U.S. Air Force Senior Airman Nicholas McGinnis, 31st Logistics Readiness Squadron fixed facilities personnel, collects a sample of liquid oxygen, April 23, 2015, at Aviano Air Base, Italy. Every 90 days, petroleum, oil and lubricants Airmen inspect liquid oxygen tank locks for leaks and test oxygen's purity. (U.S. Air Force photo by Senior Airman Matthew Lotz/Released)