Maximum Operating Depth Notes

The maximum operating depth of a gas is the depth at which the partial pressure of oxygen exceeds a safe limit. This is generally accepted as being 1.4 bar for the dive phase and 1.6 bar for the decompression phase.

What is the maximum operating depth of a 36% Nitrox mix assuming a maximum partial pressure of 1.4 bar?


We know that the partial pressure of each gas in the mixture is proportional to its concentration and we know that the partial pressure of O2 will be 1.4 bar at our MOD. Therefore we can write

1.4 = 0.36 * ATA

We can then rearrange this to give us the absolute pressure at which a 36% mix gives a PPO2 of 1.4 bar:

ATA = 1.4 / 0.36

Next we need to convert this absolute pressure into a depth. Assuming every 10 meters of water exerts the same pressure as one atmosphere

28  =  (3.8 * 10) - 10


Calculating the best mix

The best mix is the mix with the highest fraction or percentage of oxygen that can be used at the target depth.Calculating the best mix is similiar to the calculation of MOD but in reverse.

What is the best mix for a 30 meter dive using a maximum PPO2 of 1.4 bar?

Step 1: Determine the absolute pressure at the target depth:

(30 / 10) + 1 = 4 bar

Step 2: Determine what fraction will produce the target partial pressure at that absolute pressure:

1.4 / 4 = 0.35 or 35%

The two steps can be combined into a single formula:

Best Mix = (1.4 / ((Depth / 10) + 1)) * 100