This document specifies the geometry and method of use (installation and operating conditions) of orifice plates when they are inserted in a conduit running full to determine the flow rate of the fluid flowing in the conduit.This document also provides background information for calculating the flow rate and is applicable in conjunction with the requirements given in ISO 5167-1.This document is applicable to primary devices having an orifice plate used with flange pressure tappings, or with corner pressure tappings, or with D and D/2 pressure tappings. Other pressure tappings such as “vena contracta” and pipe tappings are not covered by this document. This document is applicable only to a flow which remains subsonic throughout the measuring section and where the fluid can be considered as single phase. It is not applicable to the measurement of pulsating flow. It does not cover the use of orifice plates in pipe sizes less than 50 mm or more than 1 000 mm, or where the pipe Reynolds numbers are below 5 000.