comprise the lower end of the gripping ring 54 radially outwardly as shown in fig5 b to bring shoulder 62 out of recess 92 to effect a complete release of the downhole tool 88 when an upward force is applied at the same time as the application of wellbore pressure those skilled in the art can see that the precharging of annular cavity 34 which acts on piston 42 allows a reference hydrostatic pressure to be trapped in annular passage 72 against the compressible fluid trapped in passage 34 when the downhole tool 88 is supported downhole this occurs because passage 70 is sealingly closed as illustrated by comparing fig2 b and 3 b as the recess surface 90 moves away from passage 70 and seals 102 and 104 effectively straddle passage 70 which is now fully covered by the outer body 48 with that reference pressure trapped which is generally a pressure close to the wellbore hydrostatic at the desired location for release from the downhole tool 88 applied pressure on the wellbore on the release piston 64 one end of which 108 is exposed to the trapped hydrostatic pressure in the annular passage 72 results in the release sequence just described it also moves the floating piston 42 and compresses the fluid in chamber 34 fig6 a and b illustrate that on the way up the hole annular passage 72 is still isolated from wellbore hydrostatic as passage 70 continues to be sealed.