ISO 17089-1:2010 specifies requirements and recommendations for ultrasonic gas flowmeters (USMs), which utilize the transit time of acoustic signals to measure the flow of single phase homogenous gases in closed conduits. ISO 17089-1:2010 applies to transit time ultrasonic gas flowmeters used for custody transfer and allocation metering, such as full-bore, reduced-area, high-pressure, and low-pressure meters or any combination of these. There are no limits on the minimum or maximum sizes of the meter. ISO 17089-1:2010 can be applied to the measurement of almost any type of gas, such as air, natural gas, and ethane. Included are flow measurement performance requirements for meters of two accuracy classes suitable for applications such as custody transfer and allocation measurement. ISO 17089-1:2010 specifies construction, performance, calibration, and output characteristics of ultrasonic meters for gas flow measurement and deals with installation conditions.