Abstract: A mixing apparatus utilizes piston burettes driven by stepping motors as metering pumps to deliver component liquids to be mixed into a liquid stream in a selectable constant mixing ratio. A computer-based automatic control system is used to control the regulated take up of the respective components by the burettes and the delivery of them to a mixing device. The computer calculates the rates of stepping of the respective motors so that each component is delivered over the same period of time and at a respective uniform rate. The computer also controls associated switching valves via an interface device and may be a desk top machine with keyboard or pre-programmed data input.