Diameter Indexing Safety System Style Female Fittings. The Compressed Gas Association (CGA) developed the Diameter-Index Safety system (DISS) to establish a standard for non-interchangeable, removable connections for use with Medical Gases, Instrument Air, Vacuum (Suction), and WAGD (Waste Anesthetic Gas Disposal). Non-interchangeable indexing is achieved by a series of increasing and decreasing diameters in the components of the connections. These specific diameters act in key-like fashion, so that fittings within the gas service "family" will connect only with their own "family members." In place of diameter indexing, Oxygen (DISS 1240) has been assigned the long established 9/16"-18 thread connections as its safety standards.