Orifice Flange Unions (OFU) contain a differential producing Paddle Type Orifice Plate (PTOP sold separately) within. On either side of the orifice plate are tap holes drilled into the orifice flanges which measure the difference between the upstream and downstream pressure. Depending upon the style of orifice plate used, Orifice Flange Unions (OFU) come in two different facings: Raised Face and Ring Type Joint and come in several different connection styles.