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CHAPTER 3 — Underwater Physiology and Diving Disorders 3-11
3-4.8 Oxygen Consumption. A diver’s oxygen consumption is an important factor
when determining how long breathing gas will last, the ventilation rates required
to maintain proper helmet oxygen level, and the length of time a canister will
absorb carbon dioxide. Oxygen consumption is a measure of energy expenditure
and is closely linked to the respi ratory processes of ventilation and carbon dioxide
production.
Oxygen consumption is measured in liters per minute (l/min) at Standard Temper
ature (0°C, 32°F) and Pressure (14.7 psia, 1 ata), Dry Gas (STPD). These rates of
oxygen consumption are not depth dependent. This means that a fully charged MK
16 oxygen bottle containing 360 standard liters (3.96 scf) of usable gas will last
225 minutes at an oxygen consumption rate of 1.6 liters per minute at any depth,
provided no gas leaks from the rig.
Minute ventilation, or respiratory minute volume (RMV), is measured at BTPS
(body temperature 37°C/98.6°F, ambient barometric pressure, saturated with water
vapor at body temperature) and varies depending on a person’s activity level,
as shown in
Figure 3 6 . Surface RMV can be approximated by multiplying the
oxygen consumption rate by 25. Although this 25:1 ratio decreases with increasing
gas density and high inhaled oxygen concentrations, it is a good rule of thumb
approximation for computing how long the breathing gas will last.
Unlike oxygen consumption, the amount of gas a diver inhales is depth dependent.
At the surface, a diver swimming at 0.5 knot inhales 20 l/min of gas. A SCUBA
cylinder containing 71.2 standard cubic feet (scf) of air (approximately 2,000
stan dard liters) lasts approximately 100 minutes. At 33 fsw, the diver still inhales
20 l/min at BTPS, but the gas is twice as dense; thus, the inhalation would be
approxi mately 40 standard l/min and the cylinder would last only half as long, or
50 minutes. At three atmospheres, the same cylinder would last only one third as
long as at the surface.
Carbon dioxide production depends only on the level of exertion and can be
assumed to be independent of depth. Carbon dioxide production and RQ are used
to compute ventilation rates for chambers and free-flow diving helmets. These
factors may also be used to determine whether the oxygen supply or the duration
of the CO
2
absorbent will limit a diver’s time in a closed or semi closed system.
3-5 RESPIRATORY PROBLEMS IN DIVING.
Physiological problems often occur when divers are exposed to the pressures of
depth. However, some of the difficulties related to respiratory processes can occur
at any time because of an inadequate supply of oxygen or inadequate removal of
carbon dioxide from the tissue cells. Depth may modify these problems for the
diver, but the basic difficulties remain the same. Fortunately, the diver has normal
physiological reserves to adapt to environmental changes and is only marginally
aware of small changes. The extra work of breathing reduces the diver’s ability to
do heavy work at depth, but moderate work can be done with adequate equipment
at the maximum depths currently achieved in diving.
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