MLA Citation: Bloomfield, Louis A. "Question 1223"
How Everything Works 23 Oct 2017. 23 Oct 2017 <http://howeverythingworks.org/print1.php?QNum=1223>.
1223. What is the relative insulating value of various levels of vacuum? For example, how insulating is 1/2 atmosphere as compared to full atmosphere?
Amazingly enough, air's ability to carry heat doesn't change much as you reduce its pressure and density as long as you stay above about a thousandth of atmospheric pressure and density. That's because reducing the density of air molecules may leave fewer particles to carry heat, but it also allows them to travel farther before they collide with other molecules. The reduction in molecular density is almost perfectly cancelled by an increase in the mean free path those molecules travel between collisions—there are fewer heat carriers, but they can move more easily. It isn't until you reach very low pressures and densities—so that the mean free path begins to approach the size of the enclosed gas—that reducing the air pressure and density begins to decrease the air's ability to carry heat. That's why even a small leakage of gas into a vacuum flask spoils that flask's insulating characteristics. However, you can decrease the "air's" ability to carry heat by increasing the mass of its molecules—heavier particles such as carbon dioxide or krypton travel more slowly than normal air molecules and don't carry heat as well.

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