sentences:  Cascade filling is often used for partial pressure blending of breathing gas mixtures for diving, to economize on the relatively expensive oxygen, for nitrox, and the even more expensive helium in trimix or heliox mixtures.Cascade storage is used at compressed natural gas (CNG) fueling stations.Typically three CNG tanks will be used, and a vehicle will first be fueled from one of them, which will result an incomplete fill, perhaps to 2000 PSI for a 3000 PSI tank.