Abstract: A multi-purpose controller for controlling the flow rate of material being transported in a suitable conduit for the particular material being transported includes a means for calculating an index of the measured flow rate over a selected period of time. This calculated index, such as the arithmetic mean of a number of sampled values of the flow rate, is then compared with the reference or set point input to the controller. The resulting error signal is used to calculate a correction to the previously determined control signal, which is then used as an updated control signal at the input of the flow rate actuator. The repetitive operations of calculating an up-dated flow rate index, comparing the index with the reference input, calculating an up-dated control signal, and so forth is time-coordinated by means of a wait time interval that is related to the inherent time delay of the material time in transit from the flow rate modulator to the flow rate measurement sensor location.