Abstract: A portable diagnostic system for testing gas pressure reduction equipment includes a computer arrangement 16 interfaced to an electro-pneumatic test unit 12 which provides test pressures for and receives information on displacement from the pressure reduction equipment. Unit 12 includes a number of valves including three solenoid on/off valves, a needle valve and an on/off ball valve. The position of the valve member of the needle valve is sensed by a position transducer. Displacement transducers 40 and 42 connected to the reduction equipment sense slamshut valve and regulator displacements pressure transducer 44 and a differential pressure transducer 102 are each connected to the equipment. Signals from the transducers are fed to a data acquisition system 210 which also drives the solenoid valves and the needle and ball valves, all under the control of the PC16 to give an indication for example of how far the valves are open in the regulating equipment (ie from the valve seat).