Understanding Load Cell Accuracy
This guide focuses on understanding load cell accuracy, as well as the key differences between analog and digital load cells.
TECHNICAL INSIGHT
A. Drivers of Load Cell Accuracy
This Technical Insight examines the drivers of load cell accuracy.
The accuracy of a load cell must be evaluated against the following key criteria that impact accuracy (“Accuracy Drivers”).
- Non-linearity means the load cells error when measured over its entire operating load range. The highest error is always found over the whole load range, for example at 0%-100% loading with lower errors being found at lower loading ranges for example at 0%- 20%. Quality load cells achieve +-0.03%.
- Repeatability means the maximum difference between load cell output readings for repeated loadings under identical loading conditions. This test can be done by either increasing or decreasing loads. Quality load cells achieve +-0.03%
- Temperature Effect means how much the output of a load cell changes due to the load cell’s sensitivity changing due to temperature changes. These changes are best seen when a load cell with no load on it sees the output change from reading zero to a positive or negative value, as the temperature is either increased or decreased. Quality load cells achieve +-0.01%.
- Creep means the change in load cell output over a period of time when a constant load remains on the load cell for a period of time. Quality load cells achieve +-0.01%.
- Communications means is the load cell generating an analogue or digital output. Analogue communications have a tendency to amplify errors arising from the 4 factors described above, while a digital output tends to smooth these out. Quality load cells are digital.
B. Comparison of Load Cell Specifications
The following table compares the Accuracy Drivers of a sample of load cells commonly found on African weighbridges:
The accuracy of a load cell is therefore determined by these Accuracy Drivers. With the high-temperature changes found in Africa, values for a load cell’s “temperature effect” are particularly relevant.
C. Analogue Versus Digital Output
The form of load cell communication to the indicator is critical. Digital load cells transmit a digital output which is linear while analogue load cells transmit an analogue output which generally takes on a less stable waveform resulting in “data value fluctuations”.
The following tables illustrate this fact:
D. Digitisation of Analogue Load Cells
The form of load cell communication to the indicator is critical. Digital load cells transmit a digital output which is linear while analogue load cells transmit an analogue output which generally takes on a less stable waveform resulting in “data value fluctuations”.
Significant improvements in the accuracy of analogue load cells can be achieved through the digitisation of the load cell outputs. This requires removing the existing junction box and replacing it with a digitisation junction box or “D-Link” box and the replacement of the analogue weighing indicator to a digital indicator.
In addition to digitizing the load cell outputs, certain “D-Link” boxes have the functionality to also improve the load cell accuracy by compensating for errors arising from temperature changes.
The digitisation of load cells will be the subject focus for Weighing Technical Bulletin 002.
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Jonathan is a Kenyan born British national who acquired Sasco from Avery Berkel in 2002 and took on the challenge of revitalizing the then ailing company. Jonathan has been CEO of various listed companies, spent 10 years as an investment banker in London, is committed to innovation and in a firm believer in the exceptional growth prospects for Africa over the coming decades.
