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Chapter 5 — 12
Difficulties are still encountered in obtaining adequate air supply with reasonable respiratory efforts
under the following conditions :
• low cylinder pressures (observable on contents gauges), < 50 Bar
• cylinder valve not fully opened
• resistance in the first or second stage regulators (poor design or inadequate maintenance)
• increased respiration (exertion, hyperventilation, negative buoyancy etc.)
• at greater depths where the air breathed is more compressed (dense), > 30 metres
• with other demands on the air supply (inflating buoyancy compensator, octopus reg. etc.)
Some demand valves are bulky and quite heavy, requiring continual tension on the bite and the jaw
to retain the mouthpiece. This can lead to painful spasm of the jaw muscles and a dysfunction of the
jaw (temporo-mandibular) joint (see Chapter 32). Malleable plastic mouthpieces are available which
attempt to spread the load evenly over the teeth. A soft silastic mouthpiece may be more valuable.
Lugs attached to the mouthpiece are designed to keep the mouth open slightly, in a comfortable
position and to locate and retain the demand valve correctly in the mouth. It should not be necessary
to grip the lugs tightly.
Cylinder Valve
The gas outlet from the cylinder to the regulator is controlled by a high pressure valve or tap. High
pressure "burst discs" are fitted by law to all scuba cylinder valves to minimise the risk of explosion
if the tank is over-pressurised.
A common problem with divers is when they open the valve to check tank pressure, then close it to
prevent accidental air loss
en route
to the dive site. The high pressure remains in the hoses and so the
pressure gauge reads “full”. There is enough gas in the hose to permit a breath or two as the diver
descends. Very soon he finds the supply of gas suddenly depleted, the pressure gauge then reads
“zero”, or nearly so, and a rapid and embarrassing ascent is required.
Twin Hose Scuba
The twin or dual hose unit has both a first and second stage reducing valve combined in a single
module attached to the cylinder yoke. Air is delivered by an intake hose to the diver's mouth at a
pressure equal to the surrounding water. An outlet hose exhausts the exhaled air to the regulator for
release to the water.
Since the diver's exhaust gas bubbles are released behind the head from the regulator, they tend not
to interfere with vision. The twin hose apparatus has the disadvantage of requiring two bulky
corrugated air hoses of around 2.5–3 cm diameter, and it is more difficult to purge the system of
water. These units are rarely used today, except by photographers and in sites where regulators may
freeze. The twin hoses were very prone to perishing and leakages.