Volumetric feeder
A volumetric feeder, on the other hand, does this based on volume in speed.
A volumetric feeder is a dosing system that supplies the production system with a certain volume of material in a set timeframe, based on the so-called displacement principle. In contrast to a gravimetric feeder, the dosing speed of a volumetric feeder should be selected manually, which may vary based on the nature of the raw input materials. For example, masterbatch requires a different speed compared to dosing of liquid or powder input material.
For smaller production processes and companies who are looking to save on dosing equipment, a volumetric feeder is favoured over a gravimetric feeder. Yet, a gravimetric feeder enables one to save heavily on the input material by a more precise feeding process. This means that a gravimetric feeding system usually provide for a higher return of investment (ROI) on a long-term basis.  
Volumetric feeder

Advantages
Relatively low priced dosing system
Accurate dosing cylinder
Stepper motor speed starting from 0.1 up to 200 rpm
Disadvantages
Manual recalibration required
Limited control over your product quality
No monitoring and reporting options
Cannot automatically compensate for fluctuations in density
Sensitive to material build-up
No automatic detection of material supply interruption