It needs to be “oxygen clean”—which means free from contaminants that can be the fuel for an oxygen-fed fire. Cleanliness on surfaces is measured by milligrams (mg) of hydrocarbon per square foot of surface area (mg/ft2). In the U.S., DOT (in RRC 901c) requires that the cleanliness of cylinders used in oxygen service be proven to be less than 2.5 mg/ft2, which is a level considerably below what can be seen by the naked eye. For this reason, a visual inspection after cleaning is not enough. A test must be performed to verify that the required cleanliness level has been achieved. In CFR49 173.302(b), DOT specifies an approved cleaning method and a standard test to determine the cleanliness of cylinders.