This part of ISO 17089 specifies requirements and recommendations for ultrasonic gas meters (USMs), which utilize acoustic signals to measure the flow in the gaseous phase in closed conduits. This part of ISO 17089 is applicable to transit time USMs and is focused towards industrial flow measurement. Included are meters comprising meter bodies as well as meters with field-mounted transducers. There are no limits on the size of the meter. It can be applied to the measurement of almost any type of gas; such as but not limited to air, hydrocarbon gases, and steam. This part of ISO 17089 specifies performance, calibration (when required), and output characteristics of USMs for gas flow measurement and deals with installation conditions. NOTE It is possible that national or other regulations apply which can be more stringent than those in this part of ISO 17089.