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One reliable method for inventory management of cement, fly ash, slag, and aggregate is the reciprocating bob or "Yo-Yo." These systems give the concrete producer control not available in the past.

Reciprocating bob systems use a weighted bob that is mechanically lowered by a cable into a silo or bin. A sensor records the number of times wheel slots inside a silo- or bin-mounted unit pass through a beam breaker to calculate the bob travel distance.

Bob systems are not susceptible to problems such as dust, high temperatures, fumes, foam, and angle of material repose. Most system manufacturers offer inventory management systems that instantly report material levels on multiple silos and alert the batch operator when a silo is low or can hold another load. Once the system detects a low level, the controller can send an alarmed output to the batch room computer. Or, it can access a modem in order to dial a pager or fax machine and place an order directly to the shipper.

Bob systems are subject to a few limitations. First, a bob measures the position of a single point in the silo, so it does not compensate for material density. Another problem with pinpoint measurement is an inability to sense ratholing or bridging at the load-out point. Third, cone-shaped silo bottoms require calculus equations for volume measurement.

Generally, bob measurement systems are subject to two types of mechanical problems. On older systems, the pulley cable may tangle and halt wheel travel. In addition, a weighted bob buried in material can result in inaccurate measurements, unit failure, broken cable, and lost bobs. The article includes installation guidelines that help prevent these problems.

The article includes a list of questions the producer should ask in choosing the right reciprocating bob manufacturer. It also has a sidebar article about strain-gauge technology for material inventory for silos at standalone plants.