I was at an extrusion customer site last week and they were not sure whether the feed system was truly gravimetric or volumetric….
The system uses a scale to measure the material used when filling the hopper, but has a feed screw to meter the material to the feedthroat. The system can be set to a lbs/hr or RPM of the feed screw.
Since the system can adjust the screw speed to maintain a desired lbs/hr setpoint, it can be considered gravimetric. The precision of he setpoint is not high since it has to take intermittent weight measurements as the hopper needs more material. This rate of consumption is then reevaluated each time the hopper is refilled. As a result, the system is a true closed-loop gravimetric system with a relatively long lag time.