DiviNG EBER WARD - Midwest Scuba Diving Magazine

DIVING SCIENCE
Nitrox: Air on the
Side of Caution
by RICHARD TALAGA PH.D
On a recent dive trip to Florida, a drift dive near
Palm Beach, the divemaster announced “This
will be a 30 minute dive for those diving on air
and a 40 minute dive for those of you diving
on Nitrox”. More and more divers are taking
certification courses to use Nitrox because,
among other benefits, it reduces nitrogen
exposure and allows divers to increase their
bottom time. So, what is Nitrox and what do
we need to know to use it safely? Instructors
report that students find some of the physics
concepts, as they are presented in Nitrox
courses, confusing. The goal of this article
is to clarify those concepts by presenting a
simple description of gas and gas pressure.
We’ll also work through some examples and
solve a few problems that Nitrox students
often encounter.
Nitrox is not just one type of gas. Rather,
it’s a family of gases composed of nitrogen
and oxygen. To extend bottom time, divers
use Nitrox blends with proportionately less
nitrogen and more oxygen than air. More
properly called Enriched Air Nitrox (EANx),
the proportion of oxygen and nitrogen is
custom-blended to fit a specific dive profile.
Most Nitrox gas mixes have 30% to 40%
oxygen. EAN 32, for example, is a Nitrox
gas with 32% oxygen and 68% nitrogen. Of
course the most common Nitrox gas with 21%
oxygen and 79% nitrogen is good old air.
If reducing nitrogen exposure is the goal,
why not remove all of the nitrogen and fill
the cylinder with pure oxygen? It turns out
that too much of a good thing is dangerous!
Overexposure to oxygen leads to a condition
known as oxygen toxicity, which can be
deadly. We can’t tolerate high concentrations
of oxygen in our system just as we can’t
tolerate high concentrations of nitrogen.
Oxygen toxicity is not an issue for divers if
they obey recreational diving limits. However,
Nitrox users are exposed to higher oxygen
concentrations and are trained to understand
oxygen exposure limits and to dive safely
within those limits.
The key to diving Nitrox safely is proper
planning to prevent overexposure to nitrogen
and oxygen. Divers with open water
certification already know how to use dive
tables, designed to prevent overexposure to
nitrogen when diving with air. Nitrox divers
also use dive tables that look like standard
air tables but that are specific to the nitrogen
fraction in their cylinder. Also, because the
oxygen fraction in EANx is greater than in air,
Nitrox divers must calculate their maximum
dive depth to keep oxygen exposure within
acceptable guidelines.
Nitrox calculations are easy but not necessarily
intuitive. If you have a picture of what’s
going on inside a gas container, the math will
make sense and calculations will be more
intuitive. Let’s get started by viewing gas
from a microscopic perspective: just a bunch
of molecules rattling around in a container.
It’s the molecules that are responsible for
gas pressure and different types of molecules
contribute their share to the total pressure.
That’s Dalton’s Law of partial pressure, which
we’ll use several times to illustrate Nitrox
calculations. Let’s enter the microscopic
world of gas molecules and have a look at
what’s going on.
Gas
A gas is a collection of molecules that are not
bound together. Because molecules aren’t
bound together they will quickly disperse
unless they are in some kind of container. In
some sense, molecules are like tiny ping-pong
balls. Imagine a bunch of ping-pong balls
rattling around in a lottery jar. If the glass
breaks, the ping-pong balls will fly out in all
directions, never to return. A gas has to be
kept inside something like a bottle, a balloon,
our lungs…you get the picture.
Air is a “mixed” gas composed of several
“pure” gases. Nitrogen and oxygen amount
to about 99% of air, followed by argon
(0.93%), carbon dioxide (0.038%) and other
trace gases. Since air is a mixed gas, its
molecules are not all alike. Techniques have
been developed to separate mixed gases into
their pure components, which can be packed
into cylinders for commercial use. Let’s look
at the main components of Nitrox: nitrogen
and oxygen. Nitrogen molecules are about
12% lighter than oxygen molecules and both
types of molecules are about the same size:
very small. An important difference between
oxygen and nitrogen is that their chemical
properties are different. Breathing pure
nitrogen won’t sustain life.
Nitrox contains mostly oxygen and nitrogen
molecules, along with trace amounts of other
types of molecules found in air. What are all
those molecules doing? They are moving
in random directions, with speeds averaging
close to a thousand miles per hour bouncing
off each other and off the walls of whatever
contains them. The hotter the gas, the faster
the molecular speeds. A gas is usually
characterized by its absolute temperature,
pressure and how much volume it occupies.
Temperature and volume are terms used by
people in every day life and need only a little
additional explanation, which we will get to a
bit later. Gas pressure, however, is a different
story.
Pressure
The pressure of a gas inside a container is
due to the constant barrage of molecules
bouncing off the container walls. There are so
many molecules hitting the walls at any time
that it doesn’t feel like the jerky pounding of
individual molecules. Instead, it feels like a
constant uniform force pushing outwards.
Molecules are distributed evenly throughout
the volume of the container, so that the
number of molecules bouncing off any square
inch patch of wall is the same as the number
of molecules bouncing off any other patch
on a different part of the wall. This constant
force per unit area is the gas pressure, often
expressed as the number of pounds per square
inch (psi). We conclude that gas pressure is
the same at all locations inside the cylinder.
Three factors affect gas pressure: the number
of molecules in the container, the container’s
volume and the gas temperature. Let’s see how
a change in any one of these factors changes
the pressure. Consider a rigid container,
like a SCUBA cylinder. If we add more gas
molecules into the container, the number of
molecules hitting the cylinder’s inner wall
increases, which raises the pressure. If the
volume does not change, the gas pressure is
proportional to the number of molecules in a
container.
Instead of adding more molecules, we could
heat the cylinder. Heating the cylinder also
heats the gas, so gas molecules have faster
speeds and hit the cylinder wall harder and
more frequently, thereby raising the pressure.
Cooling has the opposite effect and the
pressure is reduced. Gas pressure is directly
proportional to the absolute temperature.
This is known as Charles’ Law.
The third factor is volume. If we transfer all
of the gas molecules from a larger container to
a smaller container, the molecules now don’t
have to travel as far to bounce off a wall. They
hit the wall more frequently, thereby raising
the pressure. In addition, molecules are more
concentrated in the smaller volume, so there
are more molecules for every square inch
patch than before. This also has the effect
of raising the pressure. The end result: a
reduction of the volume causes an increase
of the pressure. Of course the converse
is also true: a bigger volume results in a
smaller pressure. In summary, gas pressure is
inversely proportional to the volume. This is
known as Boyle’s Law.
Gas Fraction, or FO2 and FN2
Since Nitrox comes in a variety of blends, you
have to know the exact percentage of oxygen
and nitrogen in the SCUBA cylinder you will
be using. Nitrox divers are required to measure
the oxygen content with an oxygen analyzer,
which displays fraction of oxygen, FO 2
as a
decimal fraction. For example, if the oxygen
fraction is 0.32, the cylinder contains EAN 32.
The fraction of nitrogen, FN 2
, contained in an
EAN 32 cylinder is equal to 0.68, since FO 2
+
FN 2
must equal 1.00. In other words, EAN 32
consists of 32% oxygen molecules and 68%
nitrogen molecules.
As Nitrox is used and cylinder pressure drops,
FO 2
and FN 2
do not change. Suppose that after
20 minutes of diving on EAN 32 your cylinder
pressure drops from 3,000 psi to 1,600 psi.
FO 2
is still equal to 0.32 and FN 2
is still
equal to 0.68. This should be obvious if you
think about diving with air. Your air cylinder
contains 21% oxygen and 79% nitrogen
18 MIDWEST SCUBA DIVING WINTER 2006