USEFUL INFORMATION WHEN DIVING ON NITROX

For diving with Nitrox mixes the most important thing is that you are qualified to use mix gasses. Basic Nitrox is covered as part of the Ocean Divers course and allows divers to use the two most common mixes 32% and 36%. These mixes allow diving at a 1.4 Partial Pressure of Oxygen (PO2) to 33 meters and 28 meters respectively.

More advanced users (Advanced Nitrox) users can use mixes up to 100% Oxygen.

The information here can be referenced by both Basic Nitrox and Advanced Nitrox users.

Most commonly the Nitrox Diver wants to be able to calculate three basic formulae:

  • Maximum Operating Depth (MOD) for a given Nitrox mix
  • Equivalent Air Depth (EAD) or Equivalent Narcotic Depth (END)
  • Best Nitrox Mix

To find all the above certain information is required:

Partial Pressure of Oxygen - PO2

When using Nitrox as a ‘bottom gas’ the value 1.4 is used

When using Nitrox as a ‘decompression gas’ then value 1.5 or 1.6 is used

Maximum Depth (expressed as absolute pressure)

The following Table gives the equivalent absolute pressure (atmospheres) for a given depth

Depth (ATA) / Depth (feet) / Depth (meters)
1 (surface) / 0 / 0
2 / 33 / 10
3 / 66 / 20
4 / 99 / 30
5 / 132 / 40
6 / 165 / 50
7 / 198 / 60
7.5 / 218 / 66 (maximum operating depth air diving)

For example should I want to dive to 25 meters the ATA would be 3.5 bar. To find the equivalent depth when the ATA is known subtract 1 and that will provide the depth in meters or subtract 33 and that will be the depth in feet.

Fraction of Nitrogen in the Mix

When calculating the EAD or END then the fraction of Nitrogen (FN2) needs to be known. In a normal air mix the fraction of N2is 79% (100-21)

Fraction of Oxygen in the Mix

When calculating the MOD the fraction of O2 requires to be known. In a normal air mix the fraction is 21% (100-79)

Calculations

1. A diver wants to calculate the best mix (Nitrox) for a dive to 27 meters using Nitrox as bottom gas. He knows:

the depth = 27 + 1/ 10= 3.7 bar

the PO2 = 1.4

he requires the % mix for the Nitrox = PO2/Depth (ATA) = 1.4 / 3.7 = 37% Nitrox mix

Please Note:When using Feet the depth (ATA) is found by knowing the depth in feet adding 33 and dividing by 33 and then subtracting 1 from the resultant

2. A diver wants to calculate the MOD for a dive using 33% Nitrox mix at a PO2 of 1.4. He knows the:

the fraction of Oxygen (FO2) = 37%

the PO2 = 1.4

he requires the MOD = (PO2/-FO2)-1 = (1.4 /0.37)-1 = 2.7 bar or 27 meters

3. A diver wants to calculate the EAD for a dive using 40% Nitrox mix at a PO2 of 1.4. He knows the:

MOD = (1.4 / 0.4)-1 = 2.5 bar (25 meters)

The fraction of Nitrogen in normal air 79%

The fraction of Nitrogen (FN2) in the mix = 100-40 = 60%

He requires the EAD = (FN2/79) x (2.5+1) = (60 / 79) x 3.5 = 2.6 – 1 = 1.6 bar or 16 meters