Regular air has stayed in some tyres for years and years without leaking out.
Nitrogen has a vapour density of 14. Air is slightly above that.
Diffusion rate of gases (like going through rubber - unless there is a defect like a puncture) increases as the vapour density reduces, so hydrogen and helium would be lost fastest. Nitrogen diffuses quite a deal faster than oxygen.PV = nRT This is the General Gas Equation, where P = Pressure, V = Volume, n = a constant (relating to the number of molecules in a given amount of matter), R = constant (for an IDEAL gas), and T = Temperature. All units would be S.I. units these days (Pascals, cubic metres, and Kelvin for the variables) so that calculations are kept relatively simple.
Now this equation will need a slightly different value of R when we are dealing with non-ideal gases, so if one uses a pure substance (well a pure gas) that constant will be fixed and not vary even ever so slightly under relatively similar conditions. Real life situations give close enough answers just using R - it is only scientists doing very precise experiments who would even consider different real values. Now, air is a mixture and contains vapours as well as gases. Vapours do not conform to the gas equations as, by definition, a vapour can be changed to another state by changing the pressure at any given temperature (an example is propane - in a cylinder it is liquid, but if you open the valve it changes to a vapour (most people erroneously say a gas!)
Water and carbon dioxide are both vapours in atmospheric air and, further, the water content can vary by a relatvely large amounts at different times. Now, to get back to the issue here. Atmospheric air will make not a jot of difference to the operation of a tractor tyre that you would ever notice.
Using nitrogen would prevent the innertube, if fitted, from getting wet on the inside - because none should ever be pumped into it from a "wet" compressed air supply as nitrogen bottles should only contain pure nitrogen. Confused? Forget it all and use plain air. It is cheap and readily available. Leave nitrogen to the extremists, who also want rediculous specific power oututs from their engines, or those that are stupid(?) enough to want to emulate them and are prepared to pay relatively exorbitant amounts of their readily available cash for a tiny improvement that they will not even notice or be able to measure. Would you check the temperature and change the tyre pressures by a few pascals every time the temperature changed by the odd fraction of a degree. Would you consider if your tractor tyres are running on dry or wet soil? I think not.
RAB
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