chambers 62 64 and the fluid pressure will go undetected when withdrawal of a sample from the formation shown at 11 in fig2 is begun the drive piston 46 is typically positioned so that either the first 58 or the second 60 pumping piston is fully extended into its respective pumping chamber 62 or 64 withdrawal of a sample is begun by application of hydraulic pressure to the appropriate drive chamber 50 or 52 adjacent to the completely compressed pump chamber into which its pump piston 58 or 60 is fully extended whereupon the drive piston 46 moves and correspondingly displaces the pumping pistons 58 60 thereby increasing the volume of the fully compressed pumping chamber 62 or 64 the first 62 and second 64 pumping chambers are connected respectively to a first 72 and a second 74 inlet check valve both of which enable flow from the probe shown as 18 in fig2 into an inlet flowline 70 connected as shown at 18a to the probe 18 on the expansion stroke of the respective pumping chamber 62 or 64 the inlet flowline 70 is further connected to a precision pressure transducer 108 which is itself connected to the electronics unit 14 which enables substantially continuous measurement of the pressure in the flowline 70 the use of the pressure measurement made by the transducer 108 in the present invention will be further explained during the compression stroke corresponding to expansion of the opposing chamber 64 or 62 discharge from the first 62 or from the second 64 chamber is conducted respectively through a first 78 or second 80 discharge check valve to a discharge line 76 the discharge line 76 can be selectively hydraulically connected.