EAN32 diver below you at 40-odd meters?
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Rainer
Contributor
Anti-Hero
Contributor
You'd think right? But (and don't ask me how it happened) one night you're walking down the street and you try to help some poor disabled guy load a sofa into his van, and before you know it, you friggin' wake up at the bottom of Temple of Doom and he's standing over the edge with a basket and it's "It rubs the lotion on its skin and then it puts the regulator in" all over again.
Giving up diving is no guarantee against toxing.
Slamfire
Contributor
I'll try to be succinct. First of all thanks to those that pointed me in the right direction to get the information. I read some of the information last night and learned several new things I did not know before. I welcome any well-based corrections to my conclusions:
Great Degree of Variability
Kenneth Donald is often cited for a study about variability. Unfortunately I have not read Donald as a primary source yet. He made a point of the individual variation to time-quantitative symptoms. There's the classic plot of the diver who was exposed to a fixed oxygen partial pressure day after day and his time quantitative symptoms ranged from five minutes to several hours -- Figure 4 page 5.
Does anybody know what was the ppO2 that Donald was using in this experiment? According to Shreeves and Richardson (2000), most of the published body of testing oxygen exposure involves ppO2s greater than 1.6 atm. This makes extrapolating to lower ppO2s difficult. SB Medical Moderator, Saturation (2003) says that several O2 convulsion studies have been made "across a range of pp02 with most at pp02 > 3". I am not denying the variability. I am just saying that just because there was great variability when people were subjected to ppO2s of, say, 3.5 atm, does not necessarily mean that you will have the same variability at 1.6 atm. And even if you do have that same variability once you get past the threshold, hasn't that threshold been moved far enough ahead to make it impractical to cross it due to restrictions like the amount of gas available for the dive and nitrogen loading?
1.6 vs 1.4 vs 1.2
Donald (1947) reports that "The Admiralty Experimental Diving Unit was unable to demonstrate oxygen poisoning in the range of 0 to 20 fsw." This is the ppO2 range of 1.0 to 1.6 atm. Shreeves and Richardson (2000) say that "against this data set, a shorter time limit of 45 minutes at the lower O2S limit of 1.6 atm, certainly seems reasonable." I am guessing that one reason why so many O2 convulsion studies are done at such high ppO2s is because they cannot get subjects to easily convulse in controlled lab situations when you keep the ppO2 to 1.6 atm or below.
The time limits for ppO2s [in the NOAA table] below 1.4 atm stem more from pulmonary oxygen toxicity concerns than from CNS (acute) toxicity (Hamilton, 1989). I'm interpreting this as being more likely to have pulmonary O2 toxicity than getting a CNS hit when you go for more than 150 mins at 1.4 atm.
So 1.6 atm is very conservative and 1.4 atm even more so. Why, then, did PADI choose 1.4? D. Richardson (2000) from DSAT/PADI says, "We should look at the database and known outcomes to answer that question. For this whole discussion, if you look at Kenneth Donald's work from 1942 to 1945, working with the Royal Navy, they did about 2,000 test dives using oxygen breathing. If you compare that database to NOAA's dose duration, it falls right in between 1.4 and 1.6 atm. PADI had no objection to 1.6 atm. We set it at 1.4 atm for recreational divers as a margin of safety for diver error. However, 1.6 atm fits very nicely under the Donald data set as well."
Bjorkmann (2000) from EnviroDive says that DCIEM allowed them 1.5 atm, but they moved it to 1.2 atm because it "allowed us to put it all on a simple table that was very easy for people to track... if you have to go beyond a 1.2 atm, we have to start getting back into the formulas and this is where we're running into problems" [with divers remembering how to use formulas].
Wells (2000) from NOAA says they created their table "thanks to the fine work of Butler and Thalmann that had been published a couple of years ago...". He goes on to say, "I've been asked many times why the Manual doesn't match the numbers in the Butler and Thalmann paper. It was considerably less than they had recommended in their paper. As you remember, the [old] Navy [manual] cut back from 1.6 atm for 30 minutes. We wound up increasing it to 1.6 atm for 45 minutes based on Thalmann's work... The 1.6 atm for 45 minutes fell within what we considered a quite acceptable risk."
Thalmann (2000) comments on why 1.3 atm was chosen for the Navy. "Because it can be dived all day long. It doesn't impact Navy operations all that much and they don't want any O2 hits in the water. The Navy has within the last month rewritten its helium tables to eliminate 100 percent oxygen in the water. Why? In the last two years there have been two O2 incidents. They don't want any of them, they've eliminated O2 in the water and everybody's happy with that."
In the 2000 DAN Nitrox Workshop, the participants made it clear that they were not aware of a single case of CNS convulsions for single exposure dives at 1.6 ppO2 or less. Single exposure dives refer to a one gas, usually within the NDL dives. Multi level exposure dives refer to staged decompression dives where the diver gets exposed to higher O2 levels several times throughout the dive. Apparently the participants where aware of 2 cases of Multi level exposure dive hits. It seems that this was taking into consideration about 10 years (1990-2000) of dives and about 250,000 dives.
Vann (2000) from DAN says, "When you're talking about a multi-level oxygen exposure as you are after a long decompression dive, it's a whole new ballgame. You can't compare that with a single limit."
Even though a quarter of a million non-controlled anecdotal dives without a hit (or with 2 hits if you include tech diving) is hardly scientific data, it does hint that 1.6atm for 45 mins may be adequate.
Risk
So why go through the big deal of trying to eliminate all risk of a O2 hit when it seems that the likelihood of a DCS hit is much higher? An underwater O2 hit can carry much more dire consequences than a light DCS hit like skin bends, or pain in your knees. It seems that'll be easier to survive a small DCS hit than an O2 hit, particularly if you're soloing.
Conclusions
Max ppO2.- I think that the 1.6 or 1.4 atm limit is perfectly suitable for single exposure dives. Think about it in the context of the limitations of an average recreational diver with a single Al 80 trying to stay within the NDL. For shallower depths, the long air NDL doesn't make it very worthwhile to use Nitrox. For deeper depths, you run out of gas before you hit the NDL or the Oxygen clock limit.
The Oxygen Clock .- The clock may have been created using what seems arbitrary rationale -- at the 1.6 atm level -- and pulmonary toxicity rationale that does not address CNS at the 1.4 atm and lower levels. But the reason for doing this seems to be simply because it is very hard to get people to convulse at 1.6 or below.
Does that make the clock useless? I don't think so. It provides a measurement parameter that may be correlated to risk. I prefer to measure %CNS and log it for every dive and have a relative idea of where I am standing. Relative to what? I don't know, but something is better than nothing.
If I were to go on a staged deco dive using nothing but ratio deco, you better believe I'll want to put my max ppO2s as low as possible (1.2 and lower) simply because I have no way of tracking my %CNS and this is a multi level exposure dive -- which, the experts say, does wonky things. If on the other hand I have V-planner giving me a %CNS figure I'll have more peace of mind because I can refer to previous dives like this and their respective %CNS. I am able to benchmark against myself. I know there could be the variability issue. But something is better than nothing, and I'm still not fully convinced that variability will play a crucial factor at the lower ppO2 levels. If it had, we should have seen much, much more O2 hits at 1.6 or below.
Great Degree of Variability
Kenneth Donald is often cited for a study about variability. Unfortunately I have not read Donald as a primary source yet. He made a point of the individual variation to time-quantitative symptoms. There's the classic plot of the diver who was exposed to a fixed oxygen partial pressure day after day and his time quantitative symptoms ranged from five minutes to several hours -- Figure 4 page 5.
Does anybody know what was the ppO2 that Donald was using in this experiment? According to Shreeves and Richardson (2000), most of the published body of testing oxygen exposure involves ppO2s greater than 1.6 atm. This makes extrapolating to lower ppO2s difficult. SB Medical Moderator, Saturation (2003) says that several O2 convulsion studies have been made "across a range of pp02 with most at pp02 > 3". I am not denying the variability. I am just saying that just because there was great variability when people were subjected to ppO2s of, say, 3.5 atm, does not necessarily mean that you will have the same variability at 1.6 atm. And even if you do have that same variability once you get past the threshold, hasn't that threshold been moved far enough ahead to make it impractical to cross it due to restrictions like the amount of gas available for the dive and nitrogen loading?
1.6 vs 1.4 vs 1.2
Donald (1947) reports that "The Admiralty Experimental Diving Unit was unable to demonstrate oxygen poisoning in the range of 0 to 20 fsw." This is the ppO2 range of 1.0 to 1.6 atm. Shreeves and Richardson (2000) say that "against this data set, a shorter time limit of 45 minutes at the lower O2S limit of 1.6 atm, certainly seems reasonable." I am guessing that one reason why so many O2 convulsion studies are done at such high ppO2s is because they cannot get subjects to easily convulse in controlled lab situations when you keep the ppO2 to 1.6 atm or below.
The time limits for ppO2s [in the NOAA table] below 1.4 atm stem more from pulmonary oxygen toxicity concerns than from CNS (acute) toxicity (Hamilton, 1989). I'm interpreting this as being more likely to have pulmonary O2 toxicity than getting a CNS hit when you go for more than 150 mins at 1.4 atm.
So 1.6 atm is very conservative and 1.4 atm even more so. Why, then, did PADI choose 1.4? D. Richardson (2000) from DSAT/PADI says, "We should look at the database and known outcomes to answer that question. For this whole discussion, if you look at Kenneth Donald's work from 1942 to 1945, working with the Royal Navy, they did about 2,000 test dives using oxygen breathing. If you compare that database to NOAA's dose duration, it falls right in between 1.4 and 1.6 atm. PADI had no objection to 1.6 atm. We set it at 1.4 atm for recreational divers as a margin of safety for diver error. However, 1.6 atm fits very nicely under the Donald data set as well."
Bjorkmann (2000) from EnviroDive says that DCIEM allowed them 1.5 atm, but they moved it to 1.2 atm because it "allowed us to put it all on a simple table that was very easy for people to track... if you have to go beyond a 1.2 atm, we have to start getting back into the formulas and this is where we're running into problems" [with divers remembering how to use formulas].
Wells (2000) from NOAA says they created their table "thanks to the fine work of Butler and Thalmann that had been published a couple of years ago...". He goes on to say, "I've been asked many times why the Manual doesn't match the numbers in the Butler and Thalmann paper. It was considerably less than they had recommended in their paper. As you remember, the [old] Navy [manual] cut back from 1.6 atm for 30 minutes. We wound up increasing it to 1.6 atm for 45 minutes based on Thalmann's work... The 1.6 atm for 45 minutes fell within what we considered a quite acceptable risk."
Thalmann (2000) comments on why 1.3 atm was chosen for the Navy. "Because it can be dived all day long. It doesn't impact Navy operations all that much and they don't want any O2 hits in the water. The Navy has within the last month rewritten its helium tables to eliminate 100 percent oxygen in the water. Why? In the last two years there have been two O2 incidents. They don't want any of them, they've eliminated O2 in the water and everybody's happy with that."
In the 2000 DAN Nitrox Workshop, the participants made it clear that they were not aware of a single case of CNS convulsions for single exposure dives at 1.6 ppO2 or less. Single exposure dives refer to a one gas, usually within the NDL dives. Multi level exposure dives refer to staged decompression dives where the diver gets exposed to higher O2 levels several times throughout the dive. Apparently the participants where aware of 2 cases of Multi level exposure dive hits. It seems that this was taking into consideration about 10 years (1990-2000) of dives and about 250,000 dives.
Vann (2000) from DAN says, "When you're talking about a multi-level oxygen exposure as you are after a long decompression dive, it's a whole new ballgame. You can't compare that with a single limit."
Even though a quarter of a million non-controlled anecdotal dives without a hit (or with 2 hits if you include tech diving) is hardly scientific data, it does hint that 1.6atm for 45 mins may be adequate.
Risk
So why go through the big deal of trying to eliminate all risk of a O2 hit when it seems that the likelihood of a DCS hit is much higher? An underwater O2 hit can carry much more dire consequences than a light DCS hit like skin bends, or pain in your knees. It seems that'll be easier to survive a small DCS hit than an O2 hit, particularly if you're soloing.
Conclusions
Max ppO2.- I think that the 1.6 or 1.4 atm limit is perfectly suitable for single exposure dives. Think about it in the context of the limitations of an average recreational diver with a single Al 80 trying to stay within the NDL. For shallower depths, the long air NDL doesn't make it very worthwhile to use Nitrox. For deeper depths, you run out of gas before you hit the NDL or the Oxygen clock limit.
The Oxygen Clock .- The clock may have been created using what seems arbitrary rationale -- at the 1.6 atm level -- and pulmonary toxicity rationale that does not address CNS at the 1.4 atm and lower levels. But the reason for doing this seems to be simply because it is very hard to get people to convulse at 1.6 or below.
Does that make the clock useless? I don't think so. It provides a measurement parameter that may be correlated to risk. I prefer to measure %CNS and log it for every dive and have a relative idea of where I am standing. Relative to what? I don't know, but something is better than nothing.
If I were to go on a staged deco dive using nothing but ratio deco, you better believe I'll want to put my max ppO2s as low as possible (1.2 and lower) simply because I have no way of tracking my %CNS and this is a multi level exposure dive -- which, the experts say, does wonky things. If on the other hand I have V-planner giving me a %CNS figure I'll have more peace of mind because I can refer to previous dives like this and their respective %CNS. I am able to benchmark against myself. I know there could be the variability issue. But something is better than nothing, and I'm still not fully convinced that variability will play a crucial factor at the lower ppO2 levels. If it had, we should have seen much, much more O2 hits at 1.6 or below.
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Rainer, yes i can cite an example of a diver toxing at 1.3, it happened during DAN's nitrox study in the early 90's if my memory is correct.
Slamfire
Contributor
If you download the pdf you can find here: Rubicon Research Repository: Item 123456789/4855
and go to pages 92-93 (95-96 on pdf) you will find a discussion in which they go around the table asking if they are knew of any single exposure CNS hits. This was the DAN nitrox workshop, and they would have brought up this case... They did mention that DAN knew about 2 cases, but one was a tech dive and another was an advil overdose... this is what it ended up with:
"M. Lang: It's duly noted. The conclusion then is that there are no documented oxygen seizures from recreational nitrox diving at 1.6 atm or less on single exposures."
Asking around for anecdotal references is hardly scientific data. For example many drownings could have been the result of undiscovered CNS hits. Nonetheless, with such a large sample, somebody would have heard something.
Was the case that you remember a tech dive?
vinegarbiscuit
Contributor
My OW instructor refused to use EAN on wall dives for that very reason. Said it limited her from safely grabbing runaway students/customers, and that she had a bit more wiggle-room with air to make the plunge and bring them back to safety.
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