a dive time up to 150 minutes or make a series of dives
to 40 fsw for up to a total of 150 minutes.
When using the NAUI RGBM Tables, you may also
switch tables from leaner to richer oxygen fractions from
dive one to dive two (or dive two to dive three) by simply
following the maximum depth constraints as you move
between the appropriate dive tables, since depth (pressure)
is the only controlling factor. For example, it is
possible to use air and the NAUI RGBM Air Table on
Dive One, then switch to EAN36 and use the RGBM
EAN36 Table on Dive Two.
Comparing this dive series to one derived from the
standard NAUI EAN32 Dive Tables: On the first dive to 24
meters (80 feet), the diver would have a maximum dive
time of 50 minutes and emerge from the dive as a “J”
diver. One hour later, the diver would be an “H” diver
and, if planning an 18 meter (60 foot) dive, would have
an adjusted maximum dive time of only 34 minutes. In
order to attain an adjusted maximum time equivalent to
the RGBM diver’s 85 minutes, the surface interval
between the first and the second dive would have to be at
least 5 hours 41 minutes. Sometimes simpler is better.
THE RULE OF HALVES
In 2003, NAUI introduced the “Rule of Halves” into
its recommended diving procedures. The NAUI Rule of
Halves is based on RGBM theory and provides an extra
margin of safety for divers by further controlling both
dissolved-phase and free-phase nitrogen in the body. It
can be used with any dive deeper than 12 meters (40 feet)
and with any dive tables or dive computer.
To use the Rule of Halves, when ending any nodecompression
dive in excess of 12 meters (40 feet),
halve the distance from the dive’s deepest depth to the
surface. Ascend to that depth and make a two to three
minute stop (with two and one-half minutes being optimum).
Then continue your ascent to the 3-6 meter (10-
20 feet) safety stop for one minute before surfacing.
For example, following a dive to 30 msw (100 fsw),
you would perform a two and a half minute stop at
about 15 msw (50 fsw) and then ascend to 5 msw (15
fsw) for a one minute stop before surfacing.
DIVE COMPUTERS AND NITROX
Dive computers perform real-time dive calculations.
Generally, their algorithms are quite conservative. But,
because they sample the diver’s depth and dive time
every few seconds and recalculate nitrogen absorption
over a range of theoretical tissue compartments, divers
enjoy extended dive times when using a dive computer.
In effect, the diver receives “credit” for the shallow portions
of the dive, which is not possible with the “squareprofile”
assumptions of dive tables. The first dive computers
were designed only for air diving, but with the
growing popularity of enriched air nitrox diving, every
manufacturer now produces a line of nitrox computers,
and many also produce trimix-capable dive computers
for technical diving.
Currently, several manufacturers have incorporated
the NAUI RGBM algorithms as well as the NAUI Rule of
Halves into their dive computers.
More and more divers are buying and using dive
computers, especially as prices are decreasing with
increased production and after recovery of initial
research and development costs. Many dive centers now
include computers in their rental equipment inventory.
Dive computers display not only current depth and dive
time, they also show dive time remaining and usually
the maximum depth thus far in the dive. They monitor
ascent rate and will sound an alarm or flash in the dis-
NAUI Nitrox Diver
62 Dive Tables and Dive Computers
FIGURE 5-9: EXAMPLES OF NITROX DIVE COMPUTERS