Miska, C. R. Check all SCHE Supply Purge Check Valves to Prevent Backflow from SCHE into Helium Supply, report, September 3, 2000; (https://digital.library.unt.edu/ark:/67531/metadc737583/m1/1/: accessed December 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .