When it comes to choosing
and using a dive computer, how
can you tell which models are the
most cautious and which the most
liberal in terms of the deco advice
they give?
The truth is, no one computer
fits the bill under all
circumstances: the computer that
seems most conservative using
one dive profile can become one
of the more liberal models when
using another.
England’s Diver magazine conducted a series of tests to
investigate the characteristics of
full-function decompression-stop
diving computers. Many American
computer makers and
American publications say little
about decompression diving,
telling divers to stay within the socalled
“recreational limits” of 130
feet and not to make decompression
dives. However, anyone who
has been on a live-aboard knows
that divers often go far beyond
that arbitrary limit.
Furthermore, every dive you
do is a decompression dive,
because you breathe air under
increasing pressure as you go
down, then breathe it under
decreasing pressure as you rise.
You're able to do a no-stop dive
only if you limit yourself to a given
time for a particular depth and
then come up at a prescribed rate
of ascent.
Computers are an essential
piece of diving equipment.
Manufacturers add peripheral
features to give them a perceived
“added value,” but the core
function remains the computer’s
ability to measure pressure
(depth) against time and relate
this measurement to “tissue
models” to represent what might be
happening to your body during a
dive. It does this with a mathematical
model — an algorithm — that
calculates the absorption of nitrogen
and the rate at which the nitrogen
comes out of solution. This is
measured in “halftimes,” a “halftime”
being the length of time it takes for a
particular tissue to become halfsaturated
with nitrogen at an exponential
rate at a particular ambient
pressure. Fast-absorbing tissues, like
blood, have about a four-minute
halftime, while heavy bones have
much longer ones, in the neighborhood
of 480 minutes.
Once a diver has gone beyond
the limits of no-stop diving, fullfunction,
decompression-stop
diving computers give information
about a diver’s nitrogen-absorption
status, including the length and
number of pauses necessary during
ascent on a stage-decompression
dive. When it comes to the core
function, we can distinguish six
distinct groups among the models
(see chart below).
In any given group, each
computer shares the same algorithm
or mathematical model. So
for the purposes of this test, Diver compared a representative from
each group on a series of dives.
Ascent rates varied. Group B
computers were set at 30 ft/min.,
whereas others had variable ascent
rates from 60 ft./min. at depth to
20 ft./min. in the shallows. They
ascended at the maximum rate
allowed by the most conservative
computer at any given moment of
the dive. They used the same
surface interval for all computers.
The Performances
First Dive: 120 feet: After 9 min. to
120 feet, all the computers were
within 2 min. of the same no-stop
time. At 10 minutes, the Group E
computer showed no-stop time left
and the Group F showed 1 min. The
others still showed two minutes of nostop
diving.
After 17 min., the Group F computer
had become the least cautious
with a 2 min. stop at 10 feet,
but Group D was asking for a 1 min.
stop at 20 feet and a total ascent
time of 6 min.
By 23 min. we had risen to 70 feet.
Group A was the least cautious, closely followed by Groups E and B.
By 28 min. and 16 min., Group C
became the least cautious.
Dive Computer Tests |
Group A |
Group B |
Group C |
Group D |
Mares Air Lab |
Suunto Eon |
Aladin Air |
Dive Rite B’Air |
Mares Guardian |
Suunto Favor |
Aladin Air X |
Dive Rite Bridge II |
Mares Surveyor |
Suunto Solution Alpha |
Aladin Air X Nitrox |
Dive Rite Nitec |
Mares Tutor |
Suunto Solution Nitrox |
Aladin Pro |
Dive Rite Nitec3 |
|
Suunto Spyder ACW |
Aladin Pro Nitrox |
|
|
Suunto Viper |
Mares Genius |
|
|
|
Monitor 3 |
|
|
Group E |
Group F |
|
|
Dacor Sportster |
Cochran Aquanox |
|
|
Hydrotech Data |
Cochran Commander + |
|
|
Ocean Reef Plus 01 |
Cochran Commander Nitrox |
|
|
Orca Pilot & Pilot Audio |
Cochran Nemesis + |
|
|
Orca Pilot Audio Nitrox |
Cochran Nemesis Nitrox IIa |
|
|
Orca Pilot Nitrox |
|
|
|
UBS Chameleon |
|
|
|
Zeagle Status II |
|
|
|
After 34 min. at 20 feet, Group C
still wanted an 11 min. ascent time,
whereas Group F required only 2
min. At 20 feet after 39 min.,
Group F reverted to no-stop diving,
while the others still required
between 4 min. (Group E) and 8
min. (Group C) of decompression
before being free to ascend to the
surface. They all eventually reverted
to no-stop diving.
Dives Two and Three (180 feet):
After 8 min. at 180 feet, the Group
B computer went into deco-diving
mode and showed a total ascent
time of 6 min. A minute later all
the other computers went into the
deco-diving mode.
Two minutes later, the order
changed. Group C became the
most conservative followed by
Groups E, D, B, A and F. During a
slow ascent, after 13 min. at 130
feet, Group D showed the most
caution and Groups E, B, and C
tied for second most cautious,
followed by Group A and Group F.
Group D stayed the most cautious
throughout the rest of the dive and
Group F the least. The others
completely reversed their order
between 100 ft. (16 min.) and 55
feet (23 min.).
Hesitating at 42 ft. meant that
Group F added deco time, while
Group E became less cautious.
Group F first reverted to no-stop
diving after 36 min. (7.6 m.).
Here the order of caution was
Group D, followed by C, B, A, E,
and finally Group F, although it was
the Group B computer that
reverted to no-stop diving soonest
after E and F.
This time Group D proved consistently
the most cautious because,
unlike the others, it refused to clear
a required 10-foot stop by doing
extra time in the 10 to 20-foot
range. This pattern was repeated on
another dive to 180 feet for 63 min.
Dive Four: This involved a quick
dash to 180 feet and a leisurely
ascent through the 100 to 30 foot
range, completing the dive after 35
min. Again, Group F was least
cautious and Group D most
conservative.
The Conclusions
All of the computers compared
(and their siblings) are equally
useful for decompression-stop
diving. One cannot make a decision
about which is most conservative
simply by comparing them during a
dive at one way point alone.
Because a computer gives you a
mathematician’s idea of what might
be happening to your body using
model tissues rather than your own
blood, skin, and bones, using one is
an act of faith. You have to believe
what the computer tells you. And
you have to realize there is no way
of knowing how close you come,
each time you dive, to a decompression
illness.
When using a computer, it’s
your choice whether you make
additional shallow water stops or
surface when your computer clears
the last stop from its display and
reverts to no-stop diving mode.
However, we suggest that if you
have a Group F computer, add
optional conservatism (up to 50
percent is available) before undertaking
this sort of diving. Those
using Group E computers might do
well to use them in “condition
hard” mode rather than lesscautious
“condition normal.”
If you use a Group B computer
and dive with a buddy who uses a
Group C or D model, you might
adapt the algorithm by selecting an
“Altitude 1” setting rather than
“Altitude 0.”
Group C computer-users
should note that their devices are
not as cautious as others when used
for a single dive.
Group A computer users
should take heed of the additional
and optional 3 min. safety stop
offered between 16 and 30 feet
(ST3 to ST1) on no-stop dives and
perhaps add a similar safety stop to
deco-stop dives.
Group D computer users
should be sure to reserve enough
air for the lengthy stops at 10 feet
that might be required and have
the necessary buoyancy control at
that depth to be able to do the stop
comfortably.
No one knows what is right for
you. If you are older, overweight, or
unfit, or if you simply have not
dived very much, there’s reason to
add an element of caution when
you use any computer.
A version of this article originally
appeared in the June issue of Diver
Magazine. Undercurrent takes all
responsibility for additions and edits.
We have converted the original data
from meters to feet. To subscribe,
contact Diver Magazine by telephone
(011) 0181 943 4288; fax (011) 0181
943 4312; e-mail 100737.2226@compuserve.com.