SCALE SYSTEMS IN BAG PACKAGING
By: Ken Goodworth, PE
This document is written to help bagging equipment operators, supervisors, and managers understand the capabilities and limitations of scale systems in the packaging environment.
An understanding of some key terms is important to understanding scale systems. These terms are accuracy and precision. The best way to understand these terms is to review the illustrations below:
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Accurate But Not Precise |
Precise But Not Accurate |
Precuse and Accurate |
Accuracy is a measurement of the deviation of weights from the average weight. Accuracy can be measured by taking a sample of bag weights (say 30). Each individual weight is entered into a handheld calculator (or computer program) with statistical functions. The calculator can then calculate sigma (?). In the packaging industry we usually quote accuracy at 2? (meaning 2 times ?). This number, obtained from a small sample, can be used to predict the performance of a system over a long period of time or a large number of weights. For example, 2? of 50 grams would indicate (based upon statistics and probability) that 95% of bags weighed would fall within a range of +/- 50 grams centered about the average weight (not the target weight). See the illustration below.
Precision is a measurement of how close the measured average (or mean) is from the target weight. Even accurate scales require some operator involvement to ensure that they track the target weight and are precise.
A typical packaging system has as many as three scales. The first scale is used to weigh the product into the bag. We will call this scale the bagging scale. A downstream scale is used to check the weight of the bags filled upstream. We will call this the checkweigher (scale). Additionally there may be a reference scale used to make sure the other scales are properly adjusted and properly functioning. We will call this the floor scale.
Perhaps the most important scale in the system is the checkweigher as it is used to ensure that only acceptable bags (proper weight) are shipped to customers. This is done by rejecting bags that are out of the acceptable range. The floor scale, if more accurate and precise than the checkweigher, can be used to calibrate the checkweigher. The goal is to ensure that the checkweigher weighs as it should according to government standards (i.e., Department of Weights and Measures in the United States).
Determining an appropriate range to separate good bags and rejected bags can be a challenge. Some industries, like cement and concrete mix, are not as concerned with accuracy. They use a wide zone for acceptable weights, often 3-5 kg; the checkweigher basically rejects broken bags and grossly off-weight bags. Other industries, such as the chemical industry and the food industry, require higher accuracy.
Accuracy begins at the bagging scale. Other factors come into play by the time the bag is weighed on the checkweigher. To determine the accuracy of the bagging scale for a given product, all outside influences must be eliminated. The product must be consistent and free of lumps. The variation in the weight of the empty bags must be accounted for. Temperature changes which can affect load cell performance must be minimized (no open doors to cold or hot environment). Air movement past the scale should be minimized – no fans on the equipment or open doors allowing air drafts (I have personally observed 40 gram swings in the displayed weight due to drafts). Vibrations should also be minimized; do not expect to weigh accurately while a freight train rumbles nearby.
Once all of the external influences have been eliminated, a test can be made for the bagging scale accuracy. Pre-weigh each empty bag and write the weight on the bag. Fill each bag from the bagging scale. Weigh each filled bag on a precision floor scale (or the checkweigher – which may have been government certified and calibrated). Repeat this process for about 30-50 bags. Subtract the empty bag weight from the full bag weight on the floor scale to get the net weight. Record the net weight of the 30-50 bags. Calculate accuracy at 2?. Also calculate the average net weight. If this is different than the target weight, a manual adjustment by the operator to the bagging scale will be necessary. In this way, both accuracy and precision are being addressed. Please note that it is impractical to expect that the target weight and the average weight will be exactly the same. Through operator attention and adjustment, it can be kept close. See the illustration below:
Now that the accuracy of the bagging scale has been determined, attention can be shifted to the Checkweigher. The accuracy of the checkweigher should be measured by itself, with no external influences. This can be done by running the same bag onto the scale 10-20 times, recording the weight, and then calculating accuracy at 2?. A good scale, mounted under a conveyor, should weigh the same bag consistently with no more than +/- 25 grams deviation at 2?. If this scale is not the governing scale, manual adjustments will be required to tune the precision (average weight) of the checkweigher to that of the governing floor scale.
Now that the accuracy of the scales has been determined, careful attention must be paid to the warning and reject setpoints. Most checkweighers classify the bag weight into one of five zones: Good, High-Pass, Low-Pass, High-Reject, Low-Reject. Setting the reject points of the checkweigher, along with checkweigher accuracy determines the accuracy of the system, and ultimately what the customer will see. It is important to remember that outside influences (see examples listed on page 2) do play a role in what the checkweigher sees. Please see the illustration below:
Additionally, the customer weight requirements play a role in determining the reject points. If the reject points are set too tight, many bags will be rejected. If the reject points are set too loose, there is little control of the operation. Because so many factors enter into the selection of the reject points, the selection should only be made by the company doing the packaging, and not by the equipment supplier. It is a management decision. A rule-of-thumb (business practice or suggestion) that accounts for outside influences and scale error (see illustration above), is to set the reject points at 6? (bagging scale), and the warning points at 4? (bagging scale). See the illustration below:
Using the rule-of-thumb would suggest that when a bagging scale provides an accuracy of +/- 50 grams at 2? , the reject points of the checkweigher should be set at +/- 150 grams (6?). So, for a 25 kg target weight, the reject points would be set at 25.15 kg (high) and 24.85 kg (low). This rule of thumb provides a good place to start. Management can adjust the reject points based upon customer demands and the severity of outside influences.
By paying attention to the accuracy of the individual scales in a system, and by accounting for and minimizing the outside influences, it is possible to operate a system that provides good system accuracy. If these things are ignored, the system accuracy will suffer.
