Quote of the Day
Hope without a plan is denial.
— Time management expert
Every Friday afternoon, the hardware and software engineers sit down in our lunch room and chat about what occurred during the week. The discussion is always lively and includes management and engineers. It is my favorite time of the week at work. This week we discussed why automotive shops put "dry nitrogen" in car tires (Figure 1) – I am pretty sure all compressed nitrogen is dry because of the way it is processed.
A couple of our engineers are motorheads and they use nitrogen to fill the tires of their personal vehicles. This post is an extension of my earlier post on the variation of tire pressure with temperature.
- Nitrogen-filled tires retain their pressure longer.
All tires eventually lose air pressure with time as the gas diffuses through the tire. Nitrogen has a lower rate of diffusion from tires than oxygen, so the tire retains it pressure longer. The permeability of a gas through a material is a function of the gas and the material. For example, the permeabilities of nitrogen and oxygen through poly-isoprene are (Source)
Where STP standpoints for standard temperature and pressure. As you can see, the permeability of oxygen through synthetic rubber is substantially greater than that of nitrogen.
- Nitrogen gas is dry -- it has virtually no water in it.
We isolate nitrogen from the air industrially by the fractional distillation of liquified air. This process removes all water from gas. Using a dry gas provides two advantages:
Removing water eliminates a source of corrosion.
Removing water results in less pressure variation with temperature. See Figure 2 for an example (Source).
- Unlike oxygen, nitrogen gas does not promote corrosion.
At least for iron compounds, oxygen promotes rust. Oxygen and water together are bad for iron-based materials.
- Nitrogen is relatively cheap.
There are lots of dry gasses available. Nitrogen is about as cheap as you can get.
After we completed our discussion of tire pressure, we then discussed how we need to liven up our discussions. Sitting around on a Friday afternoon discussing tire pressure sounds really boring. I guess that's what life is like with a bunch of engineers around.
When I worked on torpedoes, we used to backfill them with nitrogen. The main concern there was corrosion.