Important topics in Diving Medicine:

Diving Physics

The kinetic theory of gases states that atmospheric pressure is generated by the collision of the molecules in air. At sea level the pressure is 1 atmosphere. (1 atmosphere = 1 ATA (atmosphere absolute) = 0 ATG (atmosphere gauge) = 1.013 bar = 760 mm Hg)

Depth in sea water:
0 m0 ft1 ATA0 ATG
10 m33 ft2 ATA1 ATG
20 m66 ft3 ATA 2 ATG
30 m99 ft4 ATA3 ATG
...
80 m264 ft9 ATA8 ATG

Boyle's law states that pressure times volume is constant:

Pressure x Volume = K, or P1 x V1 = P2 x V2

This means that the volume of a given mass of gas is inversely proportional to the absolute pressure if the temperature is constant. Gases expand when pressure decreases and contract when pressure increases.
Pressure Volume Interaction
Depth/Altitude Pressure Volume
Sea level1 ATA10 L gas
10 m/33 ft depth2 ATA5 L gas
40 m/132 ft depth5 ATA2 L gas
90 m/297 ft depth10 ATA1 L gas
5000 m/18000 ft altitude0.5 ATA20 L gas
10300m/33400 ft altitude 0.25 ATA 40 L gas
Important concepts:
  1. Volume changes are greatest near the surface.
  2. During descent the gas compression in body cavities will result in a vacuum and/or volume decrease.
  3. During ascent the gas decompression in body cavities will result in increased pressure and/or cavity expansion.
  4. Any injury caused by pressure change is called barotrauma.

Henry's law: At a constant temperature the amount of gas that will dissolve in a liquid is proportional to the partial pressure of the gas over the liquid.

For diving physics, this means that at sea level (1 ATA) human body tissues are in equilibrium with air and contain a certain amount of N2 occupying the space of about 1 L. At 33 feet depth (2 ATA) the body will reach equilibrium and contain twice as much N2, but this double amount of N2 molecules will be compressed and occupy the same total space (1L) as half the amount of N2at half the pressure at the surface.

Each gas has a different solubility and the amount of gas that will dissolve depends on the liquid. It takes time for the equilibrium to occur. (If this was not so beer would go flat as soon as the bottle is opened. Terrible thought.) If tissue has the time to equilibrate with the atmospheric partial pressure of a gas, it is called saturated.

Gas movement in body tissues: Gas molecules enter the tissue and tend to move throughout the tissue until the partial pressure is the same at each point.
Gas dissolving depends on:

  1. the partial pressure of the gas,
  2. the distance of diffusion, and
  3. the permeability of the tissue.

Rule of thumb: The ratio of pressure during the dive to pressure after the dive has to be less than 2:1 for a diver to not have to make any decompression stops:

  1. A diver can descend from sea level (1 ATA) to 10 m (2 ATA) and ascend safely.
  2. A diver can descend from the surface of a high mountain lake (0.5 ATA) to only 5 m (1 ATA) before experiencing decompression symptoms upon ascent.
  3. A diver in Sealab at 90 m depth (10 ATA) can descend to 190 m (20 ATA) and ascend back to 90 m safely.

Here are some relevant reports from other websites:

CYBER DIVER diving accident report
Diving Accident Management
Diving accident report worksheet

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