Weighing Method

The weighing method is the most reliable and accurate in the measurement of the powder flow rate
in the bulk solids flow.1 The hopper scale (weigh hopper) and the belt scale (belt weigher or conveyor
scale) are the typical flowmeters of this kind.
The hopper scale receives powder in the hopper, weighs it, and then discharges. The frequency
of receiving and discharging is 100–400 cycles per hour, which can be changed according to the
process requirement. A combination of high speed in the early stage of the feeding and low speed
in the final stage allows a short cycle time with high accuracy.6 A hopper scale which weighs the
scale hopper after the feeding is perfectly stopped is called the net-weight mode hopper scale. For
continuous powder feeding, the hopper scale is operated in the loss-in-weight mode, in which the
powder is added before the hopper becomes empty and the discharge flow is kept continuous. The
powder adhered on the hopper wall does not cause a weigh error of the hopper scale in this mode.
The belt scale is, however, more suitable than the hopper scale for the continuous feeding of
powder, where the flow rate is obtained as a product of the weight of powder on the belt of unit