great white shark adventure - Midwest Scuba Diving Magazine

Reverse
Dive Profiles
By Dan Orr, DAN
President and
Chief Executive Officer
The ‘Well-Known Facts’
DAN Discusses Repetitive
Diving and the Twist On
Reverse Dive Profiles
On a recent dive trip, I overheard two divers talking about
planning their second dive for the day. The first dive was a
spectacular drift dive over a gently sloping wall in the warm,
clear waters of the Caribbean. The divers had chosen to remain
shallower than the rest of the group in order to photograph a pair
of turtles doing their best to keep the local turtle population from
becoming endangered.
While moving to the next location, the divemaster announced that
the second dive would take place at a similar location. Since their
attention was focused elsewhere on the first dive, they wanted
to see and enjoy the wall on the upcoming dive. This, however,
created a dilemma. They had been taught, as most divers have,
that the deepest dive always had to be made first. As one of the
divers put it, “It is a well-known fact that you are likely to get
bent if you don’t make the deepest dive first.”
This well-known fact seems logical. A shallower second dive
would appear to be a form of decompression following a
relatively deep dive. This traditional wisdom was challenged in
October 1999, when a workshop* funded by AAUS (American
Academy of Underwater Sciences), DAN, DEMA (Diving
Equipment and Marketing Association), Dive Training magazine
and the Smithsonian was held at the Smithsonian Institution to
specifically address the issue of reverse dive profiles. Those
present at the workshop represented the recreational, military,
and research communities. The findings were as follows:
• Historically, neither the U.S. Navy nor the commercial sector
has prohibited reverse dive profiles.
• Reverse dive profiles are being performed in recreational,
scientific, commercial and military diving.
• The prohibition of reverse dive profiles by recreational
training organizations cannot be traced to any definite
diving experience that indicates an increased risk of DCS
(decompression sickness).
• No convincing evidence was presented that reverse dive
profiles within the no-decompression limits lead to a
measurable increase in the risk of DCS.
After reviewing and discussing all the evidence presented, these
representatives of the diving industry came to the following
conclusion:
“We find no reason for the diving communities to prohibit reverse
dive profiles for no-decompression dives less that 40 msw / 130
fsw and depth differentials less than 12 msw / 40 fsw.”
In the final analysis, however, it is up to you and your buddy to
plan your dive in a manner that suits your objectives and gives
you the greatest level of confidence that the risk of injury will be
minimal.
Remember, dive tables or dive computers have no supernatural
ability to protect you from injury due to DCS. I’ve heard Karl
Huggins, co-inventor of the EDGE, the first electronic dive
computer, say many times that there is no talisman, electronic or
otherwise, that creates a magical shield that protects divers from
DCS.
Dive tables and computers are tools, not unlike your regulator or
DIVING MEDICINE
buoyancy compensation device. They are based on mathematical
models, founded in theory, and designed to emulate body tissues,
but those tools cannot take into account the wide range of body
and tissue types, nor factors such as cold, fatigue and exertion,
that may change the diver’s susceptibility during a dive. Diving
researchers do not completely understand the exact mechanisms
of injury in regards to DCS. Even when divers use tables or
computers correctly, there is always a risk of DCS.
Divers can take the following steps to reduce the risk of DCS:
• Dive conservatively. Don’t push the limits. Many experts
believe that the closer one comes to the U.S. Navy nodecompression
limits, the greater the risk of DCS.
• Avoid minimum surface intervals. Surface intervals allow
offgassing of residual nitrogen. Increasing your surface
interval will reduce your nitrogen load and offer greater
benefit to the slower tissue compartments.
• Make slow ascents and make safety stops. Research has
shown that slow ascents and safety stops help to reduce
the size and quantity of gas bubbles in circulation. Many
researchers believe there is a relationship between gas bubbles
in circulation and the likelihood of DCS symptoms.
• Manage additional risks. If the risks change during the dive,
it may be wise for the diver to reduce bottom time or increase
the safety stop or both.
• Avoid cold and dehydration. By being properly hydrated
and using appropriate exposure protection, you can help
decrease DCS risks. Dehydration and cold may actually
increase risk of DCS by affecting the body’s blood flow,
which influences nitrogen exchange through respiration.
• Maintain a high level of personal fitness. Exercise tolerance
is essential to safe diving. Exhaustion may significantly
increase risk.
When it comes to your well-being and the safety of your diving
partner, don’t take anything for granted. Well-known facts, once
thought to be absolute, are regularly being challenged.
Some of these “well-known facts” have been with us for quite
some time, but I wouldn’t stake my life on them.
* Vann RD, Denoble PJ, Pollock NW. Reverse Dives And
Project Dive Exploration. Reverse Dive Profiles Workshop;
1999 Oct 29-30 2000; 1999: 181-187.
Snapshot: Dan Orr
Dan Orr has been diving for 40-plus years and has held
membership and leadership positions in many notable
diving organizations such as NAUI, PADI, ACUC, YMCA,
NASE, IAND, UHMS, NACD, AUAS, the Institute of Diving
and the Explorers Club. He is the recipient of numerous
awards such as AUAS’s NOGI Award for Sports/
Education, NAUI’s Leonard Greenstone Award for Diving
Safety, the Our World-Underwater Award and Beneath
the Sea’s Diver of the Year. He was named Chairman of
the Board of the Historical Diving Society for 2004 and
currently serves on the DEMA board of directors.