Maximum operating depth
In underwater diving activities such as saturation diving, technical diving and nitrox diving, the maximum operating depth (MOD) of a breathing gas is the depth below which the partial pressure of oxygen (pO2) of the gas mix exceeds a safe limit. This safe limit is somewhat arbitrary, and varies depending on the diver training agency or Code of Practice, the level of underwater exertion planned and the planned duration of the dive, but is normally in the range of 1.2 to 1.6 bar.
The MOD is significant when planning dives using gases such as heliox, nitrox and trimix because the proportion of oxygen in the mix determines a maximum safe depth for breathing that gas. There is a risk of acute oxygen toxicity if the MOD is exceeded. The tables below show MODs for a selection of oxygen mixes. Note that 21% is the approximate concentration of oxygen in normal air.
Safe limit of partial pressure of oxygen
Acute oxygen toxicity is a time variable response to the partial pressure exposure history of the diver and is both complex and not fully understood.
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