More Gases

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When the average open water dive student receives a tank full of gas to dive with, it is filled with compressed air, but just what gases are in there? Scuba Tech Philippines What we refer to as 'air' is a mixture of about 78% nitrogen, 21% oxygen, and 1% other gases including argon and carbon dioxide.

 
If a diver breaths in an excess of nitrogen, it can result in narcosis. In the diving community, this is referred to as being 'narked' and generally can occur in divers the go deeper than 30 meters or 100 feet of sea water. Symptoms of this condition include a decrease in reasoning and seemingly drunken behavior.

If a diver breathes an excess concentration of oxygen, it can result in oxygen toxicity. Symptoms of oxygen toxicity include, but are not limited to: muscle spasms, twitching, dizziness, nausea, fatigue, incoordination, and convulsions.

Why are these conditions of concern for a diver breathing air with a seemingly fixed amount of nitrogen and oxygen?

Dalton's Law Dalton's law states that the total pressure exerted by a gas is equal to the sum of the pressures exerted by each gas in the mixture. The part of the total pressure exerted by each gas is referred to as the partial pressure. In air, the total pressure is due to the partial pressure of nitrogen, plus the partial pressure of oxygen, plus the partial pressure of each of the trace gases. PTotal = P1 + P2 + P3 +... What Dalton discovered is that all gases compress equally and therefore the ratio of gases remains the same as the pressure increases. As the diver descends deeper, more pressure is put on the breathing gas, compressing it in the hose of the regulator, yet the volume of the divers breath is remaining roughly the same. Therefore, as a diver descends, he or she is breathing higher concentrations of both oxygen and nitrogen than would be experienced at the surface. The table to the right is found in many introductory SCUBA manual and may seem confusing, but remember, the partial pressure of a gas can be found by taking the fraction of a gas in the mixture, in decimal form (percentage/100), times the depth in atmospheres absolute, where atmospheres absolute is the number of feet on you depth gauge divided by 33, plus one. This calculates the number of atmospheres a diver is experiencing from the water they are submersed in and add an atmosphere of pressure to account for the pressure of the earth's atmosphere.

Partial pressure formula Scuba Tech Philippines partial pressure = fraction of gas in mix × atm absolute atm absolute = (depth (ft) / 33) + 1

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Henry's Law Henry's law states that the amount of gas dissolved in a solution is directly proportional to the pressure on the gas. In other words, as you apply pressure to a container with both gas and liquid in it, the liquid can dissolve more of the gas. This is important to divers because if gas becomes dissolved in the body at too high of a gradient, when the diver ascends, these gases may come out of solution faster than they can diffuse through the surface of the body. This can result in tiny gas bubbles in various areas of the body and can be a serious health risk. Please refer to physiology portion of an introductory SCUBA manual for more information on these risks. Dive tables and computers incorporate Henry's law when calculating the length of time a diver may spend at depth.

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