Page 53 - AlertDiver_Winter2013

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Q: Last week I did some shore diving in the Caribbean. One of the
divers in my group was very diligent about always wading into the
water using a technique he called the “stingray shuffle.” Might that
technique actually decrease the likelihood of a stingray injury?
A:Shuffling your feet while entering and exiting the water may indeed reduce
your risk of being stung by a stingray. Stingray injuries most often occur
when the animal is accidentally stepped on or startled. Their flattened
bodies facilitate effective concealment beneath the sand, making them difficult
to see. Even stepping over a stingray may be perceived as a potential threat and
prompt defensive action. Stingrays are not aggressive animals, but their barbed tails
are effective defense mechanisms capable of causing lacerations or punctures of the
foot or lower leg. Shuffling makes stepping on or startling a ray less likely, and it
increases the chances that rays will swim away to avoid contact.
— Niles Clarke, EMT, DMT; Nick Bird, M.D., MMM
Q: A friend and I were discussing how gas supplies are consumed
more quickly at deeper depths, and he suggested that
“consumption doubles every 33 feet.” That didn’t seem right to
me. Could you clarify?
A:This is a common misconception regarding pressure-volume relations and
Boyle’s law. The doubling of pressure only occurs between sea level (one
atmosphere) and 33 feet of seawater (two atmospheres). The change from
33 feet of seawater to 66 feet of seawater (two to three atmospheres) represents
an increase of only 50 percent; the change from 66 feet to 99 feet (three to four
atmospheres) represents a 33 percent increase, and the change from 99 feet to 132
feet (four to five atmospheres) represents a 25 percent increase. Effectively, the
compression of gas at 132 feet would account for an approximate five-fold increase
in open-circuit air consumption — still critical for planning purposes but nowhere
near the 16-fold increase your friend described. It is important to remember that
the effect of Boyle’s law represents an arithmetic, not a geometric, progression.
— Neal W. Pollock, Ph.D.
JAMES CARNEHAN