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Chapter 24 — 1
Chapter 24
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BREATHING GAS
CONTAMINATION
The supply of uncontaminated breathing gas (air) is of vital importance to the diver because
of the magnifying effect on contamination by the partial pressure rise with increasing depth.
For example, 5% contamination of gas at atmospheric pressure is equivalent to 20% at 30
metres depth (4 ATA).
Contamination usually arises either from impurities in the air taken into the compressor or
from contaminants generated by the compressor itself.
PREPARATION OF COMPRESSED AIR
Atmospheric air is taken into the compressor and is compressed by one of two methods. Most
dive shops use a piston and cylinder compressor which raises the pressure of the gas in
several stages. A more advanced compressor uses a diaphragm pump similar in principle to
that in a refrigerator.
Ideally the compressed air should be treated by passing it through several purifying cartridges
(or filters) to remove contaminants. Silica gel is used to remove
water
vapour, activated
charcoal removes
oil
and
hydrocarbons
, a molecular sieve removes
water droplets and
dust
particles
and a catalyst converts
carbon monoxide
to carbon dioxide, which can be
absorbed. Less scrupulous air suppliers have been known to substitute women's sanitary pads,
instead of filters.