Basic HTML Version
Chapter 3 — 10
inspiration is about three times this value. Some regulators may have difficulty delivering gas at
this rate, adding considerable resistance to breathing.
This problem is magnified at depth because the greater pressure increases the density of the
inhaled gas, especially at depths in excess of 30 metres when air is breathed. The same effect is
seen at about 200 metres depth when helium
/
O
2
(heliox) mixture is breathed, because heliox is
less dense than air. It is likely that resistance to breathing will ultimately limit the depth to which
divers can reach.
An idea of the respiratory loads which the diver faces can be gained from the following table :
SCUBA
SWIM
SPEED*
OXYGEN
CONSUMPTION
RESPIRATORY
MINUTE VOLUME
Slow scuba swim
0.5 knots
0.8 litres / minute
18 litres / minute
Average scuba
swim
0.8 knots
1.5 litres / minute
28 litres / minute
Fast scuba swim
1.0 knots
1.8 litres / minute
40 litres / minute
Maximum scuba
swim
1.3 knots
3–4 litres / minute
70–100 litres / minute
*. a knot is equal to 1 nautical mile per hour, or 1.85 km / hr
Table 3.1
Air Consumption
O
2
consumption is virtually the same for a given amount of exercise whether it is performed at
the surface or deep under water. Because compressed air is being breathed at depth, more O
2
will
be supplied than is needed by the diver. The actual volume of gas breathed at any depth will be
the same as that which would be breathed at the surface. However, since the gas being breathed
at depth is at greater pressure, the volume breathed, if converted to atmospheric (surface)
pressure, will also be greater.
For example, during maximal effort a diver may consume 70 litres of air per minute at the
surface. If he is performing an equivalent amount of effort at 20 metres depth (3ATA), he will
still be breathing 70 litres per minute from his scuba regulator at 20 metres, but this will be
equivalent to :
70 (litres)
x
3 (atmospheres)
= 210 litres per minute at surface or atmospheric pressure.
So, the endurance of an air supply decreases with depth.
The regulator may not be able to meet the respiratory demands of a diver when certain conditions
apply (see Chapter 5). Under these conditions, the diver may be aware of an inadequate air
supply and either panic or take other dangerous action, such as a rapid ascent or omission of
decompression requirements.
Skip Breathing
It is possible for a scuba diver to minimize his air consumption by deliberately slowing his
breathing rate. This type of breathing pattern obviously limits the reserve of O
2
which will be
stored in the divers lungs and haemoglobin, and may lead to retention of CO
2
and acidosis. It