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During a dive, Suunto D5 calculates partial pressure of oxygen (pO2), central nervous system toxicity (CNS%) and pulmonary oxygen toxicity, tracked by oxygen toxicity units (OTU). 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%, Suunto D5 notifies you and the value stays in the view.
You can customize views to always show CNS% and OTU.