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This article is about the affect of the load cell to the weighing systems.
Selecting a top-quality load cell for your weighing controllers is the first step in obtaining weighing accuracy. The load cell (also called a load sensor or transducer) is a piece of machined metal that bends with the load's mechanical force and converts the mechanical force into an electrical signal. The bend doesn't exceed the metal's elasticity and is measured by strain gauges bonded at points on the cell. As long as the load is applied to the proper spot on the load cell, the strain gauges provide a proportional electrical signal.
The key specifications for a load cell that will provide accurate weight information are:
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Nonlinearity: ±0.018 percent of the load cell's rated output.
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Hysteresis: ±0.025 percent of the load cell's rated output.
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Nonrepeatability: ±0.01 percent of the load cell's rated output.
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Creep: ±0.01 percent of the load cell's rated output in 5 minutes.
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Temperature effect on output: ±0.0008 percent of the load per degree Fahrenheit.
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Temperature effect on zero: ±0.001 percent of the load cell's rated output per degree Fahrenheit.
Understanding the specifications. Although every specification won't necessarily apply to your weighing controller system installation, it's important to understand each specification to determine the load cell's combined accuracy.
Nonlinearity is the load cell calibration curve's maximum deviation from a straight line with weighing sensor, starting at zero load and ending at the cell's maximum rated capacity. Nonlinearity measures the cell's weighing error over its entire operating range. The worst-case nonlinearity specification of ±0.018 percent is seen over the load cell's full range. The smaller the change in weight on your load cell, the smaller the error resulting from nonlinearity.
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Figure 1: Typical Load Cell Calibration Curve
Hysteresis is the difference between two load cell output readings for the same applied load -- one reading obtained by increasing the load from zero, the other by decreasing the load from the load cell's maximum rated capacity. As with nonlinearity, the worst-case ±0.025 percent hysteresis specification is seen over the load cell's full range, and the error caused by hysteresis diminishes with small weight changes. In an application such as batching, where you typically need accurate weight measurements only during filling, you can ignore the error caused by hysteresis. Hysteresis error normally falls into a different region on a load cell's calibration curve than nonlinearity error, as shown in Figure 1. As a result, the specifications for these two errors are combined on some load cells into an algebraic sum, called a combined error specification, of ±0.03 percent.
Nonrepeatability is the maximum difference between load cell output readings for repeated loadings under identical loading conditions (that is, either increasing the load from zero or decreasing the load from the load cell's maximum rated capacity) and environmental conditions. The nonrepeatability specification is ±0.01 percent over the load cell's full range. Nonrepeatability can affect the weight measurement in any weighing application. You can determine the worst-case nonrepeatability specification by adding the nonrepeatability error to the load cell's combined error.
Creep is the change in load cell output over time when a load remains on the cell for a long time. In a 2- to 3-minute batch or filling cycle, creep isn't a significant problem. But if you use load cells to monitor inventory in a storage silo, you need to consider creep effects.
Temperature changes can cause weighing errors. Most load cells are temperature-compensated to reduce these errors. But if your weighing system is subject to large temperature changes during the weighing cycle -- for example, if an outdoor weigh vessel is exposed to low overnight temperatures but heats up quickly in the daytime sun -- consider how temperature can affect the load cell output. If the only significant change affecting your weighing system is between summer and winter temperatures, you can recalibrate the load cells once when the season changes to correct for any temperature-caused errors.
Temperature changes affect load cell output by changing the load cell's sensitivity, and you must consider this effect unless you perform a new calibration for each large temperature change. The temperature effect on the load cell at zero load causes the cell's entire output range to shift. But if the load cell rezeroes (that is, tares in the net-weight mode) before it starts the weighing cycle -- such as in a batching application -- you don't need to be concerned about this temperature effect on zero load.
Considering your load cell's response time. The load cell's response time is another factor to consider for some applications. The typical load cell behaves like a stiff spring that oscillates, so to achieve an accurate weight reading, the load cell must settle -- that is, stop oscillating -- in less time than the required weighing period. While load cell response time is typically not important for a batching application, a high-speed checkweighing or rotary filling machine requires fast-responding load cells. Such load cells dampen their own natural oscillating frequency when a load is applied to them. However, the load cells don't reject vibrations applied to them from outside sources, such as nearby weighing equipment, so you still need to isolate the load cells from such vibration sources (covered in more detail in the later section, "3: Environmental forces").
