on line 84 representative of the set pressure for the valve in operation at the initiation of a measurement cycle the controller 8 controls the air pressure within the airtight chamber 46 to increase so that the diaphragm expands in response to the diaphragm expansion the movable piston 34 and the force transducer 64 are raised upwardly thereby compressing the force transducer 64 between the movable piston 34 and the load nut 68 such motion is transmitted directly by the spindle extension 20 to the closure element not shown with the gradual increase of air pressure the diaphragm 36 provides a force ramp to the closure element in the preferred form of the invention the force ramp is linear with the force increasing uniformly with time other embodiments may use non linear form ramps in response to the force applied by way of the diaphragm the closure element is raised from the position where the valve seals the associated port in the pressure line and the core 79 of the transducer 76 is pushed upward by the flange 28 as a result the transducer 76 provides an output signal representative of the position of the valve closure element between a first limit position where the closure element seals the port and a second limit position where the port is open the controller 8 monitors the signal on line 76a from the position transducer 76 and detects the test time when the core 79 has been displaced by a predetermined distance corresponding to the lift of the closure element of the valve assembly from the sealing position at the time that this distance is reached by the core 79 the controller 8 obtains via.