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Fundamentally; air, oxygen and nitrogen will all behave exactly the same in terms of pressure change for each 10 degrees of temperature change. However temperature alone is not the whole story.

Ambient air contains moisture, nitrogen does not. If moisture is present it contributes to a greater change in pressure simply because at lower temperatures water condenses to become a liquid. The liquid form of water occupies very little volume and contributes only a negligible pressure to the tire. But at higher temperatures, such as those in a running tire, water evaporates inside the tire and becomes a gas which increases pressure in the tire.

Ambient air contains about 21% oxygen. Oxygenís smaller molecular size allows it to permeate through the rubber of the tire. By inflating with nitrogen, which is much less permeable than oxygen, the pressure changes due to oxygen loss are greatly reduced.

The racing industry is correct; nitrogen is more predictable. Because nitrogen is dry it has no moisture to contribute extra pressure changes with temperature. Because nitrogen permeates out much slower than oxygen pressure changes due to that leakage are almost eliminated compared with ambient air.

Letís get a little deeper into the science. Keep in mind that the air in your tires changes about 1psi for every 10 degree temperature change. This means that a significant change in temperature will create a significant change in your tire pressure. Here is a set of Ideal Gas Law calculations showing the effects of a 10F degree temperature change on truck and passenger tires. The two sets of data represent different initial temperatures of 60F and 90F. This demonstrates that the magnitude of the pressure fluctuation differs depending on initial conditions but only slightly.


Calculate pressure change expected for each 10F degrees temperature change:

The final pressure is calculated based on the Ideal Gas Law where, for this discussion, P and T change while n, R, and V are fixed or constant.

The Ideal Gas Law equation is P*V = n*R*T where: P = pressure, T = temperature, V = volume, R = the ideal gas constant and n = the amount of gas in the tire in moles.

Using algebra to isolate the variables of interest, P and T, the equation becomes P/T = (n*R/V).

Therefore Pinitial/Tinitial = (n*R/V) = Pfinal/Tfinal since n, R, and V are all constant. That is, we assume no volume (V) change (i.e., no significant stretching of the tire rubber) and we consider the amount of gas in the tire (n) to be constant because the time frame is very short compared to the time it takes for gas to permeate through the tire rubber. The ideal gas constant, R, is by definition constant and therefore cannot change.

As we have shown above Pinitial/Tinitial = Pfinal/Tfinal. This can be rearranged algebraically to Pfinal = [Pinitial * (Tfinal/Tinitial)]. This allows us to calculate Pfinal by multiplying Pinitial by the ratio of Tfinal to Tinitial.

Note: temperatures must be converted to Kelvin units (K), from Fahrenheit units (F), for this calculation.

First, assume the tires are filled at 60F, to either 100 psig for a truck tire or 30 psig for a passenger tire:


Truck Tire, Initial 100psig, 60F

Pinitial (psig)

Pinitial (psia)

Tinitial (F)

Tinitial (K)

 

Tfinal (F)

Tfinal (K)

Pfinal (psia)

Pfinal (psig)

Pchange (psig)

100

114.7

60

288.6

 

50

283.0

112.5

97.8

-2.2

100

114.7

60

288.6

 

60

288.6

114.7

100.0

 

100

114.7

60

288.6

 

70

294.1

116.9

102.2

2.2


Passenger Tire, Initial 30psig, 60F

Pinitial (psig)

Pinitial (psia)

Tinitial (F)

Tinitial (K)

 

Tfinal (F)

Tfinal (K)

Pfinal (psia)

Pfinal (psig)

Pchange (psig)

30

44.7

60

288.6

 

50

283.0

43.8

29.1

-0.9

30

44.7

60

288.6

 

60

288.6

44.7

30.0

 

30

44.7

60

288.6

 

70

294.1

45.6

30.9

0.9



Next, assume the tires are filled at initial temperatures of 90F, instead of 60F:

Truck Tire, Initial 100psig, 90F
Pinitial (psig)

Pinitial (psia)

Tinitial (F)

Tinitial (K)

 

Tfinal (F)

Tfinal (K)

Pfinal (psia)

Pfinal (psig)

Pchange (psig)

100

114.7

90

305.2

 

80

299.7

112.6

97.9

-2.1

100

114.7

90

305.2

 

90

305.2

114.7

100.0

 

100

114.7

90

305.2

 

100

310.8

116.8

102.1

2.1


Passenger Tire, Initial 30psig, 90F

Pinitial (psig)

Pinitial (psia)

Tinitial (F)

Tinitial (K)

 

Tfinal (F)

Tfinal (K)

Pfinal (psia)

Pfinal (psig)

Pchange (psig)

30

44.7

90

305.2

 

80

299.7

43.9

29.2

-0.8

30

44.7

90

305.2

 

90

30.0

 

30

44.7

90

305.2

 

100

310.8

45.5

30.8

0.8



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