How do weight scales work?

How do scales measure your weight? Traditional balance scales (also known as beam scales) operate on the principle of comparing masses. Most will have a horizontal beam connected to two platforms on each end. 

The goal is to balance the two sides by placing the object to be measured on one side, and objects where the weight is known (standard weights) on the other. If the object being weighed is heavier than the known weights, the beam will tilt downwards towards the heavier side. When enough weights are added to balance the beam, we now know the mass of the object after summing the total of the known weights. Of course, this method is fairly time-consuming, requires you to have an assortment of standard weights on hand, and can be difficult when measuring very precisely.  

 

This is why traditional scales have been gradually replaced with digital scales. The introduction of strain gauge technology helped push forward the development of digital scales, which have now become the main choice in various industries.

Digital scales generally use strain gauge "load cells" for measurement. When an object is placed onto the scale, the weight of the object bends the load cells inside the platform very slightly (nearly imperceptible to human eyes). This causes the electrical signal running through the load cell to change; a heavier weight will affect the signal in a different way compared to a lighter weight.

 

This signal is typically read by a digital indicator, and transformed into a weight value we can understand (such as in kg, lb). Whether it's an industrial scale, veterinary scale, medical scale... all digital scales essentially use the same principle.

 

Not all scales are equally accurate! The cheap bathroom scale you can find at your local electronics store is not as consistent or accurate as a medically certified scale that you will find at a hospital. The level of accuracy demanded from a scale depends on the certification level (Class I-IV); for measuring people in medical settings, Class III is the most common required accuracy level. In addition, even the most accurate scale will require re-calibration over time. (this is the same for both traditional and digital scales)

 

Of course, factors such as regular wear and tear can also affect the accuracy of a scale, which is why they need to be regularly inspected, re-calibrated, repaired, and ultimately replaced when they've reached the end of their service life.

 

Standard weights are placed on the platform, to check if the displayed weight matches the standard weight(s). The allowable difference between display weight and actual weight is set by the Non-Automatic Weighing Instruments (NAWI) Directive. If the scale fails the accuracy check by displaying results that are under or over the acceptable difference range, then re-calibration is required.

 

Although scales generally work using the same principle, there are still significant differences when it comes to design and usage. Examples include Infant Scales with measurement trays and small capacity precision, Bed Scales that have ramps allowing you to push a bed onto the platform, Handrail Scales with built-in handrails for the patient to use that won't affect accuracy, and various other types for various usage scenarios!

 

Selecting the right type of scale for your application needs is quite important, whether it's the correct form factor, accuracy level, and certification level. Consider reading our quick guide to help make your decision!