# Gravimetric and volumetric feeders: What are they? And what is the difference?

## Gravimetric and volumetric feeders: What are they? And what is the difference?

Feeders are auxiliary equipment that dispense a measured amount of an ingredient—a resin, a colorant, or an additive—into resin entering a processing machine.

The term “volumetric” refers to one way that a feeder measures and dispenses an ingredient. The other way is called “gravimetric.” Let’s briefly look at both:

Volumetric feeding. To use a volumetric feeder, the user must know the rate at which an ingredient must be dispensed into a process (e.g., 10 grams/60 sec.) and how much volume that weight represents. To set that rate in a volumetric feeder, the user turns on the feeder auger and begins “catching” samples in a container for a timed period (e.g., 60 seconds). After each catch, the user weighs the sample and then adjusts the auger speed, up or down, until the feeder delivers the correct amount of the ingredient into the sample container. The sampling process must be repeated each time that that there is a change in bulk density (e.g., a different material) or a change in the ratio of the ingredient in the process.

Gravimetric feeding. Gravimetric feeders represent a significant improvement over volumetric feeders. They utilize a computer and a load cell that continuously records the weight of the feeder and its contents.  When the user inputs a given dispensing rate, the computer automatically calculates the rate and begins releasing the ingredient into the process. Simultaneously, the load cell reports the loss-in-weight of the feeder and remaining material, enabling the computer to continuously calculate and control the amount of material being dispensed.

For the user, the beauty of the gravimetric design is in its improved accuracy and its ability to automatically calculate and dispense the correct weight of material. With this design, there is no need for sample catching and no worry about changes to materials or dispense rates. In a properly calibrated gravimetric feeder, you enter the dispense rate (weight/time) required and everything happens automatically.

This is why gravimetric feeding has largely displaced volumetric feeding as the preferred method used by processors.