Abstract: A gas separation system for extraction of gas from pressurized fluid has a gas sampling interface and gas circulation tubing to pass sampled gas to a gas sensor to determine the character or a property of the gas. The gas sampling interface includes a semi-permeable membrane in the form of tubing having an inner core volume with an internal support that provides multiple supporting surfaces to prevent collapse of the tubing wall against external pressure and provides interstitial spaces for the flow of gas. Internal supports may include a coil of spring-like configuration fitted coaxially within the tubing; or multiple filaments that form a longitudinal bundle within the core of the tubing, or both. An outer tubular conduit surrounding the semi-permeable membrane tubing provides a flow path for the external fluid to increase the rate of diffusion of dissolved gases through the semi-permeable membrane.