"Air" is actually about 78% nitrogen already, then 20.9% of it is oxygen, the last 1.1% or so is other gasses like argon. One of the characteristics of this mix of gasses is that it behaves pretty much according to the ideal gas law... which is the following description:
PV=nRT
where:
- P is the pressure of the gas
- V is the volume of the gas
- n is the amount of substance of gas (in moles)
- R is the ideal, or universal, gas constant (just a number)
- T is the temperature of the gas
So now lets talk about the purported benefits of a nitrogen-only fill. The sellers of this service claim that it:
1. Reduces oxidation on the wheel and tires
2. Reduces molecular seepage and thereby under-inflation (which has lots of other effects)
3. Reduces moisture in the tire.
There are actually lots of other benefits that are claimed but all of them basically are derived from these three. So let me address them individually. Yep, it probably does reduce oxidation of the tire and of the wheel itself. But that is almost never the cause of failure of a tire or wheel. Molecular seepage also probably happens... but the claims are that it is "normal" for a tire to loose 1-3 psi per month due to permeation (seepage)! However it has been shown that the more-realistic rate is more like 0.3 psi per month according to this ARPN Journal study. Finally, yes it does actually reduce the moisture in the air... but again the amount of moisture in the air is already minuscule. First off the air can only get into the tire as vapor so assuming 100% humidity there could only be a few ounces of water total in there. Furthermore, most air compressors have the effect of drying the air used because of the over-compression and then precipitating out of water in the compressor. This is why all air compressors have water drain systems. Plus the effect of water in the tire is not that significant either. It still behaves like an ideal gas.
Here is some math on the moisture content:
Let's first examine how much water is actually in the air that is used to fill tires. Generally the tire shop (or home shop) uses a piston air compressor and a pressure tank to source the compressed air for the tires. If we assume that the air compressed is 100-degrees f and 100% relative humidity starting at atmospheric temperature.… sort of a worst-case scenario. In terms of how much actual water is in the air, the moisture in this case works out to be 4.3% of the air mass. If we compress that mix up to 100 psi (what most shop compressors run at) then using standard psychometric charts, we see that the capacity for the air to hold water is reduced substantially. In fact the air can only then hold 0.53% mass of water. In practicality we see that in shop compressors because you have to drain water out of the air tank periodically. So then taking that air and cooling back down to 75-degrees (or basically the temperature of a normal shop), the water holding capacity goes even further down to 0.24% mass. This is also why industrial air compressors have dryers… because when the air cools down there will be more moisture dropped out. Luckily in a shop environment this happens mostly in one tank. The last step then is for the air to be inserted into tires. The pressure then changes back down to about 30 psi. So using the final mass of water in our air, the relative humidity of air in the tire will be 39% (remember starting at 100%), and the dew point will be 48 degrees.
But lets be real for a moment here… when was the last time your wheel rusted out? Or when was the last time you had a tire fail due to internal corrosion? The reason it doesn’t is because the air in you tire is already far dryer than the atmosphere. This is a simple case of saving drivers from a risk that is exceptionally low.
Pressure Variation Theory:
As noted above, pressure inside your car tires can be modeled using the ideal gas law. In that law the relationship between pressure and temperature is identified as being a linear relationship. Using a 30 PSI tire, the relationship shows that for every 10 degree change in temperature, pressure changes by about 0.9 PSI. However, in this model, it doesn’t matter what gas is used. Air, nitrogen, water vapor all can be modeled with this formula to a reasonable level of accuracy. However, the nitrogen purveyors have constructed an interesting argument. That is that they assume that the water contained in the tire air is in a liquid state at normal temperatures and then goes to gas state at driving temperature. To figure out how much this effects the tire pressure, lets do the math. Using the data above you can see that is clearly is possible to have some liquid water in the tire assuming ambient temperature is below 48 degrees and you filled up at 100-degrees and 100% humidity. So how much does that effect pressure? Lets take our 0.24% water derived above and convert that to actual amounts. Assuming the air volume in a tire is about 12-gal, that would equate to about 0.368 Oz of water. Using steam charts at 35-psi, the specific volume change in water is 696 times. So our .368 Oz of water would be .01Kg which equates to 10 milliliters in liquid form. Using our steam chart (and that assume that the water changes from 100% liquid to 100% vapor) the new volume would be 6,960 milliliters or 1.8 Gal. But remember that it now complies with the ideal gas law so in order for the change so we compress that to 35 psi it becomes 0.5 gal or 4% of the compressed air volume. Using the fact that proportionality applies to pressure mass relationships this would increase or decrease pressure about 1.4 psi. So even in the most extreme example I can think of the state-change theory of nitrogen promoters has a very small effect.
Let's first examine how much water is actually in the air that is used to fill tires. Generally the tire shop (or home shop) uses a piston air compressor and a pressure tank to source the compressed air for the tires. If we assume that the air compressed is 100-degrees f and 100% relative humidity starting at atmospheric temperature.… sort of a worst-case scenario. In terms of how much actual water is in the air, the moisture in this case works out to be 4.3% of the air mass. If we compress that mix up to 100 psi (what most shop compressors run at) then using standard psychometric charts, we see that the capacity for the air to hold water is reduced substantially. In fact the air can only then hold 0.53% mass of water. In practicality we see that in shop compressors because you have to drain water out of the air tank periodically. So then taking that air and cooling back down to 75-degrees (or basically the temperature of a normal shop), the water holding capacity goes even further down to 0.24% mass. This is also why industrial air compressors have dryers… because when the air cools down there will be more moisture dropped out. Luckily in a shop environment this happens mostly in one tank. The last step then is for the air to be inserted into tires. The pressure then changes back down to about 30 psi. So using the final mass of water in our air, the relative humidity of air in the tire will be 39% (remember starting at 100%), and the dew point will be 48 degrees.
But lets be real for a moment here… when was the last time your wheel rusted out? Or when was the last time you had a tire fail due to internal corrosion? The reason it doesn’t is because the air in you tire is already far dryer than the atmosphere. This is a simple case of saving drivers from a risk that is exceptionally low.
Pressure Variation Theory:
As noted above, pressure inside your car tires can be modeled using the ideal gas law. In that law the relationship between pressure and temperature is identified as being a linear relationship. Using a 30 PSI tire, the relationship shows that for every 10 degree change in temperature, pressure changes by about 0.9 PSI. However, in this model, it doesn’t matter what gas is used. Air, nitrogen, water vapor all can be modeled with this formula to a reasonable level of accuracy. However, the nitrogen purveyors have constructed an interesting argument. That is that they assume that the water contained in the tire air is in a liquid state at normal temperatures and then goes to gas state at driving temperature. To figure out how much this effects the tire pressure, lets do the math. Using the data above you can see that is clearly is possible to have some liquid water in the tire assuming ambient temperature is below 48 degrees and you filled up at 100-degrees and 100% humidity. So how much does that effect pressure? Lets take our 0.24% water derived above and convert that to actual amounts. Assuming the air volume in a tire is about 12-gal, that would equate to about 0.368 Oz of water. Using steam charts at 35-psi, the specific volume change in water is 696 times. So our .368 Oz of water would be .01Kg which equates to 10 milliliters in liquid form. Using our steam chart (and that assume that the water changes from 100% liquid to 100% vapor) the new volume would be 6,960 milliliters or 1.8 Gal. But remember that it now complies with the ideal gas law so in order for the change so we compress that to 35 psi it becomes 0.5 gal or 4% of the compressed air volume. Using the fact that proportionality applies to pressure mass relationships this would increase or decrease pressure about 1.4 psi. So even in the most extreme example I can think of the state-change theory of nitrogen promoters has a very small effect.
1. There probably is a benefit to having only nitrogen in you tires vs "just" 78% nitrogen.
2. The benefits however are way too small to matter especially if you're paying upwards of $70 per tire for this service.
3. You would be far better served by just checking the pressure on your tire regularly.
No comments:
Post a Comment