Counting By Weight - Industrial Scales

By Author: John Tang
Total Articles: 512
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One of the most standard problems found in industry is determining the number of parts in a batch. This is the case for hardware, electronics, plastics, pharmaceutical, and virtually every other industrial application. The usual alternatives are counting by hand, electric eye types of machine counting, or when possible the preferred method of counting by weight.

Of course, it is only possible to count by weight if the parts being counted are almost identical to each other. This is certainly the case for hardware. Nuts and bolts, screws, washers, fasteners, clips, pins and shims are usually extremely consistent. Similarly, in the electronics industry, resistors, capacitors, inductors, transistors, diodes, connectors, plugs, sockets, and integrated circuits are good examples of this type of part. But other types of components, such as molded plastic pieces, or items fashioned from natural materials such as wood or stone, will be much more inconsistent, and would generally not be suitable for counting by weight.

The typical method of counting by weight is to take a sample of the parts and place them on the scale platform. ...
... The operator must enter the number of parts in that sample. The scale will then calculate the weight of a single part, known as the piece weight. Then the entire batch of parts are put onto the scale. The scale will divide the weight of this batch by the piece weight to determine the number of pieces in the batch.

The type of scales that are used for this purpose, parts counting scales, should have a number of features. The most important is that it should have excellent sensitivity. If just a few pieces are used as a sample, the scale must be able to very accurately determine the weight of that sample. For example, a scale may have a capacity of 100 lb. It may have a resolution of .01 lb. If the parts sample consists of 10 pieces, and if each piece weighs .0103 lb, then the 10 pieces will weigh .103 lb. But the scale will only be able to detect a sample weight of .10 lb, and error of 3%. Now, when the entire batch is placed on the scale, it will show an error of at least 3%.

One method of improving the accuracy is to use a larger sample size. For the example above, if the parts sample consisted of 25 pieces, then the sample would weigh .2575 lb. The scale would detect a sample weight of .26 lb, an error of only 1%. Of course a larger sample size would reduce the error still further.

Some parts counting scales, such as the Arlyn Series 8200, allow the user to automatically enter the piece weight into memory. It may be time consuming to use a sample size of 100 pieces, but it would offer excellent accuracy. If that accurate piece weight was entered into memory, it could be recalled every time that same item was being counted. So the operator would not have to once again go through the effort of acquiring that sample piece weight.

Another way to improve the accuracy is to use a scale with better sensitivity. The Arlyn SAW (Surface Acoustic Wave) series of ultra precision scales would serve this purpose. For the same example above, this type of scale has a resolution of .001 lb. So with the same 10 piece sample, the scale would detect a weight of .103 lb. The error would only be the inherent error of the scale itself, which is 1/100 of 1%. Of course, this is a very significant improvement.

Another important feature of a parts counting is its resistance to shock load and overload. Often, when a full box of heavy parts is placed on the scale platform, it may be dropped with some force. Whereas most scales are manufactured with aluminum load cells, it is preferable to use stainless steel. This will give a much larger safety margin for this type of abuse. Even better, the SAW load cells provide a huge safety margin of 500% of the rated load. This helps to provide years of trouble free operation.

Communication features are also very useful. During an inventory procedure, hundreds, or even thousands of different parts may need to be counted. This information may be communicated to a computer system. The old method for doing this generally utilized an RS-232 channel (serial communication). Some more modern methods include communication through a USB port, or wired or wireless Ethernet.