History of the agencies

[John C. Ratliff]

@John C. Ratliff,

My oxygen exposure table (NOAA) maxes out at 1.6 ATA PO2. Do you have a source for oxygen exposure tables that go up to 3.0 ATA? For purely academic reasons, I am trying to get a sense of the implications on deco if one switches to oxygen at 60 ffw when ascending from a deep air dive to 200 ffw for 30 minutes (say).

TIA,
rx7diver

Actually, we started the decompression at 40 feet, as I recall. (That was March 25th, 1975, 48 years ago.) It may have been 30 feet. Larry Murphy was in charge of that aspect, and we had regulators set up off the oxygen storage tanks at the various depths. I just went over my dive logs, and was not able to determine which depth we started on oxygen. The one note that I had was that we used "cave bottom time," which was defined as the time we left the surface until the time of our first decompression stop, which included ascent time.

I just looked in Google Scholar, searching for "oxygen decompression in 1975," and found this article from 1975:

https://www.annualreviews.org/doi/abs/10.1146/annurev.me.26.020175.001223?journalCode=med

Now, I just found this publication, which is on-line and free.

Dysbarism-related Osteonecrosis

books.google.com

It turns out that I have that publication, and it is titled "Dysbarism-Related Osteonecrosis, A Symposium." I just pulled it from my library, and it is dated 1974. There was a lot of concern about osteonecrosis in divers and one chapter is titled "Decompression Tables in Relation to Dysbaric Osteonecrosis," by Claude A. Harvey. In it there is this quote:

Behnke (1967) has discussed the principles of isobaric ("oxygen window") decompression, a technique that theoretically avoids bubble formation but "calls for too long a decompression for dives up to one-hour duration, unless it is feasible to breathe pure oxygen for a prolonged period at the +15 psi level." He points out that lengthy half-times, such as those analyzed in Tables II, III, and IV herein, seem much too long to be consistent with the limited physiological data available on N2 elimination in man. They have been employed, he suggests, only because models used to calculate tables may have questionable physiologic bases. Behnke adds: "It is likely that decompression practice in the past has served to initiate bubble evolution which subsequently has been controlled by prolonged stage decompression at relatively shallow depth."...

So I think Larry Murphy was using state-of-the-art oxygen decompression at the time to decrease the presence of "silent bubbles" and subsequent dysbarism-related osteonecrosis.

SeaRat

 

[Akimbo]

My oxygen exposure table (NOAA) maxes out at 1.6 ATA PO2. Do you have a source for oxygen exposure tables that go up to 3.0 ATA?

This thread explains it: Oxygen Toxicity Limits & Symptoms

This is the premise of the thread:

Oxygen toxicity limits can be very confusing, especially for PPO2 (Partial Pressure of Oxygen) levels above 1.6 ATA used in chamber-based hyperbaric treatment (recompression) and decompression tables. For example, here is a chart of one of the most common DCS (Decompression Sickness) treatment tables. Note that the PPO2 of pure oxygen at 60'/18.3M is 2.82 ATA — or more than twice the normal limits recreational divers observe.

 

[Kay Dee]

Sinkhole closed for diving 1 of 2

Clipping found in Orlando Evening Star published in Orlando, Florida on 8/22/1970. Sinkhole closed for diving 1 of 2

www.newspapers.com

How about the 16 year old that he took to 300’ in mystery sink? Let’s talk about that one, then we can discuss some of the others.

Don't see anything in the article re depth Hal / PSA "killed" the teenager? Is that known?

 

[rx7diver]

This thread explains it: Oxygen Toxicity Limits & Symptoms

This is the premise of the thread:

Thanks, @Akimbo. When I was doing deep air diving (and technical nitrox diving) in the mid-1990's, my training had us switch to 100% oxygen at 20 fsw (which would be our shallowest decompression stop) and take air breaks (back gas breaks) for 5 minutes every 15 or 20 minutes, depending. We were taught that switching no deeper than 20 fsw was done to "ensure" that we would not exceed the 1.6 ATA recommended oxygen exposure limits even for longish deco times.

So, for example, it allowed one to "safely" dive two reasonably long (for open water diving) deep air dives with oxygen deco in a single day (say, two 200 fsw dives each with a 25 min BT), separated by a generous surface interval, without violating the "oxygen clock" for a single exposure nor the oxygen clock for the 24-hour maximum exposure.

@John C. Ratliff's mid-1970's switch to oxygen at 60 fsw or 40 fsw or 30 fsw (rather than at 20 fsw) had me wondering what would happen to the oxygen clocks for the pair of dives described above.

rx7diver

 

[Kay Dee]

So, for example, it allowed one to "safely" dive two reasonably long (for open water diving) deep air dives with oxygen deco in a single day (say, two 200 fsw dives each with a 25 min BT), separated by a generous surface interval, without violating the "oxygen clock" for a single exposure nor the oxygen clock for the 24-hour maximum exposure.

With all due respect - and I was taught and practiced the same / similar ('cept the such short time on o2 before the air beaks above) for a time - until the 'oxygen clock' being proven somewhat of a fallacy by numerous expeditions (some I was personally involved in) doing multi day multi dive exposures to greater depths than 200ft since the mid 90's if not the late 80's.

 

[rx7diver]

With all due respect - and I was taught and practiced the same / similar ('cept the such short time on o2 before the air beaks above) for a time - until the 'oxygen clock' being proven somewhat of a fallacy by numerous expeditions (some I was personally involved in) doing multi day multi dive exposures to greater depths than 200ft since the mid 90's if not the late 80's.

@Kay Dee,

I personally did NOT do more than a single 200 ffw+ deep air dive in a single day. And I took a day off from diving every third day when doing these 200 ffw+ deep air exposures. So, for example, for a week-long trip diving the deep shipwrecks off of Isle Royale (Lake Superior, Michigan), I would NOT dive the third day. These were my own, personal, self-imposed limits. Others in my group dove much more during these outings.

rx7diver

 

[Kay Dee]

I personally did NOT do more than a single 200 ffw+ deep air dive in a single day. And I took a day off from diving every third day when doing these 200 ffw+ deep air exposures. So, for example, for a week-long trip diving the deep shipwrecks off of Isle Royale (Lake Superior, Michigan), I would NOT dive the third day. These were my own, personal, self-imposed limits. Others in my group dove much more during these outings.

I was talking more about the oxygen clock being, how should I say, 'overated' and evidenced based 'proven' not as crucial as was once made out. However, if diving air on OC to 200 of so ft - which I often did whenever helium was not available - long after I began CCR diving, I also stuck to one 200ft or so OC dive a day with a shallower dive, say to 130ft or so, in the afternoon. But once I learned about the benefits of accelerated deco (with nitrox and O2) back in the early 90's I never ever did a dive where I used air for deco. I'd pass up a deep dive before using only air for deco!

 

[Akimbo]

We were taught that switching no deeper than 20 fsw was done to "ensure" that we would not exceed the 1.6 ATA recommended oxygen exposure limits even for longish deco times.

This is an example of over-simplified "rules" that cause great confusion. It all comes down to the compromise between oxygen toxicity, minimizing decompression, and risk management — largely convulsion.

A Scuba diver's risk is far higher than a surface supplied diver in a hat or FFM with voice communications to a diving supervisor and a standby diver ready to follow your umbilical down to help you. It is pretty hard to drown from a convulsion in a hat before a standby diver arrives. A Scuba diver's reasonably safe O2 limit of 20' can jump to 40' for a surface supplied diver.

Now take divers undergoing Sur-D-O2 (Surface Decompressing using Oxygen). They are inside a deck chamber pressurized to 60' on air and receiving oxygen through a BIBS (Built-In Breathing System) oral nasal mask, the dive super can see them through view ports and can verbally communicate. If a solo diver convulses, which is the most common, the O2 can be shut off and the chamber depressurized half an atmosphere in seconds. In the less common case where other divers are in the chamber, they just pull the O2 BIBS mask off the convulsing diver. Under these conditions, 60' becomes quite reasonable.

Add an inside tender (often a medic) for divers who are compromised by DCS and 60' is still quite reasonable. Hope all this makes sense.

 

[rx7diver]

This is an example of over-simplified "rules" that cause great confusion. ...

Hmmm. I'm not sure there is any confusion here. As explained to me, we were being taught a practical approach for the types of relatively short exposures being dived in open water, and this (perhaps "over-simplified") approach had been "proved" by many of these deep air dives having been successfully conducted using it.

As a quick back-of-the-envelope example, air at 200 fsw has a PO2 of 1.48 ATA. So, a dive to 200 fsw for 25 min BT uses up approximately 21% (= 25 / 120) of the single-exposure oxygen clock. If your decompression tables tell you to breathe 100% oxygen at the 20 fsw stop for 25 minutes when you're ascending from this dive, then you will have used up an additional approximately 56% (= 25 / 45) of the single-exposure oxygen clock. So, this dive likely is "safe" with respect to the NOAA-recommended oxygen exposure limit for a single dive. (An air break is indicated, though.)

Doing a similar quick calculation for two such dives, but using the 24-hour-maximum exposure limits, shows that you will have used up approximately 60% [= 2 * (14% + 16%)] of the 24-hour maximum exposure oxygen clock. So, these two dives likely are "safe" with respect to the NOAA-recommended 24-hour oxygen exposure limit. (A very generous surface interval is indicated, though!)

Okay. Now, we need to go back and do the calculations a bit more more carefully, this time incorporating the effect of breathing air during the decompression stops deeper than 20 fsw, the impact of residual nitrogen on the repetitive dive, and the half-life of oxygen.

There is no confusion, I think. (What's done in a dry habitat or a deck chamber doesn't apply here, I think.)

rx7diver

 

[Akimbo]

There is no confusion, I think. (What's done in a dry habitat or a deck chamber doesn't apply here, I think.)

I was speaking more generally. Risk can be reduced, even in the water and on Scuba, that would allow reasonable safety deeper that 20'. A FFM manifolded to air and O2 for example. A stage that would reduce the diver's exertion is another. The stage would reduce the OxTox risk while the FFM would reduce the risk of drowning during a convulsion. Definitely not appropriate for the average recreational Scuba diver but nothing that an advanced technical diver can't manage.