A Trip to the Past
Steven Shepard
[email protected]
16 October 2014
Few of my readers
know this, but in a
prior life I was a
professional commercial diver. I did deep-water
survey work wearing Mark V diving helmets and
Kirby-Morgan Band masks, and was the co-owner
of a dive shop in northern California called the Sea
Hut. It was an important part of my life, and it’s
where I
developed
a professional interest photography and in fact wrote my
very first book, a SCUBA diving handbook for
instructors called “Commotion in the Ocean.”
My wife and I took a short vacation recently, and among
the places we visited was Gloucester, Massachusetts, the
coastal fishing town where the movie “The Perfect
Storm” was filmed (we had a beer in the bar that was
used in the movie).
One morning while walking along the waterfront I
spotted a sign that said, “The Diving Locker.” Intrigued,
we went in, where we found Paul Harling, leaning on a
Mark V helmet. The place was a diving museum of sorts,
and I suddenly found myself transported back to my
past. Paul was the “curator” of the place; over the years
he had surrounded himself with diving paraphernalia of
all kinds, much of which at one time or another I had actually used. There were wet suits dry
suits, canvas deep-water suits, hot water suits (yes, they pumped hot water from the surface
down to the diver’s suit to maintain body temperature when diving in deep cold water), every
size and shape of tank you can imagine, special underwater tools, rebreather units, even voice
synthesizers. Why? Well, it’s kind of interesting. Here’s a little physics for you. The air we
breathe at the surface is roughly 80% nitrogen and 20% oxygen. As divers descend below the
surface, every 33 feet of seawater adds one atmosphere of pressure to the environment. To
overcome that added ambient pressure, the regulator must compensate by increasing the
pressure of the air delivered to the diver’s mouth, otherwise the outside pressure would
prevent the air from coming out of the mouthpiece. So at 33 feet, the air is delivered at two
atmospheres of pressure (2 x 14.7 PSI, or 29.4 PSI), at 66 feet, three atmospheres, and so on. This
is why it’s a VERY bad idea to hold your breath on ascent: that air in your lungs is at a pressure
that is higher than sea level air, and it WILL come out—often in life-threatening ways.
But there’s another problem. At 60 to 100 feet (it varies with the diver), nitrogen becomes a
narcotic, which divers used to call rapture of the deep. Today it’s called nitrogen narcosis, and it
can be very dangerous. So sport divers are cautioned to keep their dives relatively shallow and
to ALWAYS dive with a buddy.
And finally, there’s another issue. As it happens, oxygen can also be your enemy as a diver, but
typically not for sport divers. It turns out that when the partial pressure of oxygen exceeds two
atmospheres, it can actually become a poisonous agent, leading to the sometimes-fatal problem
called oxygen toxicity. Commercial divers are prone to this when diving deep because once
they pass a depth of about 200 feet, where the air is being delivered to them at ten atmospheres
to overcome the ambient pressure, the PARTIAL pressure of the oxygen, which is 20% of the
mix, becomes two atmospheres—and deadly. To overcome this day-ruining effect, divers use
mixed gas, where the partial pressure of the oxygen is reduced by a computer so that it doesn’t
reach levels of toxicity, and the nitrogen, which would become happily narcotic, is replaced by a
non-interactive noble gas like helium, a breathing mixture called “heliox.” But there’s
ANOTHER problem: helium is a monatomic molecule, one result of which is that it doesn’t
hold heat very well—which means that a diver breathing it will quickly become very cold—
hence the hot water suit. Want one more problem? Divers who breathe heliox sound like a
drunken Mickey Mouse when communicating over the radio from the seabed, because the
density of the gas is so much lighter than air. Their vocal chords vibrate much faster, so they get
that wonderful high-pitched sound. To make them understandable to the folks on the surface, a
synthesizer is used to make their voices sound
normal again.
And you thought diving was just for fun. By
the way, that’s me in the silver suit, teaching a
class in Monterey, California in 1979. MAN
that was a long time ago. I still had a waist.
Thanks for reading.