Scuba Q and A:
Why Do Decompression Divers Use Nitrox?
Q. Why do decompression divers need to use nitrox in the shallower part of their dive?
A diver absorbs or releases inert gas based on the positive or negative partial pressure differential of those gases in their breathing mix and body tissues.
We only have to consider inert gases (i.e. nitrogen), because they aren’t metabolised in the body. Whatever is absorbed, must be released. Oxygen is metabolised by the body and, thus, doesn’t contribute to decompression sickness.
When the partial pressure of the inert gas being breathed is greater than that gas partial pressure inside the body tissues it will be steadily absorbed – until the tissues become saturated (gas partial pressure is equal).
Likewise, when the inert gas partial pressure dissolved inside body tissues is greater than the partial pressure of that same inert gas in the breathing mix, the gas will be released. This positive over-pressure differential is called super-saturation.
The greater the pressure differential/level of super-saturation, the faster that inert gas will diffuse out of the body tissues.
Without changing the gas you breath, the only means of creating super-saturation and a positive inert gas differential is by ascending to a shallower depth.
This is why decompression divers ascend to a pre-calculated depth (to achieve super-salutation) and then stop (to allow inert gas to diffuse out).
Bubbles can form if the super-saturation gets too high – causing decompression sickness. Every decompression model/algorithm dictates a maximum amount of safe super-saturation. That is known as the “M-value” (maximum-value).
The science of decompression is to get as shallow as possible to maximise off-gassing, whilst also staying below that threshold of maximum super-saturation.
There is another way that divers can increase the partial-pressure differential of an inert gas they’ve absorbed. That is to reduce the fraction (or %) of the inert gas they’re breathing.
Divers can switch onto a different breathing gas that contains less % of the inert gas – and this increases the partial pressure differential – thus speeding up the diffusion of inert gas out of their body tissues.
Because oxygen is metabolised, it is the natural choice as a replacement for the inert gas in the divers “deco” breathing mix.
However, oxygen poses problems in regards to CNS toxicity – so we have to maintain a maximum operating depth based on the partial pressure of the O2% used. This is why it is only used in the shallower phases of the ascent.
This process of speeding up the removal of inert gas from the body tissues by manipulating the inert gas % to cause greater differential at a given depth is called “accelerated decompression”.
About the Author
Andy Davis is a RAID, PADI TecRec, ANDI, BSAC and SSI qualified independent technical diving instructor who specializes in teaching advanced sidemount, trimix and wreck exploration diving courses across South East Asia. Currently residing in ‘wreck diving heaven’ at Subic Bay, Philippines, he has amassed more than 9000 open circuit and CCR dives over 27 years of diving across the globe.
Andy has published many magazine articles on technical diving, has written course materials for dive training agency syllabus, tests and reviews diving gear for major manufacturers and consults with the Philippines Underwater Archaeology Society.
He is currently writing a series of books to be published on advanced diving topics. Prior to becoming a professional technical diving educator in 2006, Andy was a commissioned officer in the Royal Air Force and has served in Iraq, Afghanistan, Belize and Cyprus.