A gas separation system suited for extraction of gas dissolved in pressurized fluid. The system includes a semi-permeable membrane in the form of tubing having an inner core volume with an internal support present within the inner core volume. The internal support provides multiple cylindrical supporting surfaces which are presented to the inside tubing wall surface to prevent collapse of the tubing wall against external pressure. 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. The response time of the system may be enhanced by the presence of an outer tubular conduit surrounding the semi-permeable membrane tubing and providing a flow path for the external fluid to increase the rate of diffusion of dissolved gases through the semi-permeable membrane. Gas pressures may be a sensed, both total and partial, as well as the identity of respective sampled gases.