Supplies a guide to the installation, selection, calibration and operation of diagonal path transit time gas ultrasonic flowmeters (USMs) for industrial gas applications. Only applies to devices where the entire gas stream flows through the body of the USM and the transducers are in contact with the fluid (wetted or invasive type). Includes full-bore and insertion type meters employing reflective or direct paths, and covers "dirty gases", i.e, gas that has been contaminated with solids and/or liquids in small quantities. Does not apply to clamp-on/externally mounted meters.