N2 is commonly used in diagnosing air conditioning system leaks.As it is inexpensive compared to popular refrigerants(not to mention doesnt destroy the o-zone hahaha)the reason air cannot be used is it is a non condensable gas and contains moisture that will contaminate the system.another use for N2 is inflating race car tires as the N2 has minimal expansion due to heat build up,as compared to air which has substantial thermo expansion.it is easily obtained from a local welding/gas supplier with no questions.so who do you want to be today?a.c. technician or amateur race car driver?hope this helps.