Dive Computers – Insights for Divers & Professionals. Wolfgang Wild

Чтение книги онлайн.

СКАЧАТЬ To give the body time to get rid of excessive gas absorbed in the tissues during the dive – i.e. off-gassing or gas-washout, as it is called.

      Diving instructors among the readers who have ever asked their students in the classroom to walk the distance of 60 feet/18 meters in one minute know how slow that is; so strolling instead of walking might be the more appropriate expression. Also well known is that under water air in the BCD (and the dry suit) expands during ascent. This means that on the way up to the surface, especially during the last 33 feet/10 meters where the water pressure decreases by 100% and the air volume in the BCD would increase proportionally (if the diver would not deflate appropriately) – during these final feet it becomes more and more difficult for the diver to control his ascent rate. This is especially difficult for the unexperienced diver because he not only has to release air from his BCD and/or dry suit, but he also should watch up to the surface to make sure nothing blocks his way home and to ensure free airways, and at the same time he should keep an eye on his depth gauge and watch or his dive computer to control his ascent rate. This is far from easy for novice divers. And we were just talking about 60 feet/18 meters per minute, and not about 30 ft/9 m per minute, or even slower …

      In so far nothing has to be changed in the statement, which we could read in May 1989 in the US magazine Skin Diver as the headline of an editorial by publisher Bill Gleason:

      60 FEET A MINUTE IS A LONG, LONG TIME

      Following a suggestion in this Skin Diver editorial the author of this eBook conducted a small experiment with staff and candidates during an instructor training course: Along the ascent line which led from an anchored platform in 16 meters/52 feet to the surface, clearly visible depth markers were attached; clearly visible, however, only for the staff – the participating divers were asked to look into the beautiful clear waters of this Austrian lake and watch endemic fish swim by. Their only assignment was to ascend continuously and as slow as possible.

      We can make it short: most of the divers were pretty much convinced that their ascent rate never exceeded the recommended maximum of 18 meters/60 feet per minute – which had meant a total of 52 seconds from a depth of 16m/52 ft [here the quick and easy calculation in feet: 1 minute = 60 seconds for 60 ft; this equals one second for one foot, or 52 seconds from the depth of 52 feet].

      Well, for good reasons several staff was positioned at various depths next to the depth markers on the ascent line, and their assignment was to note the time which the divers needed from 52 to 40 ft, from 40 to 30 ft, from 30 to 16 ft, and from 16 ft to the surface.

      The reader will already smell the outcome: A maximum of 60 feet per minute? Not really. For the last 16 feet to the surface several voluntary participants only needed 6 seconds, which equals to possibly record breaking, hardly to believe 160 feet per minute. Not only the time taking staff was frowning, imagine the faces of the participating divers. After all, we hadn’t invited dive beginners to this self-experiment, but ambitioned, future instructors.

      The following illustration sketches the set-up of this attempt.

      (In case the reader should be puzzled by the very special fish which curiously sneaked into the picture – this is a species found only in a lake by the name of “Erlaufsee” in the mountains of Austria: Harryensis endemicus, and it is by no way instructor training staff in camouflage equipment; readers who wish to go diving in this beautiful Austrian lake find the web and email address of dive center owner Harry in the bibliography.)

      As an effort to reconcile everybody, including himself, the following day a new attempt with a different approach was offered. The participating divers should make a short stop during their ascent at 9 meters/30 feet, if necessary adjust their buoyancy, then continue their ascent to 5 meters/16 feet, stop again and adjust their buoyancy again as necessary, and then finally ascend to the surface. (The stop time was irrelevant as the dive time was far from any no-decompression times.) And now, not really surprising, satisfied faces everywhere during the debriefing after the dive. The cozy ambience in the traditional Austrian restaurant “Seewirt”, located right at the lake, with a special cup of coffee with whipped cream contributed to the reviving good mood.

      Occasional contact with participants of this practical field test indicate that a lasting impression has remained which contributed to a sustained change in diving habits.

      One question is still open: What about trying to abide by an ascent rate of, let’s say, 10 meters/33 feet per minute?

      The answer is a practical one: try it, dear reader of this eBook, try it – and have yourself be critically observed by your buddy. „If anyone wants to go slower, they should be encouraged. ... The problem lies not in the theory but in the practice.“ (Richardson, Drew, Slower Ascent Rates, The Undersea Journal, Third Quarter 1988, pp. 5-6)

      Impatient, eager readers will perhaps have the following argument in their mind: But my dive computer does show me if I am ascending too fast, and in addition it warns me with an audible signal. Good objection. Annoyingly at least in the past some dive computers issued such “warnings” – but without any consequences if ignored by the diver.

      “Virtually all of the dive computer user's manuals call for a rate of ascent that is less than 60 fpm, and during our tests, which were all conducted using a rate of ascent of 60 fpm, they beeped and blinked and did everything short of electrically shocking us. Some advise the user to use a variable rate and some of the rates are as low as 20 fpm. Anyone who has tried to achieve such a rate, let alone tried to teach it to a new diver, would be happy to hear that the dive computers performance is not linked to these impractical values. Unfortunately, while we believe that this is true, there is no practical way of testing it.”

      Source: Lewis, John & Shreeves, Karl, The Recreational Diver’s Guide to Decompression Theory, Dive Tables and Dive Computers; Santa Ana, California (USA), 2nd Edition 1993, p. 76

      And the advice for the baffled reader was in 1994 the same as is in 2014:

      “We can do nothing more than advise the reader to contact the manufacturer directly and ask for himself what, if any, requirements are a necessary element of the dive computer's design. Until advised otherwise by the computer manufacturer, you are best advised to follow the ascent procedure prescribed by the dive computer manual.” (ibid.)

      In our later parts Deep Stops – what is that? and Which stop-depth is the correct one? we will come back to this topic once again.

      Excursus – How do divers control their ascent rate?

      As stated above, the challenges during the ascent are quite complex: looking up to the surface to make sure no Manta Ray, boat or the like is blocking your way; ensuring at the same time that your airways are free so releasing excessive air from your lungs is not hindered; controlling your depth & timing device to avoid coming up too fast, and deflating your BCD and/or dry suit – all at the same time.

      Good idea trying this in practice, which we also did (again with instructor candidates). And the voluntary participants in action were of course captured on fotos. Before showing them this undisputable sort of proof some of the participants refused to believe or just denied how they had performed their ascents. A few examples only: Not looking up but rather down to the instrument console with depth-gauge or dive computer; or: in fact looking up and reaching up, with the left hand holding up the BCD hose, ready for deflation – but what about looking at the instrument console or the dive computer at the left wrist?

      The following pictures have been reconstructed with staff members to protect СКАЧАТЬ