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Re: Mixed drinks

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Thu, 26 Aug 1999 03:31:12 +0300

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 ```> Obviously, dive planning would be simpler if you could ignore > the Helium at > this point. The question is if you safely can. > > > Comments?       I like to talk with FACTS, so here it is:   First of all there is the "DALTON'S LAW" which defines that "THE TOTAL PRESSURE OF A MIXTURE OF GASES IS THE SUM OF THE PARTIAL PRESSURE OF EACH GAS" The following equation clearly states the above law.   P total = P1 + P2 + P3 + .......Pn (More info on Dalton's law in http://mot.cprost.sfu.ca/~rhlogan/dalton.html )   Second, the "HENRY'S LAW" defines the solubility of gases in a given liquid. According to this law, "THE SOLUBILITY OF A GAS INTO A LIQUID IS EQUAL TO IT'S PARTIAL PRESSURE" (More info on Henry's law in http://www.chem.ualberta.ca/courses/plambeck/p101/p01182.htm )   Combining these two laws we have the following results:   1) For the first dive our 'experimental diver' uses a mixture of 10% Oxygen, 40% Nitrogen and 50% Helium. (In other words our diver breaths a mixture of 50% Air and 50% Helium)  In the depth of 50m the TOTAL PRESSURE is 6 ATM, which is the sum of 3 ATM of Helium, 2,4 ATM of Nitrogen and 0,6 ATM of Oxygen. We know that the Oxygen is metabolized from the divers body, so the rest of the gasses are dissolved into the bloodstream (and transferred into the body tissues). As our 'experimental diver' stops it's dive and starts the ascent, the partial pressure of these gases is decreasing. As a result, the dissolved gasses start to decreased (escaping from the lungs during breathing cycles).   2) In the second dive our diver uses a mixture of 64% Nitrogen and 36% Oxygen.  In the depth of 20m the TOTAL PRESSURE is 3 ATM which is the sum of 1,92 ATM of Nitrogen, 1,08 ATM of Oxygen AND 0 ATM OF HELIUM (Because there is NO HELIUM in the mixture).  Now according to Henry's law, since the partial pressure of Helium is ZERO IN ANY DEPTH, the dissolved helium is still escape from the lungs, until it reaches the Helium's partial pressure witch is ZERO. In other words our diver will continue to release Helium from his body, nomater what the pressure of the Nitrogen is.     As we can understand from the above example, the answer to the Huntzinger's question is that "YES, WE CAN SAFELY IGNORE THE REMAINING HELIUM IN THE SECOND DIVE".     Further more: A) From the Dalton's law, we can see that two different gasses CAN coexist in a solution in their maximum dissolved quantity.   B) From the Henry's law we can see that the dissolving of a gas into a liquid depends from the type of the gas also. (It's easier to dissolve Helium in a liquid rather than Nitrogen, because Helium molecules are smaller than Nitrogen's)   C) Because of the above laws (and their example) it is possible and the common rule to the Trimix divers, to use different mixture of gasses, including Nitrox and some times pure Oxygen during their Decompression stages in order to free their bodies from the remaining Helium before ascending to the surface.     That's all.    Sotiris Tzanlis.  3 Stars CMAS Instructor, TDI Nitrox Instructor, TDI TRIMIX Instructor.  Athens - Greece. ```