Henry's Law and Diving Physics
The minimum safe lower limit for the partial pressures of oxygen in a gas mixture is 0.15 bar (15 kPa) absolute. Hypoxia and sudden unconsciousness becomes a problem with an oxygen partial pressure of less than 0.16 bar absolute. If the inspired PO2 drops much below 0.10 bar (i.e., 10% oxygen at sea level), our bodies begin to shut down On the other hand too much oxygen in the blood can result in neurological distress.
The NOAA Diving Manual recommends a maximum single exposure of
45 minutes at 1.6 bar absolute120 minutes at 1.5 bar absolute
150 minutes at 1.4 bar absolute
180 minutes at 1.3 bar absolute
210 minutes at 1.2 bar absolute.
· Note 0.2 bar is partial pressure of Oxygen at 1 atmosphere
(1+x)*0.2 =1.6 bar (atmosphere)
Thus x =7 atmospheres of O2
7*(33ft of water) ~ 230 ft
Oxygen toxicity, involving convulsions, becomes a risk when these oxygen partial pressures and exposures are exceeded. The partial pressure of oxygen determines the maximum operating depth of a gas mixture.
Nitrogen narcosis is a problem with gas mixes containing nitrogen. If the diver were to suddenly ascend to the surface, the pressure which "held" the nitrogen in solution would be greatly reduced. In this situation, the nitrogen molecules would either form bubbles, or (more likely) cause pre-existing and harmlessly small "microbubbles" in the blood to grow large enough to cause problems. Whether these bubbles cause harm directly by blocking blood flow in capillaries, or by causing clotting via the complement system, it seems almost certain that the bubbles are ultimately what leads to decompression sickness.
A typical planned maximum partial pressure of nitrogen for technical diving is 3.5 bar absolute, based on an equivalent air depth of 35 meters (115 feet).
(1+x)*0.8 =3.5 bar of N2 (atmosphere)
Thus x = 3.4 atmosphere of N2
3.4 *(33 ft of water) ~ 115 ft