Why not 60% O2 for prof during OW?

[Mike Ange]

=It notes that there was no problem with navy divers, and points out that they are very disciplined and follow their tables carefully. It then contrasts that with instructors and divemasters, suggesting that these divers are less disciplined in following their computers.

I see this assumption a lot and it is an unfortunate one that should be addressed. The issue is that instructors are less disciplined as much as that the requirements of the job require some violation of the procedures used in the commercial, military and even the non-teaching recreational segments of diving. This is something not realized or recognized by the majority of divers or even new instructors in many instances. To evaluate the efficacy of these studies you have to evaluate the realities that the publishers probably assume as baseline knowledge for the reader.

For example: the (controlled) emergency swimming ascent requires a series of fairly rapid ascents (1 for each diver in the group) in quick succession from pressure ratios ranging between 1.6:1 (20fsw/6msw to surface) and 2:1 (33fsw/10msw to surface). This is well beyond what would typically be allowed (or even possible) by a surface supplied diver diving from a staging platform for example.

In the course of a regular OW class with say 6 students, the instructor will do at least 3 normal ascents, 6 emergency swimming ascents and 12 gas-sharing ascents or an average of 5.25 ascents per dive. However, in reality, the 12 ascents will frequently be done on 1 or 2 dives (depending on agency) and the 6 esa's will be completed on 1 dive as well. Plus any mistakes made by students that require a second evaluation and therefore another ascent by the instructor. Add to this the fact that many instructors do more students per dive and more than one group of divers in a given day and it is clear how the profiles they are subjected too are much less controlled than the very conservative standards used in other industry segments. However this is not due to a lack of discipline or a disregard of safety protocols.

The other assumption I see frequently is that these depths are too shallow to create hyperbaric conserns aside from barotauma. For reference on the pressure shifts issue the Haldanian model raises concerns beginning at a pressure shift range of 1.58:1 for DCS. Granted, this relates to actual internal tissue pressures for dissolved gasses and it therefore assumes some degree of saturation. However much less is known about the bone legions and other maladies resulting from hyperbaric exposures and if this shift can create concerns for DCS it is a very real possibility that the range is sufficient to create non-clinical bubbles in the blood stream for example that could possibly result in the formation of neurological lesions.

We simply do not have any conclusive evidence to prove or disprove the possibilities. Nevertheless, there is a significant amount of data indicating that hyperbaric exposure related maladies are encountered at significantly higher rate by this population and there is a fair amount of suggestive and non-conclusive data indicating the possibility that non-clinical bubble formation may be a culprit in these injuries.

HOWEVER - I still would not advocate the use of a 60% mix due to the large number of issues that this creates. Most of these issues have already been detailed quite effectively in this thread, so I will stay off that soapbox. I do however advocate that instructors use readily available nitrox mixtures (40% and less) to the extent possible and that they use conservative (IE long) surface intervals between the dives requiring ascent related evaluations.

Safe Diving,

 

[boulderjohn]

...

For example: the (controlled) emergency swimming ascent requires a series of fairly rapid ascents (1 for each diver in the group) in quick succession from pressure ratios ranging between 1.6:1 (20fsw/6msw to surface) and 2:1 (33fsw/10msw to surface). This is well beyond what would typically be allowed (or even possible) by a surface supplied diver diving from a staging platform for example.

In the course of a regular OW class with say 6 students, the instructor will do at least 3 normal ascents, 6 emergency swimming ascents and 12 gas-sharing ascents or an average of 5.25 ascents per dive. However, in reality, the 12 ascents will frequently be done on 1 or 2 dives (depending on agency) and the 6 esa's will be completed on 1 dive as well. Plus any mistakes made by students that require a second evaluation and therefore another ascent by the instructor. Add to this the fact that many instructors do more students per dive and more than one group of divers in a given day and it is clear how the profiles they are subjected too are much less controlled than the very conservative standards used in other industry segments. However this is not due to a lack of discipline or a disregard of safety protocols.

The other assumption I see frequently is that these depths are too shallow to create hyperbaric conserns aside from barotauma.
...
We simply do not have any conclusive evidence to prove or disprove the possibilities. Nevertheless, there is a significant amount of data indicating that hyperbaric exposure related maladies are encountered at significantly higher rate by this population and there is a fair amount of suggestive and non-conclusive data indicating the possibility that non-clinical bubble formation may be a culprit in these injuries.

As an instructor, I am very interested in this, but perhaps someone can help me identify the actual population of instructors and divemasters who were in this study. They were profiled in one brief paragraph, but if the information is complete (which I find hard to believe) then they are talking about a pretty unusual population.

Their study group averaged 4-6 dives per day, but it does not say how many days per year are like that. If they mean a typical work year for the duration of their job, then all I can say is "Wow!" especially when you look at their dive profiles below.

The least experienced member of their group had been doing this for 5 years, and the most experienced for 21 years, with a mean of 13.2 years. If the above paragraph refers to someone working only 5 days a week for 50 weeks, then we are talking about a population averaging roughly 16,000 dives for their careers. They said that they looked for people who had at least 500 career dives, but the population they choose should have reached that total in a few months.

And what were their dive profiles like? Well, the mean maximum depth for the group was about 233 feet, with some going as deep as 280 feet. (OK, that could be one dive.) For their careers, they report an average maximum depth of about 80 feet. Some of them reported average maximum depths for their dives of 100 feet. That means that if they are doing mostly training dives with ESA's and the like, as you are assuming, then they are averaging at least twice the maximum depth allowed by most training agencies for half the dives, and about 33% beyond the maximum depth for the remaining dives. If these statistics are anywhere near accurate, we are looking at massive standards violations throughout the study group.

I am also trying to imagine what the effect of doing 4-6 80 foot dives per day throughout the year for a number of years would do for you, and remember that some of them had 4-6 100-foot dives per day. (I think 6 100-foot dives per day over a number of years just might have a negative effect on your body.)

Of course, I don't believe anything I wrote above can possibly be true, and maybe I just need to have someone show me how I am misreading the statistics.

I will admit that although I am an instructor, my life does not in any way resemble the lives described by this study group, if my readings are at all accurate.