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Chapter 43
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2. EQUIPMENT COMPLEXITY
Technical diving involves more complex equipment for producing, supplying and delivering the
various breathing gases, other than air. With an increase in the complexity of the equipment there
is an associated increase in the likelihood of human error at all these 3 stages.
The handling of mixtures with higher than normal oxygen percentages implies greater risk of fire
and explosions. Gas mixtures may not be as compatible as the "normal" oxygen/nitrogen mix in
air, and the heat generated during compression must be appreciated. Although not common,
explosions associated with high oxygen percentages are very destructive.
Problems and mistakes develop from the use of multiple gases and complex equipment:
• Mixing, labelling and transport of gas;
• Handling it at the dive site;
• Analysing the gases and confirming that they are the ones appropriate for
the dive to be performed;
•
Selection of appropriate gases during the dive.
•
Different gases require different cylinders together with the various
attachments; manifolds, O rings, contents gauges, high pressure hoses,
and often, separate regulators.
Because of the added complexity of the equipment, the use of multiple gas mixtures and the
increased support facilities, there are substantial initial capital outlays, operating and maintenance
costs.
3. PHYSIOLOGICAL ASSUMPTIONS
There is considerable doubt regarding some of the physiological assumptions on which technical
diving is based. It is claimed that the equivalent air depth (EAD) calculation can be used to
determine the different influence of the gas mixture on the diver, and this has been applied to both
nitrogen narcosis and decompression sickness (DCS). There is, in fact, no really good evidence
that this EAD is a strictly accurate concept. Experience in highly controlled navy diving has been
reassuring, and the implication is that the EAD concept is a valid approximate assumption.
Divers using O2/N2 mixtures decompress using tables of EAD. These calculate of the
actual partial pressure of N2 for the dive and from this calculate the depth of an air dive
that has the same N2 pressure. The diver then decompresses as if he had done an air dive to
the calculated depth, the EAD.
Thus a diver breathing 40% O2 at 30 metres (60% of 4 ATA = 2.4 ATA of N2) has an
EAD of 20 metres (80% of 3ATA = 2.4 ATA of N2).
So, after this dive to 30 metres, our
diver decompresses as if he had dived to 20 metres on air.