As most air divers will know,  air is made up of two parts: 0.21% oxygen and 0.79% nitrogen ( lets not fall out over tiny inert gases!).  So in the Nitrox diving world, this would be classed as Nitrox 21.  If we had Nitrox 36, its make up would be 0.36 oxygen and 0.64% nitrogen; this is our fraction of oxygen (Fo2) and fraction of nitrogen (Fn2).  So, all we have done is increased the oxygen and reduced the nitrogen.  As we know, nitrogen is what limits our dive times.  So increasing the oxygen, gives us more bottom time, and less decompression time; sounds good eh! So, why not just breath pure oxygen then?  No nitrogen at all; no narcosis!  Well this is where it starts to get a little more complex.  Think back to diving air;  we cannot exceed 66 meters on air as, this is where its partial pressure reaches1.6  (let alone being 'narked' out your box!).  Partial pressure tables help us to monitor this in nitrox diving.  So, how do we work this out?

To work out your partial pressure: 

If you now know the depth of your planned dive (e.g. 40 meters), convert this into absolute pressure bar (or 'Pbar' for short).   (Depth ÷ 10) +1.  *Or, in our case, (40 ÷ 10) + 1 = 5.   So your absolute pressure is 5.0 bar.  or 5 'Pbar'.*

We now know that we want to dive to 40 meters (5.0bar Pbar), and want to know if we can dive it on .32% Nitrox  (.32% Fo2).  Now, here is the next formula:  Fo2 x Pbar = partial pressure of oxygen (or ppo2)

                               Fo2 x Pbar = PPo2                                                                                                                       

                                                                                                                                                    32  x  5.0    = 1.6

Now check on  oxygen exposure table  for your 1.6 oxygen exposure time