Abstract: An orifice fitting for measuring the flow of gas through a pipeline in which the orifice fitting has a body including a pair of opposing body plates that define a body chamber. Passing through both body plates is an aligned flow passage for attachment to a gas pipeline. One of the body plates includes an access opening that is selectively covered and sealed by a hatch. A carrier having a carrier chamber is movably positioned within the body chamber so that the carrier chamber can be selectively aligned with the flow passage or the access opening. Removably positioned within the carrier chamber is an orifice disk having an orifice of a predetermined size through which gas can flow when the carrier chamber is aligned with the flow passage. The carrier chamber is sealed from the body chamber by seal rings positioned on opposing sides of the carrier and biased against the body plates.