Joules, you just beat me by a minute. Yes you just remove the Oxygen and replace that with more (20%) Nitrogen. But the benefit is also that it is completely dry as where if you use compressed air there is usually some moisture in it. Thats why they use pure Nitrogen in Aircraft Tires. If you want a bottle for the track day just get an old Scuba diving tank or talk to your local Fire Brigade (3000psi of compressed air) and it should be good for hundreds of Tires and will be much cheaper than a Nitrogen tank I suppose ;-)
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