Reaching back to 1999 and pulling an article from club news, Buoyant Lift, an article written by Gareth Leyshon. Although the use of Nitrox does not eliminate DCI it does reduce the risk.
Many of you will have seen NITROX used at dive sites which you visit, you may have also noticed that an increasing
number of Branch members are using higher oxygen mixes than 21%. Having recently been asked a number of questions by both trainees &
experienced divers alike, I hope this information sheet will answer some of the general questions.
This information is NOT a substitute for proper training. If you are interested please talk to the Training Officer.
Terms & Phrases
Nitrox A gas mix of Oxygen/Nitrogen
21 – 21% Oxygen 79% Nitrogen. – Air
32 – 32 % Oxygen 68% Nitrogen.
36 – 36% Oxygen 64% Nitrogen
MOD Maximum Operating Depth.
The safe depth to which you can use a particular Nitrox mix. This is
based on the Partial Pressure of the Oxygen at this depth (PPO2). The recommend absolute
maximum PPO2 is 1.4 bar.
PP02 Partial Pressure of Oxygen.
As the pressure increases (Depth) the Partial Pressure of the gas
increases. Nitrox Divers must check that the PPO2 remains within safe limits, see MOD.
UPTD Unit of Pulmonary Toxic Dose.
Nitrox Divers are breathing rich Oxygen mixes at varying
PPO2’s they must monitor their exposure; UPTD is one of the units of exposure, which they are
required to monitor. (Also known as OTU by the American diving organisations). There is a maximum
%CNS % Central Nervous System Dose.
Using higher mixes of oxygen Divers are breathing richer Oxygen mixture at varying
PPO2s they must monitor their exposure. %CNS is one of the units of exposure, which they are
required to monitor. There is a maximum allowable dose.
As an air diver you may find yourself diving with a Nitrox Diver. The only factor that you must be aware of is the MOD.
A Nitrox Diver MUST inform you of the MOD of his mix. The dive plan must NOT exceed the MOD. You must
remember that the Nitrox Diver will not exceed his MOD, this means he will not follow you down beyond his MOD. In
the majority of cases the MOD will exceed the depth of the dive site & therefore does not present a problem.
As a suitably qualified Nitrox Diver, it is your responsibility to ENSURE the following: –
Confirmation of the gas mix .
Correct decompression planning. If you are diving with an air diver you must follow an Air Decompression
Calculation of UPTD (OTU) & % CNS.
Informing the Dive Marshall of the MOD & %mix, it must be noted on the Dive Log. (Maximum PPO2 is 1.4
You must use mixes within your qualification level.
Ensuring that cylinders are in oxygen service & appropriately identified.
Dive Marshall’s & Expedition Organiser
Although the Dive Marshall is responsible for the safety of all divers, he is not responsible for planning each individual dive.
Especially in the case of a dive using Nitrox & more so if he is not Nitrox trained. Divers have a responsibility to the Dive
Marshall to ensure that the Gas Mix used is suitable & safe for the proposed dive.
However there are some points Dive Marshalls should be aware of:
1/ Preferably Nitrox Divers should dive together – using the same mix.
2/ Air Divers & Nitrox Divers can dive together. Decompression profiles should favour the diver using Air and therefore match the Air Divers
requirements. A important consideration is that each diver will have a different MOD the dive must conbsider the Nitrox divers MOD.
3/ The MOD is based on an absolute maximum partial pressure of 1.4 bar. It is advisable to plan the maximum dive depth
well within the MOD.
4/ The Nitrox Diver must advise you of the MOD & % of the mix, these must appear on the dive log sheet.
5/ Only Divers who are qualified in the use of are permitted to use Nitrox.
6/ Nitrox can be used as a substitute in O2 administration if the O2 has run out.
7/ Treatment of diving related injuries is NOT changed – other than Oxygen poisioning (CNS
toxicity) (whole body toxicity).
8/ Symptoms & Treatment of CNS Toxicity
Symptoms:- minor muscular twitching (particularly the lips), nausea and fatigue, metallic taste on lips, visual or auditory
disturbances, muscular incoordination, dizziness and irritability, convulsions & fitting.
Treatment:- Removal from water. Allow to breath AIR – DO NOT administer oxygen.
9/ Symptoms & Treatment of Whole Body Toxicity. It is unlikely that any diver will experience this form of O2 poisioning – long term exposure of high PPO2 is required.
Symptoms:- Bronchial Cough, Pneumonia.
Treatment:- Removal from high O2 enviroment, allow to breath air. Hospitalise