the turns of the sand screen spring 351 the formation ceases to erode and only formation fluid is passed through the sand screen spring when the formation test is completed and well bore fluid pressure again acts on the inner end of the piston shaft 349 the pressure forces on the ends of the piston shaft 349 will again be balanced allowing the bias spring 353 which was compressed by movement of the piston shaft 349 inwardly to move to its relaxed position returning the piston shaft 349 to its original position as the piston shaft 349 returns toward its original or initial position the turns of the sand screen spring 351 are wiped by the spiral flutes 361 to clean off the sand particles it will be convenient to describe the operation of the tool 11 with reference to fig2 which schematically presents certain information that is produced by a strip chart recorder and is observed by the operator at the aboveground equipment location during operation of the tool in fig2 the trace a represents hydraulic pressure sensed by hydraulic fluid pressure sensor 179 on a scale of 0 5 000 p s i the trace b represents the pulses produced each time the cam actuator 171 traverses a cam notch 169 in the embodiment shown each pulse represents a two c c volume increment of mini chamber 159 volume the digital printout column c shows in p s i at predetermined time intervals typically 5 seconds the pressure sensed by formation fluid pressure sensor 181 trace d represents the pressure sensed by the formation fluid or hydrostatic pressure sensor 181 on.