Senior Airman Charles Medley, 36th Logistics Readiness Squadron cryogenics production supervisor, takes a sample of the liquid oxygen to ensure purity Nov. 7, 2017, at Andersen Air Force Base, Guam. Liquid oxygen must have a purity of 99.5 percent or higher to be used on aircraft. (U.S. Air Force photo/ Airman 1st Class Gerald R. Willis)