For safety reasons, which gas should not be used for testing refrigerant systems?

The use of oxygen for testing refrigerant systems is unsafe due to its reactive nature. When oxygen is introduced into a refrigerant system, especially at high pressures, it can lead to combustion or explosions. This risk is exacerbated in systems that may have residual refrigerants or flammable materials. In contrast, nitrogen is an inert gas commonly used for pressure testing systems without the risk of combustion. Carbon dioxide, while also not typically used for this purpose, is less reactive than oxygen but still poses risks in terms of pressure changes. Freon is a refrigerant itself and would not be appropriate for testing purposes as it can contribute to environmental harm and is not suitable for purging or pressurizing a system. Overall, the critical factor in determining the safest option is that oxygen's involvement in testing could significantly compromise safety and integrity.