Enriched air or Nitrox is commonly used in Surface Supplied Diving.

Air has 21% Oxygen and 79% Nitrogen. With Enriched Air, the % of O₂ is increased and the % on N₂ is decreased.

When a Dive Supervisor is planning & running a dive, they must calculate the Equivalent Air Depth (EAD) on the mix of O₂/N₂ (24/76, 28/72, 36/64, 40/60).

Enriched air can be a useful tool for the Supervisor. There are advantages & disadvantages in its use:

Pros:

• The diver can have an increased bottom time due to a decreased nitrogen load.
• The diver has an increased metabolic oxygen load which is helpful in an emergency.
• There is a decreased risk of DCI due to an decreased Nitrogen load.

Cons:

• There is a reduced bottom depth due to an increased ppO₂.
• There is an increased risk of Oxygen Toxicity.
• Any gas mix with >25% Oxygen is required to be handled as pure oxygen.

The lower Partial Pressure of Nitrogen (ppN2) allows you to use a shorter decompression, but which table do you use?

This is when you need the EAD. You need to calculate what depth on air would give you the same ppN2 as you have in your nitrox mix. This would determine the air decompression table to use.

In meters of sea water (msw) the formula is:-

                          EAD =    (Depth + 10)    x    Nitrogen %   ÷    79    –     10 

In Feet of sea water (fsw) the formula is:-

EAD =  (Depth + 33)   x   Nitrogen %     ÷    79     –    33    

• IMCA D022. Guidance for Diving Supervisors Manual: Section 2