Abstract: The interface apparatus of the present invention includes a pneumatic flow control means for controlling the number of moles of a gas confined within a substantially constant volume of gas at a first pressure. A volume relay has a first chamber coupled to the control means by a pneumatic bus and a second chamber for coupling to a pneumatic transducer to be positionably controlled, the first chamber and the second chamber being in fluid flow isolation one from the other. The volume relay provides a second pressure at its second chamber which has a predetermined relationship to the first pressure. A substantially constant, confined volume is coupled to the flow control means and to the relay and the flow control means includes means for adjusting the rate of flow of gas from the substantially constant volume to a region of ambient pressure.