Volumetric feeders calibration

When solution must be pumped, consideration should be given to use of holding tanks between the dry feed system and feed pumps, and the solution water supply should be controlled to prevent excessive dilution. The dry feeders may be started and stopped by tank level probes. Variable-control metering pumps can then transfer the alum stock solution to the point of application without further dilution. Means should be provided for calibration of the chemical feeders. Volumetric feeders may be mounted on platform scales. Belt feeders should include a sample chute and box to catch samples for checking actual delivery with set delivery. Gravimetric feeders are usually furnished with totalizers only. Remote instrumentation is frequently used with gravimetric equipment, but seldom used with volumetric equipment. 

Bulk Density - Bulk density is the mass per unit volume of a fertilizer and includes the voids between particles. It is of interest in bag/package sizing, in calibrating volumetric feeders or applicators, and when considering the capacity of storage bins and transport vehicles. Bulk density is the most commonly used density value for fertilizers. 

Continuous compounding processes require continuous feeding, this being achieved by the use of volmnetric or gravimetric (loss-in-weight) feeder systems. Gravimetric systems are more accurate but are more expensive. Volumetric feeders have to be calibrated prior to use in order to ensure that if process conditions are altered, the correct feed composition can be maintained. Normally twin-screw extruders are starve fed. The main-screw and feeder-screw speeds are adjusted to obtain optimum quality. 

Gravimetric and volumetric feeders need periodic calibration. Even feeders with direct weight readout must be calibrated to ensure accurate feed is continuously maintained. The feeder, age, maintenance history, and operational experience influence the frequency of calibration. Volumetric feeders must be calibrated when a chemical batch is changed. This is because of a possible change in the consis-... 

Dry chemical feeders are calibrated by weighing the chemical delivered from the feeder for a measured time period. An example of a calibration for a volumetric feeder is described in Example 5-1. 

Volumetric feeders may need to be calibrated when new chemical deliveries are received. 

Screw feeders are essentially volumetric devices, which in general provide uniform feed rates that can be calibrated to meet specific requirements. There are various reasons for linking screw feeders with weighing devices to offer feed control on a gravimetric basis. 

Dry chemical feeders, whether volumetric or gravimetric, require similar devices for calibration. A timer, a container to collect the chemical, and a scale are needed. The timer should count in seconds. The container must be large enough to collect the chemical over the needed duration and fit in the feeder or the chemical feed throat. 

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