Uses

• Weighing scales vary greatly in the amount of weight they can measure and in the way they determine weight. For example, some scales measure very small weights, such as weights in milligrams and centigrams, in pharmaceutical applications. Common household scales measure amounts ounces or grams, in the case of kitchen scales, or in pounds, in the case of bathroom scales. Industrial scales may measure weights in metric tons. Different scale technologies lend themselves to different measuring needs, and all scales used commercially are subject to specifications established by the National Institute of Science and Technology.
  
  

Balance

• Balance scales work by attaching two platforms of identical weight to a single bar that rests on a single pivot point at the center point of the bar. When the two platforms weigh the same, the platforms remain at identical heights. That is, the bar from which they hang does not tilt toward either platform. Adding object A of known, precise weight to one side of the scale causes the scale to tilt toward the weighted side. Adding object B of unknown weight to the opposite side causes the degree of tilt to change. The amount of change in the tilt, combined with the known weight of object A, provides the figures needed to calculate the weight of object B.
  
  

Spring

• Metals formed into springs generate a known amount of resistance or tension. Tension refers to the amount of energy, or weight, required to compress the spring a particular distance. Spring scales function by using particular metals in specified spring configurations that have a known, consistent resistance. Because the resistance is constant, changes in the degree of compression to the spring correspond to particular weights. If the platform attached to a spring scale used in a kitchen lowers by one centimeter, that amount of compression equals a known weight, measured, for example, in ounces.
  
  

Hydraulic

• Hydraulic scales use the known properties of liquid displacement to calculate weight indirectly. In hydraulic scales, a weighing platform rests atop the weight calculation apparatus, which is a container of known volume that contains a liquid. Placing an object of unknown weight on the weighing platform causes the liquid in the weighing device to displace. The amount of liquid displacement the object causes corresponds to particular weights so that measuring displacement provides the precise figure for the object's weight.
  
  

Differences

• Each type of scale offers advantages and each can be adapted for particular types of uses. Spring scales can be relatively inexpensive to build, but spring tension lowers over time, so it is necessary to recalibrate the scale or replace the spring at set intervals. Balance scales can likewise be relatively inexpensive to build, but the need to suspend weight from a single bar limits the upward end of mass balance scales measure. Hydraulic scales can be more expensive to build but can measure weights substantially greater than other types of scales, and therefore may be most useful in industrial settings.