SCUBA diving basics

The dry air we breathe every day is composed of 21% oxygen, 78% nitrogen, and <1% other gases. Its average pressure at sea level is 1 atm (14.7 psi). For SCUBA, this air is compressed into a SCUBA cylinder or “tank.” SCUBA tanks can be made of steel or aluminum; each of these materials has pros and cons that impact the diver’s decision on which type to use.

The compressed air in the tank is delivered to the diver through a regulator, which reduces the pressure from the tank to match the ambient pressure. At the surface, ambient pressure is 1 atm and it increases by 1 atm for every 10 m in depth through which a diver descends.

Note:Other gas mixes such as nitrox (an oxygen/nitrogen mixture with a greater amount of oxygen than air), heliox (a helium and oxygen mixture), and trimix (a mixture of oxygen, nitrogen, and helium) or even pure oxygen are also used for technical diving, but those mixes go beyond the scope of this discussion.

 

Gas Laws Formulas & Physics For Scuba Diving

The important ones are:

Boyle’s law: Relates gas pressure to volume

  • As a diver, Boyle’s law affects you every time you enter the water. Air spaces in the body are subjected to pressure and volume change, in direct proportion to your depth.
  • Some problems that can occur with Boyle’s Law:
    • Running out of air at depth (not taking into consideration the increased gas usage at depth)
    • Rapid ascent, caused by not adjusting buoyancy quickly enough to allow expanding air to escape the BCD/ Dry suit
    • Sinus, ear and mask squeeze on descent – sometimes resulting in bleeding. Sinus or inner ear pain may be experienced on ascent.
    • From 10msw to surface the pressure halves. If you breath hold, the air in your lungs will double in volume causing a ruptured lung.

Charles’ law: Relates gas pressure to temperature

  • So what problems can occur with Charles law?
    • probably only one that will significantly affect the diver which is having less gas volume in the cylinder at the lower water temperature than you expect.

Dalton’s law: Relates pressure of a mixed gas to the partial pressures of each gas in the mix

  • So what problems can occur with Daltons law?
    • Nitrogen narcosis traditionally becomes a problem in the 20 to 30msw range, equivalent to a partial pressure of 2.37 to 3.16 bar absolute breathing air. The feeling of euphoria and well-being can lead to confusion and cause a risk to the diver. Building up dive depths at the start of the season can reduce this effect.
    • As depth increases, more inert nitrogen is absorbed in the body and longer decompression is required to release the diffused nitrogen from the body tissues.
    • A range of gas mixtures, involving various proportions of nitrogen, oxygen and helium, are used to reduce the effects of nitrogen narcosis, increase dive time and reduce decompression. Breathing the right gas at the right depth is essential to avoid problems.
    • Ensuring the partial pressure of oxygen does not exceed 1.2 to 1.6 bar absolute minimizes the risk of in-water convulsions caused by breathing too high a partial pressure of oxygen. In chambers we can go much higher for medical treatments and of course there is no risk of drowning.

Henry’s law: Relates the gas dissolved in a fluid (like water) with the partial pressure of the gas in contact with the fluid.

  • So what problems can occur with Henry’s law?
    • Insufficient decompression of dissolved gases.
    • Re-absorption of gas due to reverse profiles, or saw tooth profiles.
    • Avoid getting too cold as it effects solubility of gases.

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