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Purging is "washing" the tire of oxygen with a fill and re-empty of nitrogen before putting the main charge in, to try to wash that 21% oxygen out of the initial air in the tire. I forget the exact numbers but nitrogen filling without purging puts the oxygen percentage down to about 5% and still significantly reduced the oxidation of the rubber in tires. In their testing they felt purging was good practice, but not essential.
Topping up the tires with air introduced significantly more oxygen (than not purging), although the study warned that voicing this fact could have customers inclined to drive on partially deflated tires rather than get them filled immediately with air. The actual material cost of nitrogen fill is so low that tires could be purged and re-filled for pennies at the next opportunity. The study had a brief mention of the costs of a dedicated air compressor vs a nitrogen tank and regulator, with the costs being very close.
The study did a comparison of tires used in the Phoenix AZ area with tires baked and pressurized in an oven to compare test methods. The main effects of oxidation is hardening and cracking of the rubber, increased leakage, reduced load capacity, reduced traction and increased incidence of catastrophic failure.
Sorry for getting off topic here, just an mind spill...
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