Position: You can ensure that your weighing controller performs accurately by choosing components suited to your application and taking steps to control environmental and other forces acting on the batching system. This article discusses the factors that can affect the weighing controller system's accuracy and provides advice on selecting, installing, and operating the system to handle these factors.
Weighing to measure dry bulk material quantities and flowrates has several benefits: Unlike volumetric measurement, weighing can measure a material quantity without correction factors for the material's bulk density. Weighing doesn't require contact with the material, making it suitable for measuring corrosive materials and operating in corrosive environments. It's also a widely accepted means of quantifying packaged products for sale.
A weighing controller system can take any of several forms but typically includes one or more load cells that support (or suspend) a weigh vessel or platform, a junction box, and a weight controller. When a load is applied to the weigh vessel or platform, a portion of the load is transmitted to each load cell. Each cell sends an electrical signal proportional to the load it senses via a cable to the junction box. The load cell signals are summed in the junction box and sent via one larger cable to a weight controller, which converts the summed signal to a weight reading. This weight reading's accuracy can be affected by the system components' quality and the system's installation and operation in your environment.
To help you choose high-quality weighing controller system components, take advantage of the expertise of weighing equipment suppliers. An important part of this selection process is determining how the system will be installed and what factors can affect its operation once it's up and running in your process line. Consider how these five factors can affect your system's weighing accuracy:
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Load cell accuracy.
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Load factors.
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Environmental forces.
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Interference with signal transmission.
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Instrumentation and control.
What accuracy you can expect
Let's compute the worst-case total weighing error for an example weighing controller system to see how the system's components affect accuracy. We'll consider the worst-case total error for only the load cells and weight controller in a gain-in-weight batching system.
This system weighs 400 pounds of material in a 100-pound weigh hopper, requiring the load cells to support a total of at least 500 pounds. The hopper is suspended from three load cells, each with a rated capacity of 200 pounds, yielding a total capacity of 600 pounds. A 20°F temperature change also occurs between seasonal system calibrations.
In a batching process, we're concerned only with the weighing controller system components' specifications for nonlinearity, nonrepeatability, and temperature effect on output, and nonlinearity error isn't a concern because batching is a sequence of partial weighments.
As a result, the formula for computing the system's worst-case total error is:
[(IT)2 + (LN)2 + (LT)2]1/2
where IT is the instrument's (weight controller's) temperature effect on output (0.000027 x 600 pounds x 20°F), LN is the load cell's nonrepeatability (0.0001 x 600 pounds), and LT is the load cell's temperature effect on output (0.000008 x 500 pounds x 20°F). In this example, the worst-case total error is 0.34 pounds. Remember that this is a worst-case number; a correctly installed weighing controller system will yield a lower error.
A final caution
Achieving this kind of weighing accuracy means considering many factors affect the weighing controller system, both mechanical and operational, that can affect your weighing controller system. Choosing quality components especially suited to your application will go a long way toward ensuring that your system provides the accuracy you need. These components typically have impressive worst-case specifications, and their actual performance is usually better than the specification. As a general rule, select load cells and a weighing controller with accuracies 10 times better than your desired system accuracy. And pay close attention to how you install and operate the weighing system to prevent mechanical forces and electrical noise from reducing your weighing accuracy.
By Ted Kopczynski and Dave Ness, Hardy Weinging Instruments
This article is provided by Hardy Instruments, www.hardyinst.com.
