Comparative weighings

Two-pan balances can only be used to make comparative weighings - they compare the weight of an artefact with reference weights - by balancing moments about a beam. Single-pan balances, however, can also be used to make comparative measurements using a technique that involves sequentially interchanging the artefact and a reference weight (or weights) of similar mass - essentially using the balance's indication to enable calculation of small differences between the two mass values.

Direct measurement weighing

Only single-pan balances can be used for direct measurement weighing - where mass values are read directly from a scale, either analogue or digital.

In either case it is vital that:

• balance assessment reflects the way in which the balance is to be used in practice,
• the assessment takes place under the same conditions as those where the balance would normally be used.

State-of-the-art

Comparative measurements are generally more accurate than direct measurements. At the highest accuracy level balances are generally referred to as mass comparators and are primarily designed to make comparative measurements. If they are used to make direct measurements, their resolution will remain the same but their accuracy will be compromised.

At lower accuracy levels balances used to provide direct readings will generally have lower resolution than those designed as mass comparators but are designed specifically to work in this mode.