Pressure test using oxygen free nitrogen to find leaks in the system.
When the vehicle air conditioning system is found to be empty i.e. no pressure, then a high pressure test using oxygen free nitrogen is carried out. The system is filled with Oxygen Free Nitrogen to 10 bar and left for 10-20 minutes. If there is a drop in pressure then a leak in one of the system components can be located. This is done by sound i.e escaping gas, leak detecting liquid on the joints i.e bubbles forming.
Refrigerants can be re-charged into the system with a dye, which will be visible under ultra-violet light. Next time the vehicle is serviced any leaks can be detected.