An increase in partial pressure is indicated by a positive number, and a decrease by a
negative number. The changes in ppN2 and ppHe are displayed next to each gas mixture that
you want to switch to. Maximum operating depth (MOD) of a breathing gas is the depth at
which the partial pressure of oxygen (pO
2
) of the gas mix exceeds a safe limit. You can define
pO
2
limit for the gas.
An ICD warning is generated when:
1. Gas switch depth is greater than 10 m (33 ft).
2. Geometric mean of the partial pressure change of N2 and partial pressure change of He is
bigger than 0.35 bar.
If these limits are exceeded with a gas switch, Suunto EON Steel Black indicates the risk of
ICD as shown below:
In this example, the available gas mixtures for a deep trimix dive are:
•
Trimix 15/55, MOD 76.7 m (pO
2
1.3)
•
Trimix 35/15, MOD 27.1 m (pO
2
1.3)
•
Trimix 50/10, MOD 22 m (pO
2
1.6)
•
Oxygen, MOD 6 m
Suunto EON Steel Black highlights the dangerous ICD condition when the gas mixture
switches from 15/55 to 35/15 at a depth of 27.1 m.
If this gas switch is made, the change in ppN2 and ppHe are far beyond the safe limits.
One way to avoid the ICD risk is to increase helium content in the
first decompression gas
mixture (trimix 35/15) to get a 35/32 trimix mixture. Due to this change, the second
decompression gas (trimix 50/10) needs more helium to avoid ICD risk. The second
decompression gas mixture should be trimix 50/12. These
modifications would keep the
changes in partial pressure at a safe level and remove the danger of sudden ICD.
4.26. Oxygen calculations
During a dive, Suunto EON Steel Black calculates partial pressure of oxygen (pO
2
), central
nervous system toxicity (CNS%) and pulmonary oxygen toxicity, tracked by OTU (oxygen
toxicity units). The oxygen calculations are based on currently accepted exposure time limit
tables and principles.
By default, in Air/Nitrox dive mode, CNS% and OTU values are not displayed until they reach
80% of their recommended limits. When either value reaches 80%, EON Steel
notifies you and
the value stays in the view. In default Trimix mode, CNS% and OTU values are displayed on
the screen as scrollable info.
NOTE: You can customize views to always show CNS% and OTU.
Suunto EON Steel Black
38
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