Purge Your Hoses
When connecting to a tank… purge your hose by cracking the center connection at the manifold for a second.
When connecting to a system, crack each connector for a second and then tighten. (Unless you are using ball valves or low loss of the same refrigerant type).
Low loss brings up a different subject. If you are using them, you need to make sure not to switch between refrigerants without recovering all the refrigerant from your gauges .
Both air and incorrect refrigerant are not acceptable to add to the system… even in small quantities.