Accurate Weight Can Mean Many Different Things

By Author: John Tang
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For industrial purposes, accurate weight can mean many different things. When weighing a box for shipping by parcel service, it is only necessary to be accurate to the nearest pound. For this purpose, a scale should be chosen with at least four times this accuracy to avoid always rounding up to make sure the correct weight is not under reported. This will typically save 1 lb of extra reported weight in 50% of the packages shipped, or on average, ½ lb per package. It is evident that with even a moderate number of packages shipped, significant savings can result. Nonetheless, bench scales with accuracies of ¼ lb are not considered to be very accurate.

Other attributes should also be considered. Because the packages being weighed may be handled roughly, the scale should be able to withstand shock loading. The primary component for this purpose is the load cell. Most load cells are manufactured from aluminum. While they provide adequate accuracy, they are prone to plastic deformation when subjected to high shock loads. Once they have been deformed in this manner, they will have significantly degraded accuracy, or may ...
... not operate properly at all. The best way to avoid this type of problem is to produce the load cell from stainless steel. There are preferred alloys that will offer excellent levels of shock protection along with high accuracy. Ideally, the load cell should be heat treated after manufacture to achieve the best characteristics.

Independent of the type of load cell material used, the scale should also provide mechanical overload protection. While this type of protection cannot help when there is instantaneous shock load, it will be very helpful when loads that are in excess of the rated capacity of the load cell are placed on the scale platform. The standard way to provide overload protection is to provide a small gap between the base of the scale and the platform. That gap will decrease as the load is placed on the platform. When properly implemented, the gap will close entirely when the scale is overloaded. This will transfer the weight away from the load cell, and thereby protect it from damage.

While an accuracy of ¼ lb may be suitable for bench scales that are used for shipping purposes, if will be insufficient for bench scales that may be used for formulation. Most industrial processes require an accuracy of 1/10 of 1%, or even better. If a batch of material will weigh in the range of 100 lb, a typical industrial scale will have an accuracy of 0.1 lb. This may be suitable for batching a variety of ingredients into a formula. But as with shipping scales, it is better if the scale can have a resolution that is greater than its accuracy. Trace ingredients may have to be included in a batch. Therefore most industrial scales will provide display resolutions that are five times better than their accuracy. So the 100 lb bench scale may have a resolution of 0.02 lb, with an accuracy of 0.1 lb. It is always important to factor in additional causes of inaccuracy, such as wide changes in temperature. Good quality scales, such as the Arlyn 6200 Series Bench Scales, have compensation for changes in temperature, but this compensation is not at all perfect. Over wide ranges, all scales will show variances in their results.

Even accuracies of 1/10 of 1% are inadequate for some applications. For example, the formulation of inks, dyes and paints must be exceedingly accurate to produce standardized colors without variations that can be perceived. Many of these situations have a large base material, often white in color. Much smaller quantities of other colors are used to produce the final product. Again using an example of a 100 lb batch, some of the trace colors may only need .2 lb. If this color is to be dispensed with only a 1% accuracy, the scale will need to weigh 1% of .2 lb, or .002 lb. Standard strain gage load cell industrial scales are not at all capable of weighing with that accuracy, or even resolution. This requires using an Ultra Precision Scale, such as the Arlyn SAW Series of Bench Scales. These use the unique, patented Surface Acoustic Wave technology to provide accuracies of 1/100 of 1%, and resolutions of 1 part in 100,000 or even better.