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        Daltonís Law.  Daltonís Law states that the total pressure of a gas is equal to the sum of the partial pressures of the constituent gasses.  In essence, that means you have to add up all the parts to get the whole. Actually, thatís a little over simplified.  What Daltonís Law really means is the amount of pressure a single gas in a mixture exerts is equal to total pressure times percentage of the total mix that gas makes up. I think I managed to complicate matters instead of simplifying.  Iíll stop and move on.

         Daltonís Law deals with partial pressures of gasses.  In this example, I will use atmospheres to measure pressure.  One atmosphere is equal to 14.7 psi. At sea level (1 atmosphere of pressure), since air is roughly 21% O2 and 79% N2, the partial pressure of oxygen is .21 atmospheres (21% X 1 atm = .21 atm) and the partial pressure of nitrogen is .79 atmospheres (79% X 1 atm = .79 atm). If we go back that balloon we used to explain Boyleís Law (actually, we destroyed that one, weíll have to get a new balloon), and take it to 33 feet of sea water (fsw), it is under 2 atm of pressure.  The ratio of nitrogen to oxygen in the air in the balloon hasnít changed, but the partial pressures have. The partial pressure of oxygen is now .42 atmospheres (21% X 2 atm = .42 atmospheres). The partial pressure of nitrogen is 1.58 atm (79% X 2 atm = 1.58 atm). Youíll notice if you add up the partial pressures you get the total pressure.  The partial pressure of nitrogen (1.58 atm) + the partial pressure of oxygen (.42 atm) = 2 atm.

         Why is this important?  The effect a gas has on the human body is based on the partial pressure that gas exerts, not on the percentage of the air it makes up.  When mountain climbers climb Everest, they have to take oxygen. At the top of Everest, the percentage of oxygen in the air is still 21%, but the atmospheric pressure is lower so the partial pressure of oxygen is lower. In scuba, the effect goes in the other direction.  The partial pressures increase as you go deeper.  In fact, oxygen reaches a partial pressure of 1.6 at 218 fsw and can be toxic.  That, however, is beyond the range of recreational diving.  Nitrogen can have a narcotic effect at high partial pressures.  This condition is called nitrogen narcosis. Daltonís Law can cause problems if the air youíre breathing is contaminated. A contamination of carbon dioxide, carbon monoxide, smoke from the fry vats of the KFC next to the dive shop, or whatever might not be a problem on the surface where partial pressures are relatively low, but at depth, as the partial pressures increase, a little contamination might be a big problem. If the air in your scuba tank has a taste or smell, get a different tank or drain, clean if necessary, and refill the tank.