USN decompression 30fsw/9m with 100% O2

[rjack321]

Just to be absolutely clear, the 80% would only be for OC dives and would normally be in conjunction with the 50% deco gas.

The min gas planning calculations would be to have sufficient backgas to get to the second gas switch, e.g. 9m for 80% in the case of a failure in the 50% cylinder. Easy to calculate on MultiDeco.

Have you ever actually done a 2 hour runtime 50m OC dive? Where was this, what was the water temp, and what was the profile? What gasses did you bring? What size cylinders, how much did you use and how much was in reserve?

 

[tursiops]

I have tried to find the basis for the published limits on CNS, and I have not found it. (I have to admit that I have not tried all that hard.) I strongly suspect that is was something of a guess, somewhere in the range between "somewhat educated" and "wild."

Now think what that does to me as a technical instructor. The course materials require students to make sure that students do all the CNS and OTU calculations for their dives, and they learn, of course, that it takes some pretty big dives, dives that we won't be doing in training, to exceed those limits. But they know that people do indeed do dives that exceed those limits, sometimes by a wide margin.

So when they ask me how that happens, what do I say? I answer honestly but very carefully. I certainly don't tell them that they should go ahead and ignore those limits with my blessing. I have developed something of a spiel that walks the tightrope.

What I am really saying in this post is that it is time for someone in the scuba agency to do some kind of investigation and report on the origin and accuracy of those numbers. I am not expecting it, though, because I am sure they are walking the same tightrope I am. If they were to write anything that sounded like a blessing to ignore those limits and someone were to do that and die, ....

The current NOAA Diving Manuals (Sixth Edition-2017; Fifth-2013; Fourth-2001; Third-1991) all have the same table (15.1) in them for oxygen exposure; namely the infamous “Normal Exposure Oxygen Partial Pressure Limits” that allow 45 minutes at PPO2=1.6, 180 minutes at 1.3, etc. The table is "adapted from Butler and Thalmann, 1986." The text discussing that table says:

“single exposures somewhat longer than those shown in Table 15-1 can be conducted without episodes of central nervous system (CNS) oxygen toxicity. However, the more conservative exposure times shown in Table 15-1 take operational safety into consideration and are sufficient in duration for anticipated NOAA dives. At the same time, the limits shown in Table 15-1 extend the limits published in the second edition of the NOAA Diving Manual.”​

I’ve not been able to find a Second Edition (1979) of the NOAA Diving Manual, but presumably it had even lower limits than the admittedly conservative limits in Table 15.1.

The Butler and Thalmann (1986) work in Undersea Biomed Res, 1986 Jun;13(2):193-223, has this abstract:

"Central nervous system oxygen toxicity is currently the limiting factor in underwater swimming/diving operations using closed-circuit oxygen equipment. A dive series was conducted at the Navy Experimental Diving Unit in Panama City, FL, to determine whether these limits can be safely extended and also to evaluate the feasibility of making excursions to increased depth after a previous transit at a shallower depth for various lengths of time. A total of 465 man-dives were conducted on 14 different experimental profiles. In all, 33 episodes of oxygen toxicity were encountered, including 2 convulsions. Symptoms were classified as probable, definite, or convulsion. Findings were as follows: symptom classification is a useful tool in evaluating symptoms of oxygen toxicity; safe exposure limits should generally be adjusted only as a result of definite symptoms or convulsions; the following single-depth dive limits are proposed: 20 fsw (6.1 msw)--240 min, 25 fsw (7.6 msw)--240 min, 30 fsw (9.1 msw)--80 min, 35 fsw (10.7 msw)--25 min, 40 fsw (12.2 msw)--15 min, 50 fsw (15.2 msw)--10 min; a pre-exposure of up to 4 h at 20 fsw causes only a slight increase in the probability of an oxygen toxicity symptom on subsequent downward excursions; a pre-exposure depth of 25 fsw will have a more adverse effect on subsequent excursions than will 20 fsw; a return to 20 fsw for periods of 95-110 min seems to provide an adequate recovery period from an earlier excursion and enables a second excursion to be taken without additional hazard; nausea was the most commonly noted symptom of oxygen toxicity, followed by muscle twitching and dizziness; dives on which oxygen toxicity episodes were noted had a more rapid rate of core temperature cooling than dives without toxicity episodes; several divers who had passed the U.S. Navy Oxygen Tolerance Test were observed to be reproducibly more susceptible to oxygen toxicity than the other experimental divers."​

Note that they propose a PPO2=1.6 oxygen limit as 240 mins, not 45 mins!

It would be easy to conclude that the NOAA tables are VERY conservative, and (as they say, quoted above) appropriate for anticipated NOAA dives, but perhaps a bit too conservative for those exploring the edge of their comfort zone.

 

[Wibble]

Have you ever actually done a 2 hour runtime 50m OC dive? Where was this, what was the water temp, and what was the profile? What gasses did you bring? What size cylinders, how much did you use and how much was in reserve?

Yes. Several. Typically 18/45 or 21/35 in 12 litre twinset with ali80s of 50% and 80%. Occasionally with an ali80 bottom stage of backgas. Plenty of reserves on return. Water between 12 and 18 degrees in the summer, colder in winter. Use a heated vest.

Example 2h22 total, bottom 1h03, on the Moldavia at 48.5m:
Notes: Nice dive. Topped off bottom stage from dregs of previous day's dive 85Bar. Lasted about 15m. Bungee worked well but needs another D ring on chest as it pulls it down and hinders clipping off other things. Three ali80's is an embuggerance. Only OC diver on the boat
Wreck quite intact
7 to 10 m vis. Snotty at deco


Why do you ask?

 

[rjack321]

Yes. Several. Typically 18/45 or 21/35 in 12 litre twinset with ali80s of 50% and 80%. Occasionally with an ali80 bottom stage of backgas. Plenty of reserves on return.

Define "Plenty"

To get a 2 hour runtime (I used 50/85 GF) with 21/35 at 48m is 55mins of bottom time. I dont know anyone doing 55mins of BT on a set of 12s with enough reserve to get an OOA diver back to 21m. That's 5.7 ATA. Even at a sedate 15L/min SAC that is 85L per min on the bottom. 85L x 55mins is ~4,675L of 21/35. Unless you are pulling a vacuum on those 12s, or your SAC is significantly below 10L/min, you are pulling our leg.

 

[OTF]

This thread is fascinating, several really good posts. A valuable discussion even though the topic has strayed a few times.

I wish knew more about O2 toxicity. This has some of the best information I've found https://www.johnchatterton.com/wp-content/uploads/2013/03/MechanismsofHyperoxicSeizures.pdf although that is now 20 years old.

It's the "unknown unknowns" that would keep me from pushing the maximum OTU limits, even if there is some evidence that those limits are conservative. Eg. - does anyone know of any work on the effects of chronic / long term repeated high O2 exposure? I wouldn't be surprised if recurring high levels of oxygen radicals had some measurable detrimental effect. Never heard anything either way.

 

[grantctobin]

This thread is fascinating, several really good posts. A valuable discussion even though the topic has strayed a few times.

I wish knew more about O2 toxicity. This has some of the best information I've found https://www.johnchatterton.com/wp-content/uploads/2013/03/MechanismsofHyperoxicSeizures.pdf although that is now 20 years old.

Does anyone know of any work on the effects of chronic / long term repeated high O2 exposure? I wouldn't be surprised if recurring high levels of oxygen radicals had some measurable detrimental effect. Never heard anything either way.

Myopia associated with hyperbaric oxygen therapy - PubMed non measured and returned to normal by itself (so discount it as you see fit) but something I’ve experienced over periods of long dive duration for many days in a row (e.g 4+hr a day for five or more consecutive days). Effects lessened or disappeared when I backed off deco PO2 and kept <1.4 on 20’ stop rather than spikes higher.

 

[tursiops]

This thread is fascinating, several really good posts. A valuable discussion even though the topic has strayed a few times.

I wish knew more about O2 toxicity. This has some of the best information I've found https://www.johnchatterton.com/wp-content/uploads/2013/03/MechanismsofHyperoxicSeizures.pdf although that is now 20 years old.

It's the "unknown unknowns" that would keep me from pushing the maximum OTU limits, even if there is some evidence that those limits are conservative. Eg. - does anyone know of any work on the effects of chronic / long term repeated high O2 exposure? I wouldn't be surprised if recurring high levels of oxygen radicals had some measurable detrimental effect. Never heard anything either way.

Long, and mostly not the topic you are asking about, but definitely relevant.
Pharmacology of oxygen | Deranged Physiology

Let's find out if @Dr Simon Mitchell knows of any work post-Brian?

 

[rjack321]

This thread is fascinating, several really good posts. A valuable discussion even though the topic has strayed a few times.

I wish knew more about O2 toxicity. This has some of the best information I've found https://www.johnchatterton.com/wp-content/uploads/2013/03/MechanismsofHyperoxicSeizures.pdf although that is now 20 years old.

It's the "unknown unknowns" that would keep me from pushing the maximum OTU limits, even if there is some evidence that those limits are conservative. Eg. - does anyone know of any work on the effects of chronic / long term repeated high O2 exposure? I wouldn't be surprised if recurring high levels of oxygen radicals had some measurable detrimental effect. Never heard anything either way.

In the grand scheme of things, diver exposures are nothing compared to people getting routine HBOT for diabetic wound healing or actinomycosis. I asked nearly the same question 15 years ago, and no the O2 radicals have little or no known long term effects. Of course measuring and proving the absence of effect is really hard, cause there's always another organ or effect to try and measure in detail.

 

[Wibble]

Define "Plenty"

To get a 2 hour runtime (I used 50/85 GF) with 21/35 at 48m is 55mins of bottom time. I dont know anyone doing 55mins of BT on a set of 12s with enough reserve to get an OOA diver back to 21m. That's 5.7 ATA. Even at a sedate 15L/min SAC that is 85L per min on the bottom. 85L x 55mins is ~4,675L of 21/35. Unless you are pulling a vacuum on those 12s, or your SAC is significantly below 10L/min, you are pulling our leg.

I don’t plan for an OOA possibility, mainly because I dive solo, but more because that simply isn’t a realistic risk.

Reminder, you plan for ONE failure, that "OOA" diver would have lost their backgas, failed to do a shutdown and switch, stayed beyond their min gas, lost both of their stage cylinders. Not to mention a failure of self-sufficiency, failing to plan, to practice, no doubt use the right gas and no doubt with a ridiculous SAC. How many failures is in that scenario?

But hey ho, I’m at min gas and come across a diver that was robbed of their dive kit by marauding pirates and was still alive, then what? On CCR they’d get my bailouts, on OC it’s a question of survival of both, one or none. I would try, probably sharing and blowing off any stops up to the first gas switch (actually before), but there's just as likely to be a punch-up underwater as to whom gets the gas as it runs out.

A second casualty isn’t going to help anyone.

The deeper we dive the greater are the risks which have to be accepted and, of course, the more we practice mitigation.



What would you recommend?

 

[Wibble]

Define "Plenty" [of reserves on returning]

Whatever reserves one would expect at the end of an uneventful dive. Backgas to get from the first switch to the second, plenty — half? — in both deco stages, etc. Generally like to have 50 bar left in the backgas on return.