The UIM 4F has four paths as standard. It has been tested to meet the international standards for custody transfer metering including AGA-9 and OIMLR137 (class 0.5). Integration of the flow across the paths provides accurate measurement even with varying flow profiles caused by changes in flow velocity, gas composition and pressure or upstream pipe configuration. Fast response is maintained by the simultaneous transmission on more than one path using Broadband Continuous Wave processing. The measured flow profile provides a diagnostic tool. Should one path fail, the Path Substitution Algorithm uses historical flow data to continue reliable flow measurements until the path recovers or remedial action is taken.