ISO 11439:2013 specifies minimum requirements for light-weight refillable gas cylinders intended only for the on-board storage of high pressure compressed natural gas as a fuel for automotive vehicles to which the cylinders are to be fixed. The service conditions do not cover external loadings that can arise from vehicle collisions, etc. ISO 11439:2013 covers cylinders of any seamless steel, seamless aluminium alloy or non-metallic material construction, using any design or method of manufacture suitable for the specified service conditions. It does not cover cylinders of stainless steel. ISO 11439:2013 uses 200 bar as a reference working pressure, but other working pressures can be used. Cylinders covered by ISO 11439:2013 are designated Type 1, Type 2, Type 3 and Type 4.