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Joe Flanagan

Metrology

Key Specifications That Determine the Accuracy of a Weighing System

How to ensure you achieve optimal weighing accuracy for your application

Published: Wednesday, March 15, 2017 - 12:02

There are many factors that influence the accuracy of a weighing system. Assuming that external factors are minimized or eliminated, the most important factor is the appropriate selection of the load cell for a given weighing application. This discussion covers the key load-cell specifications that are important to ensuring that you achieve optimal weighing accuracy for your application.

Nonlinearity

This specification is typically given in terms of a percentage of rated output or percentage of full scale. It represents the cell’s measurement error over the entire output range. Looking at a calibration curve for the load cell, the nonlinearity is the amount that the curve deviates from a straight line.


Figure 1: Idealized hysteresis loop for an elastic band

Hysteresis

Hysteresis is used to capture lag time in measurements. It represents the difference in measurements that you would obtain by starting with zero load and increasing the load vs. starting at rated capacity and decreasing the load. The difference can be seen figure 1, which shows a hysteresis curve for an idealized rubber band. The blue curve shows the measurement due to loading the band, while the red curve shows the measurement due to unloading the band. Load cell specifications typically indicate the difference in these curves in terms of a percentage of rated output or full scale.

Nonrepeatability

Nonrepeatability, again given in terms of a percentage of rated output or full scale, is just what it sounds like. It is a measure of the difference in readings from the exact same load applied repeatedly, under the same conditions, taken by the same person, and within a short period of time, so as to eliminate other external factors from the measurements.

Creep

This specification is a measurement of the change in results over time. The longer a load is left on a load cell, the more of a factor this becomes. It is important to take this into account for long-term measurement applications.

Temperature effects

There are typically several different temperature specifications indicated for a load cell, including operating range, compensated range, effect on output, and effect on zero. In many applications, measurements are taken within the compensated range, and therefore temperature does not have a large impact on the measurements. However, if your weighing application is performed under fluctuating temperature extremes, these effects must be factored into your overall accuracy.

Summary

All of the load cell specifications discussed here have an additive effect on the overall accuracy of your weighing system. It is important to determine which ones will have the greatest impact on your particular application so as to ensure proper selection of a load cell. However, use caution. These factors alone do not guarantee accurate results. You must account for a wide range of external factors as well, such as electrical noise, mechanical noise, environmental effects, load distribution and alignment, and the effect of other equipment in your data acquisition system.

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About The Author

Joe Flanagan’s picture

Joe Flanagan

Joe Flanagan is an engineer at Tacuna Systems, a load cells specialist.