| Re: [GMCnet] Tire size and blow outs [message #96699] |
Sat, 21 August 2010 15:41  |
Gary Casey
Messages: 448
Registered: September 2009
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All this talk of covalent bonding had better stop :-)
Over the years I've heard the claims and wondered, and wondered. There seem to
be two claims - one is that tires with nitrogen will leak less and the other is
that nitrogen-filled tires will have less pressure change wtih temperature.
Like Emery said below, using molecular size as the reason just doesn't seem to
hold water. However, there are a couple of explanations that make sense to me.
One is that the oxygen in air can react (oxidize) with the rubber over time.
Some some of the oxygen is then removed from the air in the tire, reducing the
pressure. Does that effect have enough clout to noticeably reduce the pressure?
I'm not sure, but I doubt it - the Chemists will have to chime in.
The other explanation that I think makes more sense is most compressed air
contains water vapor. In fact, unless descacent (sp) dryers are used the
compressed air will be close to saturated with water vapor. Take air from an 80
psi tank and expand it into a 60 psi tire and the amount of water vapor is still
substantial. Water vapor has a molecular weight of about half that of air (does
it have a smaller radius, Emery?) and so could migrate through the rubber. The
second thing caused by water vapor is that at low temperatures it can condense,
and this would significantly lower the pressure. The biggest advantage of
nitrogen might be that it is relatively dry.
No, I don't use Nitrogen, but do have some of those blue valve stem caps:-)
Gary Casey
> Nitrogen is a smaller molecule and will not leak down as fast with butyl
> tubeless tires. Usually used in truck tires. I wouldn't pay anything extra
> for the use of it. I don't use it and can get it free.
>
> Ken O'Rourke
> Greenville, SC
Hi Ken
I think you mean "is a larger molecule" although barely.
Molecular size a bit tricky. As a quick comparison, one can use the covalent
radius defined as 1/2 the distance between two identical covalently bonding
nuclei. This is measured in picometers (1 pm= 1x 10-12 m). Nitrogen's covalent
radius is 75pm so the length of a nitrogen (N2) molecule ought to be 4 X 75pm or
300 pm. A molecule of oxygen (O2) ought to be just a shade smaller 4 X 73pm or
292pm. So an oxygen molecule is a little less than 3% smaller than a nitrogen
molecule.
Emery Stora
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| Re: [GMCnet] Tire size and blow outs [message #96706 is a reply to message #96702] |
Sat, 21 August 2010 16:27  |
emerystora
Messages: 4442
Registered: January 2004
Karma: 13
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On Aug 21, 2010, at 2:50 PM, Desmond's GMC wrote:
> There seems to be a pretty good following of the "temperature doesn't
> change tire pressure" school of thought.
>
> Nitrogen allowed you to have a predictable tire pressure no matter the
> temperature.
>
Then that school should be given a flunking grade!
Even in high school they taught the Ideal Gas Law: PV=nRT
Air, nitrogen, oxygen, etc. all follow this gas law.
This means that the pressure is directly proportional to the temperature. However the temperature is in K or Kelvin degrees. So if you are working in F then it has to be converted.
So, if you raise the temperature 10% (in Kelvin) then the pressure goes up 10%. Yes, you can predict the tire pressure change with the temperature but if you are saying that nitrogen allows you to have a constant pressure no matter what the temperature change then that is purely fiction.
The reason I am being such a nit picker on this is probably because those statements just fly in the face of all the technical training that I have ever received.
Say you have a temperature of 80°F...
°C = (°F - 32) / (1.8)
°C = (80°F - 32 )/ 1.8
°C = (48°F) / 1.8
°C = 26.6
K = °C + 273.15
K = 26.6°C + 273.15
K = 299.75
Then say your temperature goes up to 100°F...
°C = (100°F - 32 )/ 1.8
°C = (68°F) / 1.8
°C = 37.8
K = °C + 273.15
K = 26.6°C + 273.15
K = 310.93
Note that a temperature increase from 80 to 100 F is a 25% increase
However that is only a 3.7% increase in K.
If you started with a pressure of 60 then
310.93 / 299.75 * 60 = 62.2
Your pressure would go from say 60 psi to 62.2 psi
This doesn't matter if the gas in the tire is air or nitrogen.
In a car tire if you started from a pressure of 30 psi then it would only go to 31.1 psi.
The reason some may have thought that their pressure didn't go up is that a 1 psi raise in pressure is probably not that easily detectable with the normal home tire gauge. But, the real point here is that it would raise the same whether the gas in the tire is nitrogen or air.
Emery Stora
77 Kingsley
Santa Fe, NM
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| Re: [GMCnet] Tire size and blow outs [message #96718 is a reply to message #96706] |
Sat, 21 August 2010 18:08 |
mlincoln
Messages: 107
Registered: August 2006
Location: Salt Lake City
Karma: 0
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Senior Member |
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On my GMC Acadia (a great car by the way) there is an integrated TPMS; the dash computer can display the individual tire pressures on the dash display. I see these pressures consistently increase from the recommended 35 psi when cold to ca. 37-38 psi on a typical long range drive. That amount of change is consistent with expectations based upon the ideal gas law, as Emory has pointed out. Any available inflation gas (pure oxygen, nitrogen, argon, helium, carbon dioxide, etc) will behave like an ideal gas at the sorts of pressures and temperatures we're talking about. For a given temperature change, the pressure change will be no different with nitrogen inflation than air inflation.
I think the main reason some use nitrogen is because it apparently permeates out of tires at a ca. 30% slower rate than oxygen. I was able to Google a Consumer Report blog verifying this experimentally (http://blogs.consumerreports.org/cars/2007/10/tires-nitrogen-.html). They inflated tires to 30 psi with nitrogen or ambient air, set them aside for one year and then remeasured pressure. The ambient air-filled tires dropped an average of 3.5 psi and the nitrogen-filled tires dropped 2.2 psi (1.2 psi less is about 1/3 less). The report concluded that periodic inflation checks were still needed as under-inflation with nitrogen (even if it tended to develop at only 2/3 of the rate of air) could still be a key factor in unnecessarily reduced mileage and premature tire failure.
CostCo inflates with nitrogen at no extra cost, for what it is worth. You also get cool green valve stem covers.
Mike Lincoln
1978 Center Kitchen Royale
Ann Arbor, Michigan
On Aug 21, 2010, at 5:27 PM, Emery Stora wrote:
>
> On Aug 21, 2010, at 2:50 PM, Desmond's GMC wrote:
>
>> There seems to be a pretty good following of the "temperature doesn't
>> change tire pressure" school of thought.
>>
>> Nitrogen allowed you to have a predictable tire pressure no matter the
>> temperature.
>>
>
> Then that school should be given a flunking grade!
>
> Even in high school they taught the Ideal Gas Law: PV=nRT
>
> Air, nitrogen, oxygen, etc. all follow this gas law.
>
> This means that the pressure is directly proportional to the temperature. However the temperature is in K or Kelvin degrees. So if you are working in F then it has to be converted.
>
> So, if you raise the temperature 10% (in Kelvin) then the pressure goes up 10%. Yes, you can predict the tire pressure change with the temperature but if you are saying that nitrogen allows you to have a constant pressure no matter what the temperature change then that is purely fiction.
>
> The reason I am being such a nit picker on this is probably because those statements just fly in the face of all the technical training that I have ever received.
>
>
> Say you have a temperature of 80°F...
> °C = (°F - 32) / (1.8)
> °C = (80°F - 32 )/ 1.8
> °C = (48°F) / 1.8
> °C = 26.6
> K = °C + 273.15
> K = 26.6°C + 273.15
> K = 299.75
>
> Then say your temperature goes up to 100°F...
> °C = (100°F - 32 )/ 1.8
> °C = (68°F) / 1.8
> °C = 37.8
> K = °C + 273.15
> K = 26.6°C + 273.15
> K = 310.93
>
> Note that a temperature increase from 80 to 100 F is a 25% increase
> However that is only a 3.7% increase in K.
>
> If you started with a pressure of 60 then
>
> 310.93 / 299.75 * 60 = 62.2
>
> Your pressure would go from say 60 psi to 62.2 psi
>
> This doesn't matter if the gas in the tire is air or nitrogen.
>
> In a car tire if you started from a pressure of 30 psi then it would only go to 31.1 psi.
>
> The reason some may have thought that their pressure didn't go up is that a 1 psi raise in pressure is probably not that easily detectable with the normal home tire gauge. But, the real point here is that it would raise the same whether the gas in the tire is nitrogen or air.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
>
>
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
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Mike
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