS sympthoms be used for building some sort of custom-cut table? This is effectively what Memo just did anyway, right?
Maybe some of those oddballs cases have already been seen in freediving, and are allegedly happening at exposures that are far short of predictions from existing models extrapolated from other types of diving. This is what has been suggested. That is why I am saying we need to look objectively at those cases in the past, and any cases in the future, and come to a consensus as to what they are. I am not denying they are DCS, just let’s take a scientific approach and collect information and put it into perspective. If weird things are happening and free divers are experiencing paralysis, visual changes, blackouts etc, I don’t think we should ignore it. If there is enough data we can determine if these are totally random events, or if there is dose-response relationship to the dive exposure that fits our expectation from established nitrogen models. Are these cases unique to free diving? Is there something about free diving physiology that increases susceptibility? Are these events really outliers, or should they be expected at a certain probability if we do enough dives or go deep enough. Can’t answer that yet. I am suggesting: (1) define what these cases are - what is the disease, and (2) use statistical modeling to see if free diving DCS cases, assuming that’s what they are, tend to occur inside or outside of expected boundaries. The tools exist to do this.
One problem is that divers are discouraged from sharing any DCS incidents because the AIDA medical form for competitions asks the diver 'have you ever suffered DCS?' and if the answer is 'yes', then the diver is disqualified from competing in competitions.
I have already several times posted the rules of thumb that my model/data produced, but each time people criticize it as excessively conservative.
Personally when I do sprint ascents (1.5 - 1.7m/s), it is unbelievably easy to produce symptoms. Two bounces to 40m with sprint ascents reliably produces tingling/numbness in my fingers. Looking at a single dive with sprint ascent, less than 60-62m I get no symptoms; 63m or more and I (usually) get at least tingling/numbness in my fingers combined with pain in my knuckles/elbows, as well as 'sparking' in my skin all over my body, combined with exhaustion. 71m with a sprint ascent followed by O2 deco (5m for 5min) produced no symptoms. 100m with a slower ascent and O2 deco (5m/5min) produced no symptoms. 104m with a slower ascent (1.2-1.3m/s) with O2 deco (5min at 5m) produced pain in my ankles and pains in the 'knuckles' of my toes. Two dives to 71m with slow ascents and a 12min interval caused tingling/numbness in my fingers. Enough repeated dives in the 35-50m range will also cause tingling/numbness in fingers/toes, pain in elbows, knuckles, skin sparking and exhaustion/weakness.
The generally accepted rule for single dives with moderate ascents is that 70m or less requires no post-dive deco. More than 70m produces some risk and deco should be done. In my case I have done as much as 88m on a single dive with no post-dive deco and escaped without any symptoms, but again there is a luck factor as well, I might not be able to do that every time.
Herbert did a single dive to 110m with a rather slow ascent and no post-dive deco and got major symptoms.
My friend Tyler did two no-fins dives to 35m followed by one dive with fins to 71m (no deco) and got significant symptoms very similar to mine.
Looking at Herbert's recreational profiles with symptoms and comparing them to mine, it certainly appears that 9 long deep dives (35-50m) is too much and extremely likely to cause DCS. Memo recently said he tried reducing it to just 5 such dives and had no symptoms. In my case five long deep dives (35-50m) will produce symptoms most of the time but not always.
In 2005 during recreational diving I started slowing my ascent in the last 10m and even trying to stop at 6m for a few seconds if possible, and if not possible then to go back down and do apnea deco for 30-60 seconds right afterwards. This allowed me to drastically violate my previous rules and still escape without symptoms, and Memo agreed that he too was able to escape symptoms using this method. However I do know that this method is not invincible. In the Bahamas in November 2008, several times I tried doing four dives 55-65m, each time doing 'apnea deco' after each dive (around 10-15min between dives), and still got minor symptoms. However I was able to do seven dives to 40m (apnea deco after each one) without symptoms.
The symptoms I got after the 104m with O2 deco show (to me) that for deep enough dives O2 deco at 5m for 5min is not enough. My current model calls for a first deco stop of 9m for such a dive, which agrees with my intuition, that I need to go deeper to crush the bubbles and dissolve them again, then ascend up to 6m later-- this of course means that the deco mix must be less than 100% since 9m is too deep for O2.
So for single dives with moderate ascents (<= 1.3m/s), I would say:
< 70m, no deco needed
70m-90m, O2 at 5-6m for 5min
> 90m, 80% O2 at 9min for 2min, followed by 80% O2 at 6m for 5min
For repetitive dives I would say that if no deco of any kind is to be done, then two long dives to 40m is already the limit if you want total safety. You might get away with as many as 5-6 but this is pushing it. Nine dives and you are likely toast. If you do apnea deco after each dive you may be able to get away with extra dives but eventually you will still get bent.
I have already several times posted the rules of thumb that my model/data produced, but each time people criticize it as excessively conservative.
Personally when I do sprint ascents (1.5 - 1.7m/s), it is unbelievably easy to produce symptoms. Two bounces to 40m with sprint ascents reliably produces tingling/numbness in my fingers. Looking at a single dive with sprint ascent, less than 60-62m I get no symptoms; 63m or more and I (usually) get at least tingling/numbness in my fingers combined with pain in my knuckles/elbows, as well as 'sparking' in my skin all over my body, combined with exhaustion. 71m with a sprint ascent followed by O2 deco (5m for 5min) produced no symptoms. 100m with a slower ascent and O2 deco (5m/5min) produced no symptoms. 104m with a slower ascent (1.2-1.3m/s) with O2 deco (5min at 5m) produced pain in my ankles and pains in the 'knuckles' of my toes. Two dives to 71m with slow ascents and a 12min interval caused tingling/numbness in my fingers. Enough repeated dives in the 35-50m range will also cause tingling/numbness in fingers/toes, pain in elbows, knuckles, skin sparking and exhaustion/weakness.
The generally accepted rule for single dives with moderate ascents is that 70m or less requires no post-dive deco. More than 70m produces some risk and deco should be done. In my case I have done as much as 88m on a single dive with no post-dive deco and escaped without any symptoms, but again there is a luck factor as well, I might not be able to do that every time.
Herbert did a single dive to 110m with a rather slow ascent and no post-dive deco and got major symptoms.
My friend Tyler did two no-fins dives to 35m followed by one dive with fins to 71m (no deco) and got significant symptoms very similar to mine.
Looking at Herbert's recreational profiles with symptoms and comparing them to mine, it certainly appears that 9 long deep dives (35-50m) is too much and extremely likely to cause DCS. Memo recently said he tried reducing it to just 5 such dives and had no symptoms. In my case five long deep dives (35-50m) will produce symptoms most of the time but not always.
In 2005 during recreational diving I started slowing my ascent in the last 10m and even trying to stop at 6m for a few seconds if possible, and if not possible then to go back down and do apnea deco for 30-60 seconds right afterwards. This allowed me to drastically violate my previous rules and still escape without symptoms, and Memo agreed that he too was able to escape symptoms using this method. However I do know that this method is not invincible. In the Bahamas in November 2008, several times I tried doing four dives 55-65m, each time doing 'apnea deco' after each dive (around 10-15min between dives), and still got minor symptoms. However I was able to do seven dives to 40m (apnea deco after each one) without symptoms.
The symptoms I got after the 104m with O2 deco show (to me) that for deep enough dives O2 deco at 5m for 5min is not enough. My current model calls for a first deco stop of 9m for such a dive, which agrees with my intuition, that I need to go deeper to crush the bubbles and dissolve them again, then ascend up to 6m later-- this of course means that the deco mix must be less than 100% since 9m is too deep for O2.
So for single dives with moderate ascents (<= 1.3m/s), I would say:
< 70m, no deco needed
70m-90m, O2 at 5-6m for 5min
> 90m, 80% O2 at 9min for 2min, followed by 80% O2 at 6m for 5min
For repetitive dives I would say that if no deco of any kind is to be done, then two long dives to 40m is already the limit if you want total safety. You might get away with as many as 5-6 but this is pushing it. Nine dives and you are likely toast. If you do apnea deco after each dive you may be able to get away with extra dives but eventually you will still get bent.
I would also add that time spent on the bottom may make a drastic difference. Running my Xen computer with deco calculations, a dive to 40m (bounce) results in a very different tissue result than the same dive to 40m with a 20-30 second hang on the bottom.
If AIDA is asking about DCS and using it to exclude participation, that would be wrong. I can’t verify if that is fact, so don’t quote me. If divers are being asked about DCS for statistical and research purposes, that is fine and good. But to my knowledge that data is not being compiled or used. There is no convincing evidence that if you get DCS once then you are more susceptible in the future. Statistics with n=1 are not valid. If you roll dice and get snake eyes, it doesn’t mean you are skilled at throwing dice. If your process has an element of randomness, you can’t draw these conclusions with small numbers. If you get DCS several times, then you can begin to make a statement about susceptibility with more confidence.
People will obviously not report symptoms if there is punishment or negative consequences to participation or reputation. But this goes on everywhere. Commercial divers failed to report symptoms for decades. Astronauts have had DCS on space walks, but didn’t report until after retirement for fear of being grounded. Same as pilots and G-LOC. It is totally counter-productive. It is like winning a battle but losing the war. Industries have learned from this, and now people can often report problems openly or anonymously without fearing consequences.
My other comment is that it can be very challenging to sort out subjective symptoms like pain and numbness or tingling. There can be other causes besides DCS. If you want to construct a predictive model for DCS, there will be disagreement as to what constitutes credible symptoms. I say this with no bias. There will be differing opinions on how high or low to set the bar, and what should be the burden of proof that an incident is truely DCS (subjective symptoms versus objective neurological exam). There is lack of consistency and validity if different people are reporting symptoms in different ways without at least considering alternate diagnoses.
Decompression modelers used to define observed DCS outcomes as a binary variable with x=0 for no bends and x=1 for bends, then tune the models to fit. Now some are advocating x=0.5 for marginal or vague symptoms. Carried further you could place confidence indices on any reported symptoms, where x is a continuous variable between zero and one. Mild isolated numbness would get a low score, focal paralysis would get high score. Subjective perhaps, but just a thought.
People will obviously not report symptoms if there is punishment or negative consequences to participation or reputation. But this goes on everywhere. Commercial divers failed to report symptoms for decades. Astronauts have had DCS on space walks, but didn’t report until after retirement for fear of being grounded. Same as pilots and G-LOC. It is totally counter-productive. It is like winning a battle but losing the war. Industries have learned from this, and now people can often report problems openly or anonymously without fearing consequences.
My other comment is that it can be very challenging to sort out subjective symptoms like pain and numbness or tingling. There can be other causes besides DCS. If you want to construct a predictive model for DCS, there will be disagreement as to what constitutes credible symptoms. I say this with no bias. There will be differing opinions on how high or low to set the bar, and what should be the burden of proof that an incident is truely DCS (subjective symptoms versus objective neurological exam). There is lack of consistency and validity if different people are reporting symptoms in different ways without at least considering alternate diagnoses.
Decompression modelers used to define observed DCS outcomes as a binary variable with x=0 for no bends and x=1 for bends, then tune the models to fit. Now some are advocating x=0.5 for marginal or vague symptoms. Carried further you could place confidence indices on any reported symptoms, where x is a continuous variable between zero and one. Mild isolated numbness would get a low score, focal paralysis would get high score. Subjective perhaps, but just a thought.