pipeline contents from its cylinder at this stage the return valve 50 is closed and the inlet valve 40 is opened the controller 72 then releases the pressure at the top of duplex cylinder 18 34 and pressurizes the chamber 26 34 at the top of duplex cylinder 19 34 this reverses the process so that the piston 34 is drawn to the top of cylinder 36 thereby inducing pipeline liquid to enter the probe at inlet 4 34 and pass through the sampling tubes 5 34 and 6 34 to recharge the working cylinder with fluid simultaneously the inlet probe 3 34 is retracted across the diameter of the pipeline 1 34 by the movement of hydraulic fluid between the duplex cylinders via the working cylinder 30 34 the controller 72 continues to operate the two duplex cylinders and the inlet and return valves to cause liquid to be circulated from the inlet 4 34 on the pipeline probe through the apparatus to be returned to the pipeline via the return tube 9 34 with each cycle the inlet probe 3 34 is traversed across the diameter of the pipeline in such a way that its movement is synchronized with the movement of the working piston 34 having purged the apparatus of residual liquid from the previous sample the controller 72 with the pump cylinder 36 fully charged with liquid from the pipeline 1 34 closes the inlet and return valves 40 and 50 and opens the delivery valve 60 the controller then operates the duplex cylinder 18 34 so that the working piston 34 discharges the contents of the pump cylinder 36 through the sampling tube.