A metering device that includes a housing defining a cylindrical chamber containing
a reciprocating shuttle. The two opposed end parts of the chamber are interconnected
by fluid flow ducts to a cylindrical valve bore. Contained with the valve is a
rotary valve rod driven rotationally by a stepping motor. The valve rod has part
cut away to form channels or recesses. In one rotational position of the valve
rod a flow path is established from an inlet to one end of the chamber and a separate
flow path is established from the other end of the chamber to an outlet. In an
alternate orientation of the valve rod a flow path is formed from the inlet to
the other end of the chamber and the first end of the chamber is connected to an outlet.
