Q: The air that you are breathing in “the real world” is pretty much the same as the air that Dr. MacBain is breathing in academia. The water in the air is already a vapor. It will not “vaporize” more when heated. I know the racers use nitrogen, and I’d like you to expand on your previous answer to help me understand why.

A: Will do. The moisture inside the tire is liquid, not a vapor, initially. As air is compressed, it heats up. As it cools, moisture in the air condenses into liquid. Have you ever bled the air out of a tire or air compressor? The amount of water blown out with the compressed air is amazing. That’s why you never adjust air pressure in subfreezing temperatures — the tire valve can freeze open in an instant, allowing the tire to fully deflate right in front of your eyes! In fact, most professional shops, particularly auto body/paint shops, have large in-line dryers installed to trap this moisture and prevent it from entering their air tools and paint guns.

From David W. Drummond, Ph.D.: “The biggest culprit in tire pressure changes is water vapor because it changes volume more — and condenses — in the normal range of tire pressures. In fact, Honda tells its dealers to use dry air instead of nitrogen (perhaps to save money). Two other components of air have a disproportionate effect on tire pressure. Carbon dioxide changes volume more with temperature than nitrogen or oxygen and helium diffuses through the tire because of its small molecular size. In addition, ordinary air compressors are likely to introduce oil vapor and additional carbon dioxide to compressed air. Nitrogen and oxygen make up 99 percent of dry air. Pure nitrogen is best in racing applications because it is inert, while oxygen might attack the inside of the tire at high temperatures. Also, nitrogen is cheap because it is a byproduct of producing oxygen for medical use. Both nitrogen and oxygen change volume in direct proportion to temperature in the tire temperature range (in science-speak they behave like “ideal gases”) while water vapor, carbon dioxide and oil vapor show much greater volume changes with temperature. Molecular size is not an issue except for helium. Size increases with molecular weight. Here are the approximate molecular weights as they exist in air: Helium-4, Nitrogen (N2)-28, Oxygen (O2)-32, Water (H2O)-18, Carbon dioxide (CO2)-44. My information comes from Chemistry 101 and air compressor information from friends who are scuba divers.”

From Todd Fukushi: “I’m a chemist at 3M. I have been working on rubber materials more than 20 years and permeation resistance of rubber materials is one of our focuses. I totally support your reply to John. He said “nitrogen obeys the universal gas law as oxygen, hydrogen and every other gas known to man.” It is true, if it is treated as ideal gas. However, for example, gas permeation is different among gases. John can’t believe, probably, but nitrogen and oxygen gas permeation through rubbers are very different.

“Typical automotive tire uses butyl rubber or halogen butyl rubber as inside layer of the tire because butyl rubber has lower gas permeation than natural rubber or other typical rubbers. Permeability coefficients (cm3mm/cm2seccmHg x 10-10) of oxygen and nitrogen for butyl rubber are 10 and 2.5 respectively based on a literature. If it is hydrogen, the coefficient goes up to 55. Even 20 percent of oxygen in the air would have huge impact on permeation. So regarding permeation what kind of gas is the matter. John ignores the matter. Also since nitrogen is inert gas, we don’t have to think about oxidation effect of materials.

Nitrogen-filled tire has been used for not only racing field, but also aircraft industry. You can imagine that aircraft need to maintain tire pressure precisely. So nitrogen-filled tire is proven technology in the industries.”

Whoever thought discussing air could be so interesting!