Sounds simple enough, but what about the capacity of the Scale? The Scale must have capacity to handle the largest unknown we expect to encounter in our testing plus the container holding it for weighing. It is important to remember that when using Tare to zero out a weight, that weight is still deducted from the Scale’s capacity. In the era of mechanical balances capacity generally was in a 1, 3 sequence by decades; such as 100.g, 300.g., 1000.g., 3000.g, 10,000.g, 30,000.g. With the introduction of electronic weighing it became easier to offer a wider variety of ranges and the sequence changed. It became a 1, 3, 5 sequence, then a 1, 2, 3, 5 sequence, and now it is often a 1, 2, 3, 4, 5 sequence. Are all these capacities necessary? Users buy them and perhaps that is the justification.

The classification of a device is related to its accuracy. It is very important not to confuse the resolution of a device with its accuracy. In this era of the electronic scale it is easy and inexpensive to produce scales with high resolution and poor accuracy. This is an area where the buyer must beware. The system of scale classification provides a reasonable amount of protection in this area. For instance a Class II scale with a resolution of 1.0mg will have a linearity of at least +/- 2.0mg (which provides an accuracy of +/- 2.0mg when the scale is calibrated).

Balances break down into 3 major categories in terms of accuracy; 1. Analytical Balances (Class I) which measure to 1 part in several million, such as 0.1mg in 200.g with a likely linearity of +/-0.2mg. These are very high accuracy devices and are usually used in research or other precision work. 2. Laboratory Scales (Class II) which measure to 1 part in several hundred thousand, such as 1.0mg in 300g with a likely linearity of +/- 2.0mg. These are the workhorses of the laboratory and are used for many purposes. 3. Industrial scales which are generally higher capacity scales and can provide high resolution, but usually with much poorer linearity and repeatability specifications.

All of this leads to broad product lines from a variety of manufacturers.