The Compressed Gas Association (CGA) uses CGA numbers to identify connections for specific gas service. All hex nuts used in medical gas applications are manufactured to CGA standards as defined in CGA E-1 “Standard Connections for Regulator Outlets, Torches and Fitted Hose for Welding and Cutting Equipment”. Many common fittings are referred to as A-Size, B-Size, C-Size and D-Size. This letter identification system, developed by the International Acetylene Association (IAA), predates the CGA numbering system and is still used today. CGA limits pressure ratings on E-1 connections to a standard maximum working pressure of 200 PSIG (1380 kPa) for safety.