is held in position by the shear pin 48 when fluid is required to be filtered such as on pulling out the tool 10 from the well bore 42 a ball 52 is dropped from the surface into the axial bore 14 ball 52 travels under fluid pressure to the seat 46 where it blocks the passage of fluid through the bore 14 pressure then builds up behind the ball sufficient to shear the pin 48 and move the seat 46 downwards the seat 46 will fall to the stop 50 whereupon fluid within the bore can now flow through port 32 to outlet 34 and fill the seal 36 seal 36 consequently expands by inflation to fill the annulus 38 and prevent fluid flow down the outside of the tool 10 between the sleeve 20 and the casing 40 the fluid flow to the seal 36 is regulated by a check valve 54 located in the port 32 to prevent over inflation of the seal 36 seal 36 now engages the casing 40 as shown in the right hand side of fig1 seal 36 has a surface which is suitable for continuous contact to the casing 40 while the tool is moved within the casing 40 this surface is typically a roughened rubber surface such as knobbles which reduce the surface contact area without reducing the quantity of fluid.