Loss-in-weight feeders on the overall process accuracy

Loss In Weight Feeder require periodic refill from an external device such as a screw conveyor, bin activator, pneumatic conveyor, slide gate valve, or rotary valve. Capacity requirements of the refill device and desirable refill frequencies are often overlooked by process engineers, yet they can have a major impact on the overall process accuracy.

A loss in weight feeder consists of three components: a scale with a dedicated controller to weigh material; a feed mechanism (screw, vibrating tray, etc.) with a variable-speed drive to introduce material into the process; and an integral weigh hopper for material storage. As the name suggests, the unit measures a loss in weight over a certain period of time and adjusts the feed mechanism output accordingly. In reality, of course, today’s loss-in-weight feeders do not sample weight every hour but approximately every 50 milliseconds to achieve the precise second-by-second control required by modern processes.

A typical accuracy statement for a loss-in-weight feeder specifies repeatability as a percentage of the sample-size at a given statistical confidence level. For example, a repeatability of 0.5 percent of sample size at 2 sigma means that if a sufficient number of consecutive 1-min. samples are taken at a 6000 lb/hr feed rate, then 95 percent of the samples will weigh between 99.5 and 100.5 lb (assuming perfect linearity). Implicit in this statement, however is an assumption about the refill rate. Compared to the feeding time, the refill time needs to be as short as possible to maximize the percentage of time the feeder is under gravimetric control. Although higher percentages are preferable, the following example will assume that a loss-in-weight feeder needs to be under gravimetric control for at least 90 percent of its cycle time for the accuracy expectation to be valid. Remember that the time in volumetric mode is defined as refill time plus scale stabilization time.