or more than the cleansing sweep pressure therefore the capital cost and the operating cost would be several times the respective costs for a fan and compressor combined as described above in loops a and c the invention must use purification to maintain the desirable properties of helium and remove impurities that would harm the coating or substrate the invention also can use psa tsa membrane catalytic oxidation and cryogenic separation to remove impurities however any impurity in the helium can become a process limiting agent for csf therefore depending on the application the purification system must remove nitrogen oxygen water carbon monoxide carbon dioxide hydrogen and possibly light hydrocarbons the majority of impurities will occur when parts enter and leave the csf chamber some csf applications will process larger parts that are placed in the csf chamber one at a time as the parts are placed in the csf chamber helium will escape and air will enter into the chamber the same will occur when the parts are removed from the csf chamber an evacuation of the chamber before and after the parts are placed in the enclosure would minimize the amount of helium lost and the amount of air that enters into the enclosure even though an evacuation of the enclosure would improve helium recovery the time cycle extension and capital required for vacuum capable equipment may not justify the effort higher molecular weight impurities will significantly lower the sonic velocity of helium higher sonic velocities than heavier gases is one of helium 39 s several.