Loss in weight feeders consistent of a screw feeder that doses powder from a hopper. The hopper is periodically refilled by a much larger storage silo. The hopper is on weigh cells that continuously measure the weight of the powder in the hopper. The controller will determine how much weight it loses in a given amount of time. This loss in weight is interpreted as a flow rate.

The flow rate is continuously calculated while in gravimetric mode, which is when the hopper is not being filled. When it is being filled, it enters into volumetric mode, which is when the feeder has to make assumptions what the flow rate is. It has to make an assumption because there is a net gain in weight as it fills. The controller will make an educated guess as to what the screw feed needs to be. After the hopper is refilled, it enters back into gravimetric mode where the flow rate is continuously calculated and the screw speed adjusted accordingly.

This theory is sound, but data is not always so perfect in real life. When conditions are perfect, there is still some variation of the actual flow rate.

And when conditions are not perfect, the loss in weight feeder can only perform as it’s programmed. If there is a suction pulling the powder through the screw, it can cause the motor to slow down to an actual stand still. If there’s enough of a suction then powder can flood around the screw resulting in a flow rate even though the screw speed is 0% (stopped).

And when conditions turn really sour, the loss in weight feeder can give misinformation. Small increases in hopper weight can throw the feeder into a mode that is not gravimetric mode where it doesn’t actually know what the feed rate is.

Watch the video for a much better explanation :)