Saturday, 6 October 2012


This article is designed to allow the reader to correctly mark up their stage/decompression cylinder.  Due to only owning a certain quantity of cylinders, I have decided to use masking tape to mark mine however if diving using ‘standard gasses’ all the time then permanent stickers could be used such as those made by Dive Signs. 

One of the biggest issues concerning any diver (technical or otherwise) using Nitrox is breathing the wrong gas as it will almost certainly lead to oxygen toxicity (as the maximum operating depth (MOD) has been exceeded) leading to death.  To reduce this risk we can do a number of things including analysing the gas upon filling AND re-analysing PRIOR to diving (if the cylinder has been left unattended for a period of time) and ensuring the cylinder is marked up correctly.  The stage/decompression cylinder must:
1. Be free of all unnecessary clutter.
2. Have markings that are clear and easy to read.
3. Have the MOD clearly marked in large numbers on BOTH sides of the cylinders (MOD or m is optional*) where it’s visible to the divers buddy.
4. Have analysis tape on the shoulder of the cylinder including percentage (to the decimal), MOD, pressure, date and initials/signature of the analyser.
*To the untrained, the number could be the percentage or the MOD.  For example if we look at probably the 2 most common numbers you may see on the side of a cylinder, 50 and 21, 50 could be 50% with an MOD of 21m or 50m with a Nitrox mix of 26%.   21 could be air with an MOD of 50m or 21m with a Nitrox mix of 50%. 

Why do we mark the MOD rather than the percentage?  Surely you want to know what gas you’re breathing?  This is correct however the last thing I want my brain to do when I have a number of cylinders to choose from is to try and work out which one I need by calculating the MOD in my head.  All I need to do is look at the cylinder and I automatically can see which one I need to breathe from next.   

Analysing the gas
This article is not designed to teach you how to analyse Nitrox or work out an MOD, however it will be briefly covered.  If you wish to learn more about this then please feel free to contact me here. 

Using my analyser I find the Nitrox percentage in my cylinder (instructions here).
Using Daltons Triangle I am going to work out the MOD for my gas mix. 
In this example:
1.6 ÷ 0.508 = 3.15 BAR (abs) = 21m
(Most agencies tend to use a PO2 of 1.4 for the dive phase and 1.6 for the decompression phase however check with your agency standards first)


Using masking (or gas analysis) tape I then record the details as mentioned below:
1. Percentage (to the decimal*).
2. MOD.
3. Pressure.
4. Date.
5. Initials/signature of the analyser.
*The reason for displaying the decimal is to prove it’s been analysed, rather than just writing down what you want from the filling station.
Finally as mentioned in the second paragraph the MOD is clearly marked in large numbers on BOTH sides of the cylinders.
No additional information is required or needed on the cylinder.  The only exception is if you decide to use 100% oxygen then you may wish to have an oxygen sticker down the side instead of/as well as the MOD (6m).

If you wish to have your initials on your cylinder, they can be placed on the on lower end of the cylinder so they do not interfere with the MOD or cause confusion. 

Well done, your stage/decompression cylinder is now correctly marked up for use.  If you don’t know how to correctly fit a rigging kit click here otherwise next we need to ensure the stage/decompression regulators are set up correctly (here).
The boring bit!
All opinions expressed in my articles are my own and may differ to other instructor’s and agency guidelines; by no means are they wrong and I would not wish to disrepute any of them.  This article is for information only and should not replace proper training.
Safe diving!

Timothy Gort
BSAC, PADI & SDI/TDI diver training
l Mob: 07968148261 l Email: l