33
English
CHROMIS USER MANUAL
4.7.4
Diving with Nitrox
Nitrox is the term used to describe
breathing gases made of oxygen-nitrogen
mixes with oxygen percentage higher than
21% (air). Because Nitrox contains less
nitrogen than air, there is less nitrogen
loading on the diver’s body at the same
depth as compared to breathing air.
However, the increase in oxygen
concentration in Nitrox implies an increase
in oxygen partial pressure in the breathing
mix at the same depth. At higher than
atmospheric partial pressures, oxygen
can have toxic effects on the human body.
These can be lumped into two categories:
1- Sudden effects due to oxygen
partial pressure over 1.4bar.
These are
not related to the length of the exposure
to high oxygen partial pressure . Sudden
effects can vary and depend on the exact
level of partial pressure they happen at. It is
commonly accepted that partial pressures
up to 1.4bar are tolerable, and several
training agencies advocate maximum
oxygen partial pressures up to 1.6 bar.
2- Long exposure effects to oxygen
partial pressures over 0.5bar due to
repeated and/ or long dives.
These can
affect the central
nervous system, cause damage to lungs or
to other vital organs. Long exposures can
be divided to more severe Central Nervous
System effects and less dangerous long
term Pulmonary Toxicity effects.
Chromis treats high ppO
2
and long
exposure effects in the following ways:
1- Against sudden effects: Chromis has an
MOD alarm set for a user-defi ned ppO
2
max.
As you enter the oxygen concentration for the
dive, Chromis shows you the corresponding
MOD for the defi ned ppO
2
max. The default
value of ppO
2
max from the factory is
1.4
bar.
This can be adjusted to your preference
between
1.0
and
1.6
bar. It can also be
turned
OFF
. Please refer to chapter Gas
settings for more information on how to
change this setting.
2- Against long exposure effects: Chromis
“tracks” the exposure by means of the
CNS O
2
clock. At levels of 100% and
higher there is risk of long exposure effects,
and consequently Chromis will activate an
alarm when this level of CNS O
2
is reached.
Chromis can also warn you when the CNS
O
2
level reaches 75% (see section CNS
alarm). Note that the CNS O
2
clock is
independent of the value of ppO
2
max set
by the user.
The CNS O
2
clock increases when the
oxygen partial pressure is higher than
0.5bar, and decreases when the oxygen
partial pressure is lower than 0.5bar.
Hence, while on the surface breathing air
you will always be decreasing the CNS O
2
clock. During the dive, the depth at which
0.5bar is reached for various mixes is as
follows:
• Air: 13m/43ft
• 32%: 6m/20ft
• 36%: 4m/13ft
NOTE: For oxygen concentrations of
80% and higher, the ppO
2
max is fi xed
at 1.6bar and cannot be changed.
NOTE: Repetitive very long exposures
(technical and rebreather diving)
with high ppO
2
may cause long term
Pulmonary Toxicity effects that can
be tracked with OTUs. SCUBAPRO
recommends model Galileo TMx for
completing such dives.
4.7.5
Altitude diving
4.7.5.1
Altitude classes, altitude
warning and no-fl y time after
a dive
Going to altitude is in a way similar to starting
an ascent from a dive: you expose your
body to a lower partial pressure of nitrogen
and consequently you start offgassing. After
a dive, given the higher nitrogen loading
in your body, even reaching an otherwise
negligible altitude can potentially cause
decompression sickness. Consequently,
Chromis constantly monitors the ambient
pressure and uses it to evaluate your
nitrogen loading and offgassing. If Chromis
notices a drop in ambient pressure not
compatible with your current nitrogen
loading, it will activate a warning to alert
you of the potentially dangerous situation.
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