Senior Airman David McMillen, 28th Civil Engineer Squadron heating, ventilation, and air conditioning technician, monitors the amount of refrigerant entering a chiller during maintenance at the base air traffic control tower on Ellsworth Air Force Base, S.D., June 28, 2012. By adding refrigerant, the Airmen are preventing the chiller from possibly freezing up and costing thousands of dollars to replace. (U.S. Air Force photo by Airman 1st Class Kate Thornton/Released)