Instructor Manual
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Ocean Diver
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Adapting to the underwater world
Copyright © BSAC 2017
13
• Air (21% oxygen)
Standard air from the environment is compressed and filtered into scuba
cylinders. Air has an oxygen content of 21% and contains 79% nitrogen.
• Nitrox (32% or 36% oxygen)
A nitrox mix has extra oxygen in it in place of some of the nitrogen. Nitrox 32
has 32% oxygen and 68% nitrogen whereas nitrox 36 has 36% oxygen and
64% nitrogen. The reasons that these nitrox mixes might be chosen are covered
later in the course.
Key marks and labels
Several of the additional markings contain essential information about the cylinder.
• “Breathing Air” or “Nitrox” labels
Scuba cylinders should be labelled to indicate whether the contents are air or
nitrox. This could be a large sticker around the whole cylinder.
• Cylinder size
Cylinders can have different capacities. Capacity is measured in litres, or put
another way, how much water the cylinder could hold if you took the valve off
and poured water in. Hence, the term water capacity (WC)
• Working and test pressures
The working pressure, labelled WP, indicates the maximum operating pressure
of the cylinder. Usually this is 232 bar or 300 bar. Some cylinders may have this
value marked as CP, charging pressure.
There is another mark, TP, which stands for test pressure. This is a value higher
than the WP and is used during the hydrostatic test.
• Test dates
The manufacturer tests the cylinder and stamps it with the date of this test.
Subsequent test dates are also stamped on the cylinder and these are looked
at by compressor operators to ensure the cylinder is ‘in test’ before being filled.
If a cylinder is not ‘in test’, it should not be filled. Test dates may be of the
format YYYY/MM or since 2013 just YY/MM. Where the date refers to a visual
inspection then there will be a ‘V’ stamped next to the date.
Cylinders used for nitrox need periodic cleaning
Cylinders used for nitrox mixes that are produced using partial pressure mixing
techniques need to be in oxygen service. which means they need to have
oxygen compatible parts in the valves and need to be oxygen clean to ensure no
hydrocarbons are present in them. Nitrox mixes below 40 per cent produced by
continuous blending or by using premixed gas do not need such cleaning. Cleaning
is recommended every 15 months.
