body is driven by a spool valve plunger 22 pivotally mounted on a rotating crank 24 by an eccentric pin 26 preferably the pivot pin 12 and the eccentric pin 26 are mounted perpendicular to the plate 14 and parallel to one another however to compensate for any imperfections in the alignment of the two pins the eccentric pin 26 has a double sleeve not shown with an inner layer made of any convenient flexible material such as an elastomer a motive source as for example an electric motor 28 energized by a power line 30 provides the rotational drive for the crank 24 the electric motor 28 may be controlled by a manual switch not shown in power line 30 or may have an automatic control designed for a particular operation automatic control circuitry for a diluting system is disclosed in u s patent application ser no 557 681 incorporated herein a first conduit 32 connects a first chamber 120 fig6 to the spool valve body 18 and through said spool valve body in a first position to a supply conduit 34 shown disposed in a fluid supply vessel 36 and through said valve body in a second position to an output conduit 38 connected to a probe conduit 40 the probe conduit is shown in solid lines inserted in a bracket 42 for storage when not in use the bracket 42 is shown mounted on pivot pin 12 but it could of course be mounted in any convenient location or even deleted the probe conduit is shown at 40 39 removed from bracket 42 ready for use a second conduit.