Drastically reduce your oxygen cost for E tanks of oxygen. Buy your oxygen in large H or M tanks and fill your small E or D tanks directly from the large tank. Device typically pays for itself after about 20 E tanks have been filled. Simple, safe and easy to use.One side is a CGA-870 (medical oxygen fitting), with the standard 2 pins to attach a standard E or D tank and the other side is CGA-870 (standard oxygen fitting) that screws directly onto your large tank in place of your regulator. The unit allows you to transfill between equipment that has a CGA-870 (medical) fitting and a standard CGA-870 fitting. All connections are air-tight and the small tank side include a high quality brass/viton tank seal washer for repeated use.