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Hi Chris: > I don't quite grok "large thermal > scattering cross section". Molecules in gas at pressures in the few-ata pressure range conduct heat from warm (you) to cold (water) by collisions with one another. The "hot" molecules transfer energy to "cold" ones by collisions--like dominoes falling. The last molecule in the chain of collisions gives up its energy to the dry suit when it bounces against it. Even though the net direction of heat flow goes from hot to cold, a collision is a randomizing process that tends to impede direct flow by scattering molecules away from the gradient. The bigger the molecule (the larger its X section), the more scattering impedes the conduction of heat along the gradient, and the warmer a diver stays. >Is energy, in the form of heat, "moving" thru or > across the argon molecules , or "stored" by them? Both! Each molecule stores some heat in the form of motion. As noted above, a molecule can transfer some of this heat to others in a collision. A measure of how much energy a molecule can store is its "specific heat." The more complex molecules have lots of ways to store energy by moving, rotating and vibrating. The simplest molecules (really just single atoms like Ar, He) can only store energy by moving. This is the main reason why argon is a better insulator than air: air is made up of more "complex" diatomic (dumb bell shaped) molecules, which can soak up your body's heat by rotational motion in addition to translational motion. Air's specific heat per molecule is 67% greater than Argon's at diving temperatures. Note that conductivity is just one part of the insulation story. About a year or two ago B. Aspacher reviewed other considerations (such as convection) in the IANTD journal. Regards, EM _____________________________________________________________ Eric Maiken email: ebmaiken@ea*.oa*.uc*.ed* Dept. of Physics o: 714 824-6621 U. of California fax: 714 824-2174 Irvine, CA 92715-4575
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