Weighting Scales Have Been Needed For Thousands Of Years

By Author: Larry Tang
Total Articles: 512
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Weighing scales have been used for thousands of years. Among some of the earliest sets of laws known to civilization, it was declared that people must use fair scales and weights. These rudimentary scales were typically a bar that was suspended in the middle by a string. There were pans on both ends of the bar. A known weight was placed on one pan, and the other pan was used to receive the material that was being weighed. So if a measure of grain was desired, the appropriate weight standard was used on one side, and the grain was poured onto the other side. Of course, the accuracy of this type of scale was very limited. It was dependent on attaching the string to the very center of the bar. It was also necessary to make sure that the two pans were of equal size and weight. Furthermore, the standard weights were typically anything but standard.

As technology advanced, this type of scale became more sophisticated, and more accurate. The bar was connected to a pivot instead of being held by a string. Once the bar was fixed, it was easier to adjust it for better accuracy. Of course the quality of the pivot and the bearing ...
... that it rested on helped determine the accuracy of the scale. As the pivot and bearing was used over time, friction would increase as the materials started to wear.

Equal arm balances were eventually supplanted by sliding arm scales. Instead of requiring known weights on one pan, a weight could be moved outwards from the center of the bar to match the unknown weight on the other pan. Markings on the bar corresponded to the weight that would be balanced on the other pan. So if a marking of three inches from the center balanced out a weight of one pound, moving the sliding weight to six inches would balance out a weight of six pounds on the other pan.

Electronic scales were developed with the invention of the strain gage. This device is a thin resistor that can be attached to a bending beam inside the scale. When this beam, also know as a load cell, has a force applied to it, it will bend to an extent that is proportional to the force it receives. The attached strain gage will bend by the same amount. As the strain gage bends, it will stretch. When a resistor stretches, its resistance will change. By accurately determining the change in resistance, it is possible to accurately determine the amount that the load cell is bending, and therefore the force that has been applied to it.

There are a number of characteristics of a load cell that will determine its accuracy and suitability for use in a digital scale. One of the most important is the material that is used. Very often, aluminum is chosen for bench scales and counting scales because it is easy and inexpensive to machine into the desired shapes. Arlyn Scales has chosen to use stainless steel instead for its entire range of scales, including bench scales and counting scales, but also for its platform scales and floor scales. A major advantage of stainless steel is that it has a higher modulus of elasticity, and is therefore much less prone to damage from shock load and overload. Of course another reason to use stainless steel is that it will not be subject to the same type of corrosion as aluminum or even nickel plated steel.

Another important feature is the number of strain gages that are used on each load cell. Lower accuracy scales may be constructed with just a single strain gage on each load cell. Fixed resistors are often used to balance the electronic circuit. Arlyn Scales and other high quality scale manufacturers use four strain gages on each load cell. These strain gages are wired in the form of a bridge. This is an excellent method of compensating for a number of characteristics that would otherwise contribute to inaccuracy in an industrial scale. This includes errors caused by changes in temperature and the resultant expansion or contraction of the load cell material.

The digital indicator is also very influential in the overall accuracy of an electronic scale. While lower accuracy analog to digital converters may be used to change the analog voltage from the load cell into a digital number, Arlyn Scales utilizes the modern delta sigma devices for the best levels of accuracy. This type of circuitry will result in the most reliable readings for all types of digital scales. Arlyn Scales uses this technology throughout its range of industrial scales, animal scales and high precision scales.