The rate of “free air” flow has to do with the air compressor. There are several ways to measure the capabilities of an air compressor, CFM, SCFM (Standard cubic feet per minute), FAD (Free Air Delivery). CFM is the imperial method of describing the volume flow rate of compressed air. It must be defined further to take account of pressure, temperature and relative humidity. SCFM is the flow in CFM measured at some reference point but converted back to standard air conditions (Standard Reference Atmosphere) 14.7 psia, 60°F. Free Air (Free Air Delivery FAD) is the actual quantity of compressed air converted back to the inlet conditions of the compressor. The units for FAD are CFM in the imperial system and l/min in the SI system. The units are in general measured according the ambient inlet standard conditions. Table 1305.2 requires the Nitrogen flow rate to be based on the free air delivery rating. The other gases in the Table are to be rated by CFM or SCFM. An example of rating a compressor by FAD. As an additional bit of information, nitrogen used in medical settings is more apt to be used for powering pneumatic tools than any therapeutic application.