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Chapter 5 Dive Tables and Dive Computers 57
LEARNING GOALS
Comprehension Objectives
Learn about NAUI EANx Dive Tables and NAUI
RGBM EANx Dive Tables.
Explain what Equivalent Air Depth is and how it
is used with air dive tables.
Be able to determine and calculate Equivalent
Air Depth.
Learn how to use the NAUI RGBM Nitrox Dive
Tables.
Define the “Rule of Halves” and how to use it in
all of your diving.
Understand about using dive computers for
nitrox diving.
DIVE TABLES
In your entry-level Open Water Diver course, you
learned how to use dive tables to plan and execute dives.
You learned that dive tables are used to monitor and
control the amount of nitrogen in our bodies in order
to minimize the risk of decompression sickness. This
chapter will build on that knowledge and acquaint you
with using dive tables designed for diving with oxygenenriched
air.
There are many different dive tables in use today.
They are based on varying models of nitrogen absorption
and elimination. If you were certified as a NAUI Open
Water Diver, then you probably used either the NAUI
Dive Tables or the NAUI RGBM Dive Tables to plan
your dives. Or, you might have learned to use the U.S.
Navy Tables (on which the NAUI Dive Tables are
based), the Canadian Defence and Civil Institute for
Environmental Medicine (DCIEM) Tables, or tables
developed by Dr. A.A. Bühlmann of Switzerland (which
were also designed to accommodate altitude diving). All
three of these tables are endorsed for use by NAUI in
NAUI courses. Another algorithm, which was developed
under the auspices of the Diving Science and Technology
Corporation, is used in the PADI Recreational Dive
Planner. The British Sub Aqua Club has its own tables;
the British Royal Navy has its; Stolt Offshore has its. In
fact, many navies and most commercial diving operations
have developed proprietary dive tables for their own use.
Most have also developed tables for use with oxygenenriched
air.
If you want to learn more about decompression
theory and dive table development, you can advance
your knowledge in a NAUI Master Scuba Diver course.
There are also texts that present advanced decompression
theory, and good information can be found on many
websites. In this course, we will consider only the classic
NAUI Dive Tables and the NAUI RGBM Dive Tables,
which use the more modern concept of dual-phase
modeling and the Reduced Gradient Bubble Model
developed by Dr. Bruce Wienke. We also assume that you
already know how to use dive tables to plan your dives. If
you are out of practice in the use of dive tables and dive
table terminology, you should go back and review the
information in your basic scuba textbook.
Remember that if you are diving at higher altitudes
(above about 300 meters or 1000 feet), you will have to
use special altitude dive tables or apply a conversion for
theoretical depth. NAUI RGBM Tables are available that
cover three altitude ranges up to a maximum of 3048
meters (10,000 feet) above sea level.
Also, planned required-decompression diving,
whether breathing air or nitrox, is an advanced skill
that requires additional training. Recreational divers
use oxygen-enriched air to increase the safety margin
of no-required-decompression dives or to extend
maximum dive time without encountering a mandatory
decompression obligation.
Enriched Air Nitrox (EAN)
Dive Tables Theory
So-called “standard dive tables” are designed for diving
while breathing air. The mathematics and the formulas
that were used in their development assume the diver
is breathing a mixture that is 79% nitrogen/inert gas
and 21% oxygen. When NOAA introduced procedures
for diving with an enriched air nitrox mix of 32%
oxygen/68% nitrogen (NOAA Nitrox I) in 1979, they
also published a set of derivative nitrox dive tables that
took into account the reduced percentage (and therefore
partial pressure) of nitrogen. The tables were based on
the concept of equivalent air depth. That is, because the
NOAA Nitrox I diver would be breathing only 68/79ths
of air’s partial pressure of nitrogen at any depth, the diver
could enter the dive tables at a depth that is equivalent to
only 68/79ths of the absolute pressure at the actual dive
depth. (Note that the fraction is applied to the absolute
pressure at depth, not just the depth.)
The NOAA Nitrox I tables looked exactly like the
U.S. Navy tables from which they were derived. Only
the numbers changed because of the time credits for the
reduced nitrogen partial pressures. EAN32 turned out to
be a very convenient oxygen fraction because in the depth
range from the surface to 33 meters (110 feet), the NOAA
Nitrox I tables exactly corresponded to the U.S. Navy air
tables for the next shallower depth increment. In other
words, an EAN32 diver diving to a depth of 21 meters
(70 feet) had 60 minutes of bottom time, which was the
USN no-decompression limit for an air diver diving to
only 18 meters (60 feet). An air diver descending to 21
meters (70 feet) would have only 50 minutes of bottom