Abstract: Disclosed herein is a dual chamber orifice fitting including a first chamber maintained in fluid communication with a pipeline and a second chamber maintained in fluid communication with the first chamber. The fitting includes sealing member formed as an eccentric plug member that is selectively rotatable from a first position wherein the sealing member seals the first chamber from the second chamber, and a second position wherein the sealing member permits the first chamber to be placed in fluid communication with the second chamber. The sealing member includes a curved sealing surface having a pocket recessed therein for containing a non-metallic insert seat. The non-metallic insert seat sealingly engages a seat plate when the sealing member is in the first position. A retainer plate retains the insert seat in the recessed pocket.