DISS Nitrogen 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 Nitrogen. 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." DISS nitrogen fittings are also referred to as DISS 1120.