Course Outline

The table below shows that with a fixed gas percentage (%), the partial pressures (pp) will change in open circuit diving.

Table 4
Depth Pressure Oxygen (%) Oxygen (pp) Nitrogen (%) Nitrogen (pp) Total Pressure
Sea Level 1 bar/ata 21% 0.21 79% 0.79 1 bar/ata
10 m/33 ft. 2 bar/ata 21% 0.42 79% 1.58 2 bar/ata
20 m/66 ft. 3 bar/ata 21% 0.63 79% 2.37 3 bar/ata
30 m/99 ft. 4 bar/ata 21% 0.84 79% 3.16 4 bar/ata
40 m/132 ft. 5 bar/ata 21% 1.05 79% 3.95 5 bar/ata

These calculations are very simple. You just need a calculator, and you need to refer to Table 4.

  • What is the total pressure at sea level? Answer: 0.21 + 0.79 = 1 bar
  • What is the pp of oxygen at 20 meters/66 feet/3 bar? Answer: The fraction of oxygen = 0.63
  • How do you get 0.63? Answer: 0.21 oxygen (at 1 bar/sea level) x 3 (3 bar) = 0.63

Questions (using normal air)

  1. What is the pp of oxygen at 30 meters/99 feet?
  2. What is the pp of nitrogen at 40 meters/132 feet?
  3. What is the total ppO2 and ppN2 at 20 meters/66 feet?

Answers

  1. 0.21 × 4 = 0.84 ppO2
  2. 0.79 × 5 = 3.95 ppN2
  3. 0.63 + 2.37 = 3 bar

Table 4 is in reference to open circuit scuba, air is a fixed fraction of gas in the cylinder which cannot change while underwater. You will notice the percentage of gas (fraction) does not change. If the percentage of gas remains the same, then the partial pressure will change. This has some advantages, although there are more disadvantages, and you can read more about this in the “Physiology” unit.

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