Unproper Belt Scale Results?

Belt Weighing

Unproper Belt Scale Results?

10 reasons, why your perfect belt scale might under-perform
Belt weighing is common practice in lots of bulk solid handling operations. Although it’s a long established technology, there are still bits and pieces regarding installation and maintenance which might lead to erroneous results. Following you can find the reasons why.
(ed. wgeisler - 31/3/2017)
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Problem #10: Calibration not performed frequently enough – or calibration method is incorrect

When a scale is installed, a “zero calibration” is performed to determine the weights and forces of everything other than the material being weighed. For example, if no belt-related weight is recorded when doing the initial calibration, then this weight will not be subtracted when actual measurements are made.

That initial calibration might be perfect. But the dynamics can change significantly – whether an hour, a day, a month or several months later – as a result of a variety of factors, such as material building up, the onset of conveyer belt issues or environmental changes. This means that you will need to recalibrate your scale at specific intervals to ensure accuracy, repeatability and linearity. You may have to calibrate your system twice a day if it involves a high-accuracy scale or a critical application. Once a week tends to be sufficient for most applications. Some might find once a month is acceptable for their particular situation. The key is to have performed enough calibrations in order to establish a clear predictability pattern.

Each case must be looked at individually. Let us consider temperature to illustrate this point. Belts at outdoor sites need to take different temperature conditions into account. A belt running early in the morning will have different properties than later in the day. In the morning, there might be extra weight from condensation that will dry off later on in the day. And rubber typically seen in belts will be less stiff in the afternoon after it has been running for a while with warmer temperatures and the sun shining on it. (Before any belt scale is calibrated, the conveyor should be warmed up for about half an hour.) Similarly, seasonal temperature changes will have an impact, as a belt in the summer will have much different characteristics than a belt in the winter. Repeatability might be affected if the system is not calibrated at various times to take these types of fluctuations into account.

Extreme temperatures can be especially tricky. Belt scales from Siemens, for example, can be operated from -50°C all the way up to +60°C. But there are occasional circumstances – such as a mine in the far north – where it might get colder than that. In applications such as these, in addition to carefully calibrating the system to take into account these extremes, the installation may have to be modified to protect the integrator from cold.

In addition to frequency, the method used for calibration is also an important consideration. You need to make sure you are choosing the right solution for your particular application. Using the actual material being conveyed for verifying calibration is the best method to ensure that the scale can achieve the highest possible accuracy.

If this is not possible, using test chains (representing 40-60 percent of the load) is the next best option to achieve optimum accuracy because these most closely simulate material flow on a belt. Static test weights (representing 25 to 40 percent of load) can be used, but will not be as effective as chains because all the dynamic forces of material in motion will not be captured. Typically, chains should be used for critical applications or those demanding a high accuracy. Weights are more acceptable for less accurate applications.

A third choice is electronic calibration, which does not involve physically using weights or chains. This is only acceptable for applications where just a very low accuracy is required.
As a general rule, electronic calibration results in a one to five percent impact on accuracy, weights, 0.5 to five percent and chains, 0.5 to two percent.

Make sure you choose the right scale from the right manufacturer

After you have understood and managed all of these 10 problems, you still need to make sure you have a high-quality belt scale. Once again Siemens experts can also provide support here, as they have been offering technically advanced belt scales for a long time now.

Fig. 8: Belt weighing equipment from Siemens                                                       

Siemens systems feature the simplest mounting and lowest maintenance requirements on the market. They have no moving parts, are equipped with corrosion-resistant load cells, and are designed for 300 percent of ultimate load cell capacity. Together with Siemens high-resolution speed sensors, you can achieve high measuring accuracy,
repeatability and linearity.

Not all scales are created equal. Just consider the maintenance issues if you are looking at a model with bearings, leaf springs, pivots, bushings, counter balances or check rods. These mechanisms require much more attention in order that they operate correctly, and they also have slower reaction times to changes in load levels as well as hysteresis as a result of temperature and long term wear.

Best of all, everything sold comes backed by the knowledge and experience of a team of leading industry experts who are always available to help you.

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