Nitrox diving calculations made easy Nitrox sticker

The air we breathe is Nitrox 21, which basically means it has 21% oxygen and 79% nitrogen. When we dive the pressure increases by 1 bar every 10 meters so the partial pressure of the oxygen (PO2) and the partial pressure of nitrogen (PN) increases.

                               PO2         PN        

Surface pressure    0.21  and  0.79

10 metres               0.42  and  1.58

20 metres               0.63  and  2.37

30 metres               0.84  and  3.16

As you can see the partial pressure of Nitrogen is increasing with depth. To reduce the Nitrogen we can add oxygen. Standard Nitrox mixes are Nitrox 32 and Nitrox 36. Nitrox 32 is 32% oxygen and 68% Nitrogen and Nitrox 36 is 36% oxygen and 64% Nitrogen. Increasing the oxygen increases your no decompression times and reduces the fatigue factor.

The extra oxygen gives longer no decompression times compared to no decompression times associated with Nitrox 21 (compressed air). But the extra oxygen gives us other limits. The first limitation with extra oxygen is depth. How deep can you dive with Nitrox 32 and Nitrox 36 First we need to look at Daltons Law of Partial Pressures?

                    Dalton’s Diamond 

daltons diamond                            

Terms used here

FO2 is the fraction of oxygen or the % Oxygen

PO2 is the partial pressure of oxygen

P is the absolute pressure   (depth divided by 10+1)

The forward slash  is divided by.

FO2 x P = PO2  =   Fraction of oxygen x pressure = partial pressure of oxygen   = Oxygen Dose 

PO2 / FO2 = P   =   partial pressure of oxygen / Fraction of oxygen = pressure   = Depth

PO2 / P = FO2   =   partial pressure of oxygen / pressure = Fraction of oxygen . = Best nitrox

Assuming a max PO2 of 1.4 Bar for cold water

 

.So is there to much Oxygen in the nitrox ?

As you can see the formula for oxygen dose is FO2 x P = PO2 so how can we work out the oxygen dose with Nitrox 32 at a depth of 30 meters if we put the numbers in the formula

We get   0.32 x 4.0 = PO2   0.32 the % Oxygen x P the pressure (depth divided by 10+1) = partial pressure of oxygen

So          0.32 x 4.0 = 1.28   The oxygen dose is less than 1.4 Bar so its ok to dive.

so how can we work out the oxygen dose with Nitrox 32 at a depth of 40 meters        

We get   0.32 x 5.0 = PO2    0.32 the % Oxygen x P the pressure (depth divided by 10+1) = partial pressure of oxygen

So          0.32 x 5.0 = 1.6 .    The oxygen dose is 1.6 Bar and 1.4 is the max so its not ok to dive.

 

How deep can I safely dive with this nitrox ?

As you can see the formula for depth is PO2 / FO2 = P so how deep can you dive with Nitrox 32? ..if we put the numbers in the formula

We get    1.4 / 0.32 = P  1.4 is the partial pressure of oxygen divided by 0.32 the % oxygen = P the pressure.

So           1.4 / 0.32 = 4.375   so the pressure is 4.375 Bar ...how do we get the depth from 4.375 Bar

 

The formula to find the depth is (P-1) x10 which is the pressure we just worked out 4.375 Bar minus 1 multiplied by 10.....which = 33.75

So the max depth you can dive with Nitrox 32 is 33.75 Meters........(when you have the pressure move the decimal point and subtract 10)

 

So how deep can you dive with Nitrox 36?... Well if we put the numbers in the formula as above

we get     1.4 / 0.36 =  1.4 is the partial pressure of oxygen divided by 0.36 the % oxygen = P the pressure.

So           1.4 / 0.36 = 3.888  So the pressure is 3.888 Bar ...how do we get the depth from 3.888 Bar

The formula to find the depth is (P-1) x10 which is the pressure we just worked out 3.888 Bar minus 1 multiplied by 10.....which = 28.88

So the max depth you can dive with Nitrox 36 is 28.88 Meters........ (when you have the pressure move the decimal point and subtract 10)

 

The wrecks at 25m what nitrox do I use ?

As you can see the formula for best nirtox is PO2 / P = FO2 So how can we work out the best nitrox for a dive to 25m?... If we put the numbers in the formula

We get      1.4 / 3.5 = FO2  1.4 is the partial pressure of oxygen divided by 3.5 the pressure (depth divided by 10+1) = the % oxygen

So             1.4 / 3.5 = 0.4   So the % oxygen   is 0.4 which x 100 is the percentage. So the best nitrox for a depth of 25meters is 40%

 

The wrecks at 35m what nitrox do I use ?

As you can see the formula for best nitrox is PO2 / P = FO2 So how can we work out the best nitrox for a dive to 35m ....If we put the numbers in the formula

We get      1.4 / 4.5 = FO2  1.4 is the partial pressure of oxygen divided by 4.5 the pressure (depth divided by 10+1) = the % oxygen

So             1.4 / 4.5 = 0.31  So the % oxygen   is 0.31 which x 100 is the percentage. So the best nitrox for a depth of 35meters is 31%

 

            .                       These examples are just to show how easy the calculations are, and can in no way replace a nitrox course.