setting to nitrox to reduce over conservatism on dive computers

[doctormike]

I think that is partly due to the fact that with computers you can't see the entire progression of N build up. You set your depth and it spits out a time number. One number. So you are good this side and bad that side.

Yup, exactly. The tables give you a bigger picture of N2 loading at various times and depths.

As for computers being beneficial, I agree. However, as you describe, many divers use them to eek every last minute out of their dives. It's the pushing limits, even by new divers, that is the problem. They teeter on the edge of decompression obligations but imagine they are immune because the computer still says they are this side of the NDL's. One look at the tables however, shows the decreasing margin of safety they would be experiencing. It's very hard to demonstrate diminishing margins of safety on a PDC.

That's a good point. Because if you dive tables - which assume square profiles - every bit of time above the maximum depth is adding to the conservatism of the profile. So one would think that all other things being equal, running your bottom time right up to the NSL (I'm going to start using that abbreviation!) on a dive computer would result in more nitrogen loading than running right up to the table NSL (unless you really were diving a square profile).

---------- Post added July 1st, 2014 at 09:04 PM ----------

As I stated above, I've been in communication with DoctorMike about his chamber dive, and told him that I would like to do a bit of analysis on it.

Terrific work, John… thanks for looking into this. The main reason that I spent so much time writing that up was for exactly this reason, so that other people could learn from my sloppy technique and understand that there is more than one way to get bent!

Before I get into the table calculations, let me say that I was startled by the profile he presented in his blog (see below). Note that he went deep twice, then shallow, the even deeper still. When I was taught, a long time ago, we were told to make your deepest dive first, then shallower dives later.

Although I take full responsibility for this ugly profile, here is one point that I would argue with you about (with all due respect!). Reverse profiles have traditionally been shunned as a risky practice, but while this is still an area of controversy, it seems that many in the hyperbaric medicine community no longer feel that they are a problem, based on current research. For an excellent summary, see Deco for Divers by Mark Powell, page 82-84.

What I found was very interesting, in that the last dive under the older tables from NAUI (1990) started with a repetitive group designator of H, …. he was required to have a 8 minute decompression stop at 10 feet.

Using the current U.S. Navy Diving Table 9-7 and 17-9 ….. The decompression required was 9 minutes at 20 feet, and the repetitive group designator after this dive was Group L.

If I had been tracking actual dive times for this dive as a single dive, it would have been a 70 foot dive of sixty minutes duration with a mandatory decompression stop of 9 minutes at ten feet, and a repetitive group designator of K at the end of the dive, using the current Table 17-9 U.S. Navy Dive Table. If we used the U.S. Navy diving table for Air/O2 Decompression, the decompression would have been reduced to five minutes at 10 feet.

Yes, and this ties in with my last post (i.e. what the computer does is give you credit for time above the maximum depth).

John’s table calculations (assuming square profiles) do result in a modest deco obligation. I ran the “four dives” using the same assumptions through Multi-deco (30/70) and got only a short (1 minute at 10) deco obligation. Of course, running the whole dive as a square 60 foot, 70 minute dive on the “slightrox” that I was using (EAN22) gave me significantly more deco (5 minutes at 30, 9 minutes at 20, 23 minutes at 10), but that illustrates how important your assumptions are when planning a dive.

So since my profile was about as far from square as you can get, I think that what the computer was tracking was closer to my actual nitrogen loading than what you would get using tables and square profiles (either one dive or four separate dives). That’s why I think that my hit was more the result of the multiple ascents and “bubble pumping” than of a lack of deco stops to clear the actual nitrogen load.

The danger of the “yo-yo” profile is apparent when we consider bubble formation, specifically the silent bubbles which develop on ALL ascents. Those silent bubbles, which cause no trouble if you exit the water after an ascent, can cause problems with a yo-yo profile in two ways. One is that they act as a catalyst for further bubble formation, and two is that they can make off gassing less efficient than anticipated by the model.

Now, we are talking about altering the computer's profile to make it less "conservative," but when compared in two ways against the repetitive dive tables and the single dive tables, we see that the older way is actually more conservative than the the dive computer. I think we are using the dive computer inappropriately sometimes, and ignoring dive practices from the past of planning the dive and diving the plan, making the first dive the deepest, and watching the no-decompression limits (as defined by the U.S. Navy in my above post).

Yes, and this gets back to the OP (remember him?). Deco theory and human physiology are so complex and poorly understood, that to try to “reverse engineer” a dive computer looking only at a single number seems like a bad idea. Three cheers for planning the dive, and for safe practices. Thanks again, John…

 

[John C. Ratliff]

I had several reasons for doing this calculation the "ol' way."

First, I wanted people to know that they really don't need a dive computer to dive; they need a set of tables, a good depth gauge, a diving watch, and the vigilance to dive a plan. Dive computers are a relatively new tool that can have great benefits. I enjoy seeing my dive profile, as it triggers things in my dives that I can write in my log, such as that boil that sent me toward the surface, swimming head-down but not getting there for a minute or two.

Second, I wanted people to start thinking, really thinking, about how they dive. I have asked several times about dysbaric osteonecrosis, and only DoctorMike has replied, as a concern for all divers. There is another concern in the papers I found, and that is higher blood pressure on one side of the heart verses the other due to micro bubble formation. So micro bubbles are not something you really want to have, and apparently dive masters and dive instructors are at fairly high risk for this micro bubble formation.

The third reason is selfish--I needed the refreshing on calculating repetitive dive profiles. I feel that doing this exercise helps people understand the relationship between multiple dives and residual nitrogen times for the next dive. The computer does this automatically, but because it is "out of sight," it is also "out of mind" for most divers. And that can lead to mistakes that cause a decompression problem. This is pretty critical for some resorts, such as the one in the Philippines or other remote locations, as once a "hit" happens, options are limited. Prevention is the best way, and understanding the theory behind our computers can help.

But messing with the calculation to make it "less conservative" can be a prescription for either acute problems (DCS) or chronic problems (DON). DoctorMike, thanks for your blog entry, and for this discussion. I think it will help emphasize how critical these little instruments can be for our health and welfare.

I have one question for DoctorMike in his discussion above. DoctorMike, do you think that if you had a good map, and good landmarks underwater that can help to orient a diver without having to surface, that you could have avoided the "yo-yo" effect that you feel triggered the micro-bubble formation? I say this because a friend of mine, Bruce Higgins, has created an underwater park (Edmonds Underwater Park) which has guidelines to underwater formations (including sunken boats for reefs) and maps of the park. If so, maybe this is something you can discuss with the people at Dutch Springs.

SeaRat

 

[drrich2]

John C.

I addressed the issue of chronic concerns you raised in my last post, a few up from this, as to why I think we don't make a 'bigger deal' out of such, which would include dysbaric osteonecrosis.

Richard.

 

[doctormike]

I have one question for DoctorMike in his discussion above. DoctorMike, do you think that if you had a good map, and good landmarks underwater that can help to orient a diver without having to surface, that you could have avoided the "yo-yo" effect that you feel triggered the micro-bubble formation? I say this because a friend of mine, Bruce Higgins, has created an underwater park (Edmonds Underwater Park) which has guidelines to underwater formations (including sunken boats for reefs) and maps of the park. If so, maybe this is something you can discuss with the people at Dutch Springs.

SeaRat

Oh, for sure... at this point, I have been to Dutch Springs SO many times that I can navigate around that area with my eyes closed! They have a pretty good map as well, I was just taking my buddies to an area that we don't usually go to, away from the attractions....

That was one of my major take-home points in the writeup, that just wandering around and doing navigational ascents is NOT good technique.

 

[John C. Ratliff]

DrRich2,

I was busy posting my own post, and didn't see yours until later. You raise concerns which people may have, but which are not necessary. Government regulation is not necessary, and indeed NAUI, PADI and other certification agencies exist so that government regulations don't. The industry is self-policing at this point, except for DOT requirements concerning cylinders themselves.

It has been industry standard for many years to have oxygen available for treatment of decompression illness (see DoctorMike's blog for an example). At the Warm Mineral Springs Underwater Archeological Project, Sonny Cockrell and Larry Murphy set up the oxygen decompression, not in response to regulations, but in response to diving deep and having researchers on decompression schedules. It would not be that hard to set oxygen decompression up on charters, in my estimation. It takes a recognition of the hazard, and the need to follow through with preventive measures.

In the Industrial Hygiene field, we (the American Industrial Hygiene Association, or AIHA) have a concept called ERAM, which stands for Exposure Risk Assessment and Management. The first part of that is to recognize the hazard, and then to put into effect controls, using the Hierarchy of Controls (Eliminate, Substitute, Engineering Controls, Administrative Controls and finally Personal Protective Equipment). Oxygen decompression could be considered an engineering control if the system was correctly engineered and available for the individual divers off the boat.

SeaRat

John C. Ratliff, CSP, CIH, MSPH

 

[drrich2]

A lot of low wage subsistence dive instructors & guides working mainly for tips with low margin charter boat op.s would fine most any added expense a burden, and the gear, maintenance, inspections, oxygen fills, etc…, would probably add considerable burden, especially to 'budget' operations.

I was under the impression you were mainly referring to such charter op. instructors and guides who do large numbers of dives per year, though the large majority are within recreational limits and don't violate NDL (at least the NDL's generated by liberal computers) for the gas used.

Tech. divers often do use oxygen or oxygen-enriched gases to speed safe deco., from what I've read (never had tech. training).

So, just what operations do you propose to set up oxygen decompression on? Traditional deco. diving (requiring deco. stops)? Or were you proposing using oxygen for the recreational instructors & guides?

This is what led to my impressions -

Abstract

INTRODUCTION:

Dysbaric osteonecrosis (DON) is a type of aseptic bone necrosis of long bones such as the humerus, femur, and tibia. It is observed in workers who perform in high-pressure environments.
METHODS:

There were 58 volunteer divers included in this study who had performed at least 500 dives, were working as a dive master or instructor, had never performed industrial and commercial dives, and did not have a diagnosis of osteonecrosis. Radiological evaluation was performed according to the guidelines suggested by The British Research Council Decompression Sickness Panel. A total of eight X-rays were taken per patient. When suspicious lesions were detected, MRI of the region was performed.
RESULTS:

Of the 58 divers, 2 were eliminated because of inadequate X-ray studies. A total of 18 DON lesions were detected in 14 of 56 (25%) divers. Age was the only variable independently associated with the development of DON (P < 0.05).
DISCUSSION:

The DON prevalence of 25% in this study is high considering the dive instructors had thorough diving training and strictly practiced the decompression rules. We believe this high prevalence is a result of frequent and sometimes deep dives for many years. Our findings raise the question of whether these divers can be seen as "sports divers" or should be seen as "occupational divers." If the latter description is approved, dive masters and instructors should be kept under periodic screening for DON lesions just like professional commercial divers to help reduce the morbidity associated with this disease.
(emphasis added, jcr)
Dysbaric osteonecrosis in experience... [Aviat Space Environ Med. 2007] - PubMed - NCBI

 

[John C. Ratliff]

DrRick,

Note that I did not highlight the portion of that study that gave you the impression about regulation. I simply wanted people to be aware of the prevalence of DON within this group.

Concerning who would set up oxygen decompression, I have no expertise in that since I have not been to any resort anywhere. My aim is simply awareness of dysbaric osteonecrosis (DOC) as a diving malady that they may be subjected to (the part of ERAM above called "Anticipation and Recognition"), and the control measures that could potentially be used. Maybe people such as DevoneDiver could add to this part of the discussion. And maybe you or the other physicians who post here could talk about the debilitating effects of DOC in later life too.

Part of my concern is that Dive Masters and Instructors seem (at least some of them) to be "setting to nitrox to reduce the conservatism..." of their dive computers without realizing the potential long-term health effects of doing so.

SeaRat

John C. Ratliff, CSP, CIH, MSPH

---------- Post added July 2nd, 2014 at 09:01 PM ----------

I have just reviewed a publication in my library that I procured many, many years ago titled Dysbarism-Related Osteonecrosis, A Symposium which was published by the National Institute for Occupational Safety and Health in 1974. It had a lot in it about diagnosis and treatment of this disease, but not much about prevention. It talked about one person's experience with DON, where he was driving and suddenly could not lift his leg to use the gas pedal on his car. He had to lift it with his arm, then push with his arm to make the accelerator work. There were a lot of X-rays of knees, shoulders and hips showing necrosis of these areas, and discussions about cause mechanisms.

But that was 1974, and now it's 2014, forty years later. So I've been looking on Google Scholar and PubMed.gov for more current information, and apparently regular scuba divers are not at much higher risk than the general population. It is instructors and dive masters who are at higher risk, and professional divers such as those Japanese divers who harvest marine life on the ocean floor at 60-80 feet for a living. One study of submarine escape instructors, and another of military divers, showed them at no more risk than the general population too, probably because of stricter enforcement of decompression schedules and dive protocols. But apparently commercial divers are at a higher risk. There have been studies of pigs and sheet, and that regular compressions/decompressions can cause DON, and that oxygen pre-breathing can lessen the onset of DON. There is also some documentation of healing of necrotic bone with time (years), if that bone has not fractured or separated.

Given this brief study, I now don't see oxygen decompression as something that is really necessary. I raised the question because we were using it on that project in the 1970s, and was wondering whether it could apply to resort diving as a preventative measure, especially where there is no chamber for hundreds of miles.

However, I do think that trying to "trick" your dive computer by telling it your are on nitrox when you are not is setting you up for asymptomatic DON.

SeaRat

 

[industrious95]

The whole point of the computer is to provide you with information. If you want to cheat with the computer, just overstay it's recommended time, go into deco mode, and at least it'll give you good data about where you are and what your risks are.

The computer's only calculating what your safety margin is. It's the nitrogen dissolving in your bloodstream that's the real issue. You can't cheat your way out of that.