A question almost as old as recreational diving
By-Dr Simon Mitchell
I must confess that I don’t go back to the beginning of recreational diving, but the question in the title of this article is certainly as old as my participation which began in 1974. Back then, that question was ‘what is the best dive table?’ What people really meant was âwhat is the safest dive table?’ Today, it’s usually ‘what is the best dive computer?’ and this more contemporary question focuses on much more than just safety. Nevertheless, uncertainty about the relative safety of dive computers is frequently expressed, and it remains the subject of extensive debate among those who promote one brand over another. In this article I will try to point out why arguments over the relative safety of different dive tables and computers are often unrewarding and frequently misleading. Warning: I won’t provide you with an answer about safety (especially with regard to computers), mainly because there isn’t one. But I will provide you with some perspectives that will help you distinguish signal from noise in any debates you read or hear on the topic.
Let’s first confine our discussion to dive tables. This is an appropriate place to start since even though tables are much less frequently used these days, it must be remembered that all dive computers are based on a dive table of some description.
Dive tables seek to predict (and avoid) the conditions under which nitrogen (or another inert gas) will form bubbles during or after ascent in sufficient number and / or size to cause the symptoms of decompression illness. Just that short ‘statement of intent’ embodies most of the problems that have beset dive table designers since Haldane had the first really serious go at it in the early 1900s.
Discussion of such problems could occupy an entire book, but there are two important stumbling blocks in dive table design. First, we are still not clear about the precise conditions required for bubble formation. Although we know it is all about dissolved inert gas tensions exceeding the ambient pressure (a state often referred to as ‘upersaturation’, we are still not really sure where, under what conditions, or how bubbles form. Second, we are not entirely sure how these bubbles actually cause the disease we refer to as decompression illness. It may stagger you to know that we still (hotly) debate the mechanism of the most common (musculoskeletal) and most disastrous (spinal) manifestations of this very complex disease. Added to these problems, there is ample evidence for marked inter-individual inconsistency between decompression stress and bubble formation, and between bubble formation and development of symptoms. Thus, different individuals exhibit different degrees of bubble formation after the same dive, and similarly, some individuals appear able to tolerate a degree of bubble formation without developing symptoms, whereas others would become unwell with the same bubble exposure.
To summarise this, it should be clear to you at this point that dive table designers face the near impossible task of trying to model a process (bubble formation) that is not understood, in order to prevent a disease whose interaction with that process is inconsistent and is itself still not understood. Bit of a nightmare really!
So how do we get around these seemingly insurmountable problems? Various strategies have been tried in dive table design, with most tables having arisen from attempts to mathematically model the processes of gas uptake and elimination, and bubble formation. As I have pointed out, even if bubble formation can be predicted accurately in this way (which is difficult), this is still one big step away from accurately predicting the occurrence of decompression illness. Nevertheless, these mathematical exercises produce a set of numbers which purport to guide our times, depths and decompressions during diving.
Given the constraints and difficulties in what dive table designers attempt to do, it is not surprising that the history of table design is littered with examples of tables that required modification when they started to be used in the real world. This was usually because the incidence of decompression illness associated with their use proved to be too high. Many of the tables used over the last 50 years underwent a bit of tweaking based on early trends when they were used in the field.
The alert reader might well be asking themselves ‘why weren’t tables simply subjected to careful testing programmes to determine their safety before release?’ The answer is that such testing is extraordinarily hard to do. The problem is that you are trying to prove that an undesirable event (decompression illness) is very rare when the table is used properly, and therefore that the table is safe. To demonstrate that an event is rare with any statistical certainty requires a very large number of test repetitions. Moreover, apparent success in one dive profile does not necessarily indicate that all profiles generated by the table will be equally safe. A wide range of dive profiles need to be tested. Finally, the ‘tests’ (dives in this case) need to be standardised, realistic (eg in-water dives rather than chamber dives) and carefully controlled. Giving tables to lots of divers to use and keeping a vague eye on their outcomes does not constitute valid testing of a table.
Thus, you must appreciate that adequate testing of a dive table will involve thousands of carefully controlled dive repetitions. Perhaps now it will come as no surprise to learn that very, very few dive tables have undergone enough formal testing to statistically prove the safety of any of their profiles. One exception to this is the Canadian Navy or DCIEM table. Computers are even worse, because of the myriad of combinations and permutations of dive profile they will permit. The reality is that none of them has undergone sufficient testing to allow an accurate quantification of safety (or risk) associated with their use. The same most certainly applies to the various software packages that allow calculation of decompression profiles for technical divers. Indeed, some of these packages are the least tested of any of the dive planning tools. It is always amusing to read the vituperous debates over the relative merits of these technical dive planners that are played out on internet chat lines by participants who often have little, if any, understanding of the concepts they are getting so uptight about. In arguments over the theoretical attraction of the various deep dive planning packages, the respondents always seem to forget that the number of recreational bounce dives to depths greater than 100m using any planner or computer, performed anywhere in the world, constitutes nothing more than a statistical ‘handful’. How could we possibly have any idea how safe these packages are?
So where am I going to with this? Am I saying that dive tables and computers are untested and therefore unsafe or no good? Absolutely not. But what I am saying is that the ‘question almost as old as recreational diving’, viz: ‘which table or computer is the safest’, cannot really be answered accurately, and there is little point in arguing about it (although I have no doubt that divers, and particularly technical divers will continue to do just that). By and large, the tables and computers being used out there appear to be doing a good job. The incidence of decompression illness is low (about one case in 10,000 recreational dives, and much less for serious decompression illness). Divers still occasionally get sick when diving within the limits of their table or computer, but this is almost inevitable given the apparent individual variation in susceptibility that I mentioned earlier. Indeed, to further reduce the number of such incidents the limits provided by our tables or computers would have to be more conservative, perhaps much more, and therefore much less useful.
I reiterate that this is not an anti-table or computer message. I personally own four dive computers: a Suunto Stinger, a Uwatec Air Z, and two VR3s. However, I have not ed any of them on the basis of perceived safety. I do admit to being cognisant of the fact that worldwide experience indicates none of them are likely to be radically unsafe! It is their other features that are the keys. I can wear the Stinger as a watch and it has a depth gauge that will read to 150m. That is a rarity in modern gear. I use the Air Z for normal scuba air diving because of its fantastic display and ease of use. The VR3s I use because they resolve dive profiles for mixed gas closed circuit rebreather diving. All of these devices are downloadable (which I think is a fantastic feature). No, I am not sponsored by any of the manufacturers! I name these devices merely to point out the features that are especially important to me. I would never try to argue the safety merits of any of them against each other, or against similar devices on the market. There are many excellent dive computers out there.
I hope this article provides more enlightenment than confusion. I suppose the main point is that it is fine to argue about tables and especially computers on the basis of ease of use, and various features, but claims of superior safety should always be treated with suspicion. They might be true, but it is hard to know.
I suppose I should finish by saying that there is at least one bright light on the horizon with regard to dive table and computer design for recreational diving. This comes in the form of Project Dive Exploration being run by Dr Richard Vann at DAN America. This programme involves the gathering of accurate decompression stress and outcome data from volunteer divers all around the world. These divers wear a downloadable dive logging device which precisely records their dive profiles. These profiles are downloaded to DAN along with reports of the post-dive outcomes. Obviously, the most common outcome is that there are no problems, but occasionally decompression illness will arise, and when it does, DAN will have an exact record of the decompression stress that caused it. When this database reaches millions of dives, there will be hundreds of cases of decompression illness, and the knowledge of patterns of decompression stress leading to problems may prove a powerful tool in the development of tables and computers that truly are safer.
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