DAN Medical Frequently Asked Questions
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Specific Tips and Tricks
>You should know not only which buttons to push to make your computer work, but also which mathematical model or model derivation it employs in making its decompression computations. There is a surprising range in models, from conservative to liberal, and these differences may not be evident at first glance. For example, a computer may establish conservative limits for an initial dive but liberal limits for repetitive diving. It is best to learn enough about the various available models and derivations before you select a dive computer, so you are sure to choose one that is compatible with your own level of risk tolerance. Choosing one purely based on familiarity may not be the best strategy. Even if you have had good outcomes on previous dives with a computer, it does not guarantee that it will be the best one for your future diving. Accumulating knowledge takes commitment, but informed planning for decompression safety should be a top concern.Failing to turn on your dive computer (or to take it with you on a dive) may sound like a joke, but it does happen and can create real problems. No computer can factor in the exposure profile of a previous dive if it was not there. And any decompression model is invalid unless you start using it when you are "clean" — fully off-gassed from any previous dives. If you forget to take your computer with you on a dive early in a repetitive series, you are then restricted to using tables for the duration of that series (assuming that you are able to manually compute the exposure of the unmonitored dive). And do not even think about hanging your computer on a downline during a surface interval in an effort to compensate for having forgotten it on an earlier dive; there may be stories about that happening, but it is not a responsible practice. The only person who does not have to worry about taking a dive computer on every dive is the one who uses it solely as a datalogger — that is, only to record time and depth information instead of to calculate decompression profiles. Remember, however, that using your computer simply to log your time and depth data means that you must still plan all your dives using dive tables and must recompute your repetitive group status afterward, as appropriate. You cannot move in and out of relying on your computer's decompression computations unless it has recorded all of your exposure profiles.Dive computers are wonderful at carrying out programmed mathematical computations, but they are blind to the many insights you may have before, during and between your dives. For example, your dive computer knows nothing about your personal health status, your level of physical fitness or your individual susceptibility to decompression stress. It also knows nothing about your thermal stress or physical efforts during or between dives. The fact that many dive computers display water temperature might suggest that thermal stress is factored into the device's algorithms. A water temperature reading, however, provides no useful information regarding thermal stress, since the diver carrying the device could be wearing anything from a bathing suit to a wetsuit without a hood to a cold-water drysuit with a hood, gloves and cold-water undergarments. More important, it is not yet possible to directly compute the impact of differences in thermal status during different parts of a dive, even if the computer was able to measure the diver's core temperature and skin temperature in key spots.
>We do know that being warm (rather than cool or cold) during the compression and bottom phase of a dive promotes inert gas uptake (not optimal), and that being warm during the decompression phase promotes elimination (optimal). While impractical for the comfort-loving diver, decompression safety is optimized by being neutral or cool during the inert gas uptake phase of descent and bottom time and warm during the inert gas elimination phase of ascent. While the concept of thermal changes on decompression stress is clear, we are still years away from being able to quantify the real-world effects of these factors for dive-planning purposes. Similarly, while some computers are able to track gas consumption, we have much to learn before this information can be meaningfully incorporated into decompression models. Variations in air consumption can reflect differences in the depth of a dive or in the diver's experience, level of anxiety or degree of physical exertion. The bottom line is that interpreting the precise physiological impact of the interactions among these diverse factors is exceedingly difficult, requiring thoughtful practice by divers.Divers need to pay attention to their dive computers if the information provided is to be of any use. Be aware that confirmation bias can promote risky behavior. "Getting away with" a risky exposure once, twice or even many times may eventually catch up with you. It may not truly be safe for you or for a partner who might have a higher degree of susceptibility to decompression stress. Those who wish to worry less about their exposure will have greater peace of mind if they choose a computer that employs an extremely conservative decompression model. It is also important to pay attention to your dive computer. If you are diving with a group, do not forget that there can be considerable variability in the guidance provided by different computers or computers with different user-selected settings. That means there is considerable benefit in diving with others who use a computer with a similar decompression model and settings, because if modest discrepancies arise, following the most conservative directive will likely not be terribly burdensome for the group. But if members of a group are using dive computers with substantially different models, and each diver wishes to follow his or her own device, it can lead to a breakdown in the buddy system.Although heeding your computer is important, do not take its advice unthinkingly. The same profile can sometimes be conducted without problem again and again, right up to the dive where it does not prove safe. Divers often try to blame a specific factor, such as dehydration, for the development of symptoms following one dive but not another. This approach is not productive. The range of variables in play during a dive are rarely identical, and there is a probabilistic element to decompression risk — that is, chance can play a role in the manifestation of DCS.
>The best approach is to avoid the extremes of either fatalistic resignation or smug focus on a single supposed magic bullet. There are many, many small steps you can take to make any dive safer. The most important one is to stay within a reasonably conservative time-depth profile and to add safety stops to every dive. Other important steps are to minimize your exercise intensity and avoid overheating during the gas-uptake phase of your dive, to choose the right breathing gas, to practice enough that you are able to perfectly control your buoyancy, to remain well-rested and well-hydrated, choose more conservative user-adjustable settings on the computer, and to dive with a partner who has similar goals and follows similar practices. Adding small safety margins to each step can help to provide a comfortable security cushion. Dive computers are powerful tools, but sound knowledge of diving physiology, good physical conditioning and adherence to thoughtful practices offer the best protection for divers. If you do develop DCS symptoms, you should keep your computer with you when you go for medical evaluation. Some facilities may have the ability to download or review your profile to aid in the evaluation of your case. The medical staff will surely appreciate seeing confirmation of your description of the events that precipitated your symptoms.