current to drive the relays and inverts the voltage from its input to its output in other words 5 volts at the input gives 0 volts at the output it should be remembered that actuation of the needle valve 82 can either close the valve fully open the valve or set the needle to some intermediate position depending upon the requirement of the software and the particular test being carried out also the position of the needle is monitored by the position transducer not shown incorporated in the actuator 110 and the resulting electrical signal is used by the computer together with a signal from the pressure transducer 44 or from the transducer 102 in order to calculate the correct setting of the needle in this embodiment the diagnostic system is designed to perform sixteen tests on the skid unit 10 the tests are by way of example the operation of the valves shown in fig5 will now be described in order to carry out test no 2 and test no 7 the needle valve actuator 110 is controlled to open the needle valve 82 by 50 the feed solenoid valve 76 is opened when 35 millibar pressure is reached as monitored by the pressure transducer 44 the ball valve actuator 112 is controlled to open the ball valve 86 flowrate is then monitored by the transducer 102 passing in reverse through the stream wafercheck valve 158 from the right hand side of the valve to the left hand side after 10 seconds the ball valve 86 is closed.