Two-pan balances

In scientific circles the word balance has many meanings but in the context of weighing it originates from the abbreviation of beam-balance - where the weight of a sample or artefact contributes to the balance of moments of a beam about a central fulcrum. The beam has a pan hanging from each end and hence this type of balance is known as a two-pan balance. By definition, two-pan balances are always used to make comparative weighings.

Single-pan balances

Other balances used for measuring weight, such as simple spring balances and more sophisticated electrical force balances, have a single-pan and can be used either to make comparative weighings or direct weighings.

Traceability

Comparators - either two-pan or single-pan - obtain their traceability from the calibration of the mass standard against which the unknown mass is compared, plus an analysis of the balance's performance (essentially its sensitivity and repeatability). When used as direct reading devices, traceability for single-pan balances is also provided by reference masses but the characteristics of the balance introduce other significant factors that have to be evaluated including its linearity, repeatability, reproducibility, and temperature dependence.