As described in German patent 2,900,851 filed 11 Jan. 1979 by H. G. Volz et al a standard apparatus for dosing a liquid has a housing formed with an inlet connected to a supply of the liquid, an outlet, a filling chamber, and a bypass chamber having an outlet side from which the outlet opens and an inlet side connected to the inlet.  A pumping piston subdivides the filling chamber into an inlet compartment into which the inlet opens and an outlet compartment.  A valve element in the housing can move between a filling position blocking the outlet and connecting the inlet through the bypass chamber with the outlet compartment and a feed position exposing the outlet and blocking flow between the inlet and the outlet compartment through the bypass compartment. 
Such a dosing apparatus is operated by alternately retracting the piston through a back stroke while the valve element is in the filling position to draw the liquid into the outlet compartment from the inlet through the bypass compartment and advancing the piston through a predetermined advance stroke while the valve element is in the feed position to eject the drawn-in liquid from the outlet.  In this manner it is possible to control the dose size very accurately. 
A major disadvantage of such a system is that pockets of little or no flow are formed in the housing.  In particular the inlet compartment often is quite stagnant so that, although the flow through the housing is considerable, the material in this inlet compartment can be quite stale.  When the system is used, as is common, with a foodstuff, this stale material can contaminate the fresh material being pumped with bacteria.  It is also possible for the rear end of the outlet compartment to constitute a dead pocket holding stale material.