I know there are differences of opinion when it comes to stage cylinder marking. I have set out my thoughts on this below. This is what I teach but I would be interested in an serious arguments aganst this approach.
Edit - I have edited the post beloow to clarify a few points that I didn't make clearly enough and to take into acount the comments up to page 4.
Stage Cylinder Marking
One of the biggest dangers facing the technical diver is breathing from the wrong cylinder and in particular, breathing a rich nitrox mix at depth which will almost certainly lead to oxygen toxicity. For this reason it is essential that all stage cylinders are analysed before use and labelled accordingly. This marking should take 2 forms;
1. The maximum depth at which the gas can be breathed, known as the maximum operating depth or MOD, should be clearly marked on the cylinder in a position where the divers buddy can clearly see it.
2. The contents, MOD and other information such as date analysed should also be marked on the neck but this can be smaller and less obvious.
It can cause significant confusion of the diver or the team use a mixture of percentage and MOD markings on cylinders, i.e if one diver marks up their cylinder with the percentage and another marks theirs with the MOD. For example if a cylinder is marked up as 50 then does this cylinder contain 50% and can be breathed at 21m or does it contain a mixture with 26% Oxygen that can be breathed at an MOD of 50m? Similarly a cylinder marked 21 could also be 50% that can be breathed at 21m or it could be Air .
In order to avoid confusion it is essential that the cylinder is primarily marked with the maximum operating depth (MOD). This means that we can instantly tell that a cylinder marked 21 is breathable at 21m. To be even more explicit it can also help to write 21m or even MOD 21M rather than just 21.
Where the contents are marked, this should be smaller and include a percent sign to confirm that it is a contents reading, e.g. 50% or 50% 21M or 50% MOD 21
There is occasionally reluctance amongst some divers to use the MOD as the primary marking and they will sometimes use the contents. This doesn’t really make sense. They claim that it is important to always know what you are breathing. This is only partly the case, what we really want to know is the partial pressure of the gas we are breathing, or rather that we are not exceeding a partial pressure of 1.6. Without doing a partial pressure calculation in our head the only way to be sure of this is to know the depth at which we hit the 1.6 maximum partial pressure, in other words the MOD. If knowing the MOD is the ultimate goal then why not just write it on the cylinder!
Some divers will mark up their MOD using permanent markings. This is fine if you always have the same mix but can cause problems with incorrect fills, non standard gasses, air top ups, switching cylinders or travelling abroad. For this reason temporary markings using gaffa tape or simillar can offer greater flexibility.
Edit - I have edited the post beloow to clarify a few points that I didn't make clearly enough and to take into acount the comments up to page 4.
Stage Cylinder Marking
One of the biggest dangers facing the technical diver is breathing from the wrong cylinder and in particular, breathing a rich nitrox mix at depth which will almost certainly lead to oxygen toxicity. For this reason it is essential that all stage cylinders are analysed before use and labelled accordingly. This marking should take 2 forms;
1. The maximum depth at which the gas can be breathed, known as the maximum operating depth or MOD, should be clearly marked on the cylinder in a position where the divers buddy can clearly see it.
2. The contents, MOD and other information such as date analysed should also be marked on the neck but this can be smaller and less obvious.
It can cause significant confusion of the diver or the team use a mixture of percentage and MOD markings on cylinders, i.e if one diver marks up their cylinder with the percentage and another marks theirs with the MOD. For example if a cylinder is marked up as 50 then does this cylinder contain 50% and can be breathed at 21m or does it contain a mixture with 26% Oxygen that can be breathed at an MOD of 50m? Similarly a cylinder marked 21 could also be 50% that can be breathed at 21m or it could be Air .
In order to avoid confusion it is essential that the cylinder is primarily marked with the maximum operating depth (MOD). This means that we can instantly tell that a cylinder marked 21 is breathable at 21m. To be even more explicit it can also help to write 21m or even MOD 21M rather than just 21.
Where the contents are marked, this should be smaller and include a percent sign to confirm that it is a contents reading, e.g. 50% or 50% 21M or 50% MOD 21
There is occasionally reluctance amongst some divers to use the MOD as the primary marking and they will sometimes use the contents. This doesn’t really make sense. They claim that it is important to always know what you are breathing. This is only partly the case, what we really want to know is the partial pressure of the gas we are breathing, or rather that we are not exceeding a partial pressure of 1.6. Without doing a partial pressure calculation in our head the only way to be sure of this is to know the depth at which we hit the 1.6 maximum partial pressure, in other words the MOD. If knowing the MOD is the ultimate goal then why not just write it on the cylinder!
Some divers will mark up their MOD using permanent markings. This is fine if you always have the same mix but can cause problems with incorrect fills, non standard gasses, air top ups, switching cylinders or travelling abroad. For this reason temporary markings using gaffa tape or simillar can offer greater flexibility.
