Checkweighers are weighing systems used to verify that the weight of a product is within prescribed limits. They weigh items that move along a production line, classify items by weight, and then sort or eject items by weight class. Checkweighers consist of a feeder, scale, line divider, discharge unit, and computerized control system. The quality and capabilities of checkweigher components varies by manufacturer, application, model, and cost. Most checkweighers that are designed for food, beverage and pharmaceutical processing applications are washdown capable and able to withstand extremes of temperature and pressure. In some manufacturing environments, checkweighers include an integral printer or provide an interface to a programmable logic controller (PLC) or supervisory control and data acquisition system (SCADA).
Performance specifications for checkweighers include linearity, mean error, repeatability, and accuracy. Checkweighing linearity measures the closeness in weight between an actual package and a test package. The difference or error between these two measurements indicates the degree of linearity. Mean error is the absolute value of the difference between an item’s actual weight and the average weight as reported. Checkweighing repeatability is measured with standard deviation in order to describe the variance in weight from weighing the same mass multiple times. As a rule, checkweighers with greater repeatability or precision have lower standard deviations. Accuracy is a measure of checkweigher uncertainty. Because weigh cells vary in terms of settle times, most manufacturers offer products with a range of accuracies. Higher accuracy systems are more expensive, but may provide cost savings by reducing rework and overfilling.
Checkweighers are suitable for a variety of applications and industries. Some products are used to identify underweight or overweight packages. Others are used to check process performance and measure production line efficiency. In the printing industry, checkweighers can be used to capture production totals by day, shift, batch, or production run. The food, beverage, and pharmaceutical processing industries use checkweighers to capture information such as out-of-tolerance conditions for regulatory agencies. In the United States, checkweigher suppliers often indicate whether products meet Food and Drug Administration (FDA) or U.S. Department of Agriculture (USDA) reporting standards. Handbook 133 from the National Institute of Standards and Technology (NIST) establishes content specifications for packages and defines standards such as maximum allowable variation (MAV). NIST Handbook 44 establishes standards for automatic weighing equipment such as checkweighers.
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