One method of purging a propane tank involves bleeding out the shipping air and then filling the tank with propane vapor instead of liquid propane. This process is repeated for several cycles. Another method involves using a vacuum pump to create a vacuum in the tank of about 26 inches of mercury, or 2 PSI absolute pressure, and then filling the tank with propane vapor to 1 atmosphere of pressure.
A new propane tank needs to be purged by a professional technician before it is used for the first time. This is the only time this needs to be done, unless older designs of tanks without back-check valves accidentally become filled with air. Purging eliminates moisture and impurities that could contaminate the propane or damage the tank. Propane is odorless and colorless, and a chemical odor is added that makes it smell bad. The moisture in a propane tank that is not properly purged oxidizes the chemical odor and neutralizes the smell. As a result, users are unable to detect dangerous leaks.
Once propane tanks are purged, they can be safely filled with liquid propane. They should only be filled to 80 percent capacity, because propane expands with temperature increases. Liquid propane forced into appliances creates a fire hazard and damages regulators.