Diving Physics  

During my diver training it struck me that a lot of approximations and arbitrary figures were being used without much justification. This page is my attempt to answer the question "well, what are they really?"

Composition of Air

Composition of Dry Air by Volume

To the nearest 0.01%:

Note: normal air will also include significant amounts of water vapour (H₂0) - typically 1% by volume.

Composition of Exhaled Air by Volume

Respiration exchanges oxygen in the inhaled air with carbon dioxide produced by the body's metabolism. Other gases are unaffected (note that absorption or release of nitrogen due to compression/decompression is slow enough that it does not significantly affect the volume of nitrogen in the lungs).

A person at rest, at atmospheric pressure, will produce typically 3.7% CO₂ by volume in exhaled air. Increased exertion will produce greater amounts of CO₂ - a figure of 5% by volume can be assumed for moderate exertion, thus to the nearest 0.5%:

(neglecting water vapour)

Composition of Air at Greater Pressure

During SCUBA diving, air is breathed at ambient pressure, which increases with depth. Since the volume of any gas is inversely proportional to its pressure, the composition of inhaled air by volume remains the same, although for a given volume the number of molecules of each component is increased in proportion to the absolute pressure.

If we assume that the rate of metabolism and respiration are the same at increased pressure (in fact, they are slightly increased), approximately the same number of molecules of carbon dioxide will be produced by the body at 2 atmospheres pressure as at 1 atmosphere. The volume of carbon dioxide will thus be decreased in proportion to the depth, so at a depth of 10m (2 atmospheres pressure) the composition of exhaled air by volume to the nearest 0.5% is:

Note that the partial pressure of CO₂ (= total pressure × fractional volume) thereby remains constant regardless of depth, which is fortunate since it is the partial pressure of CO₂ which controls the desire to breathe!

Artificial Ventilation (AV)

Also known as Expired Air Resuscitation (EAR).

Since it is impractical to perform AV while submerged, only the composition of exhaled air at 1 atmosphere pressure is relevant. Approximately 12% O₂ by volume would be sufficient to sustain consciousness in a healthy person - 15% O₂ allows for some respiratory dysfunction. Thus the transfer of 16% O₂ from the rescuer's exhaled air to the casualty's lungs is ample for resuscitation purposes.

Measurement of Gas Pressure

What this amounts to in practice is that 1 bar is about 1% smaller than 1 atm, and 1 kg/cm² is about 3% smaller than 1 atm.

Buoyancy

Density of seawater varies with temperature and salinity (it also varies with pressure, but not significantly within diving depths). Both the Mediterranean and the Red Sea have significantly higher levels of salinity than open ocean due to limited currents and high evaporation:

If water density is 3% higher, you will therefore need to be 3% heavier to maintain neutral buoyancy. Assuming that you and all your kit weigh about 100kg, this explains the rule-of-thumb that 2-4kg more weight is needed in seawater than in fresh water.

Measurement of Depth

To be completed...

Heat Loss in Water

The thermal conductivity of air at 20°C is 0.025 W/mK, that of water at 20°C is 0.60 W/mK - 24 times as large. This does not account for heat loss due to convection and radiation, however. Thermal radiation is not affected by surrounding fluid, so this will be the same in air and water. Convection, however, does depend on the density of the fluid - convection coefficients are highly variable [water 10 times air?] [also heat loss via evaporation in air only]

I could use some help with this one!

Smoking and Carbon Monoxide (CO)

A moderate to heavy smoker (20-30 cigarettes per day) will have an average level of carboxyhaemoglobin (COHb) in the blood of around 6% of total Hb, compared to 1.5% or less in a non-smoker.

Effects of increased levels of COHb are as follows:

Note that at 45% COHb, the capacity of the blood for carrying oxygen is 100-45=55% of maximum - thus the amount of oxygen received by the body tissues from normal air will be reduced to 55%; this is equivalent to breathing air with just under 12% O₂ by volume, agreeing with the minimum level of O₂ required to sustain consciousness quoted in the section on AV above.

With good air, for a typical smoker, the reduction in oxygen supply to the tissues due to COHb is relatively small - in practice this is likely to be exceeded in effect by the long-term physical damage caused by smoking to lung function.

The main effect of an increased level of COHb prior to diving is that carbon monoxide poisoning due to CO contamination of the cylinder air will take effect sooner. Note that the peak level of COHb is determined entirely by the level of CO in the air supply - the diving smoker will be subject to quicker CO poisoning, but not necessarily worse (CO is absorbed quite slowly, so a person with an initially low level of COHb could complete a dive with potentially toxic air before symptoms occur).

COHb amount at equilibrium is determined by the Haldane equation:

The rate of CO release by the body can be approximated by the following formula:

Thus a typical smoker would need to refrain from smoking for 5 hours prior to the dive in order to reduce COHb to 3%, 10 hours to reduce it to 1.5%.

These figures notwithstanding, empirical evidence (i.e. my instructor's anecdotes!) indicates that smoking immediately prior to a dive does have bad effects - personally, I don't smoke for at least 1 hour before a dive.

Figures used here have been taken from and verified against various web resources including NASA, Oak Ridge National Laboratory, Britannica Online and several academic sites.

Home Page

SCUBA Index