Equivalent Narcotic Depth (END) — What It Is and Why It Matters
Equivalent narcotic depth (END) is a way to estimate the narcotic effect of a gas mix at depth by expressing it as the depth at which air would produce the same narcotic impairment. It is primarily used by technical divers breathing trimix to verify that their mix provides adequate narcosis reduction for their planned depth.
END is calculated based on the nitrogen fraction in the mix (helium is considered non-narcotic). For example, a trimix blend with 35% helium breathed at 60 meters might have an END of 30 meters — meaning the diver's narcotic impairment is equivalent to diving on air at 30 meters, a manageable level.
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Frequently Asked Questions
What END is considered safe?
Most technical diving training agencies recommend keeping END at or below 30 meters (100 feet), which corresponds to a narcotic effect most divers can manage effectively. Some conservative divers aim for an END of 24 meters or less.
Is oxygen narcotic?
This is debated. The traditional calculation treats only nitrogen as narcotic. Some newer approaches consider oxygen to be mildly narcotic as well, leading to slightly different END values. For practical purposes, most divers and dive computers use the nitrogen-only method.
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