Volumetric glassware laboratory measuring devices, such as burettes, which are also called burets, are manufactured to Class A and Class B measurement standards. The accuracy of a burette manufactured to Class A standards can be expected to have a tolerance level small enough for use without calibration. A Class A 50-milliliter burette, for example, has a tolerance range of plus or minus 0.05 milliliters.
A Class A glassware measuring device typically has a tolerance range that is half that of a similar Class B device. A 50-milliliter Class B burette, for example, will have a tolerance range of plus or minus 0.10 milliliters, compared to the 50-milliliter Class A device's 0.05 range.
Class A devices are manufactured to tolerance specifications that are determined by organizations such as the American Society for Testing and Materials and the National Institute of Standards and Technology. The tolerance range of a burette is usually imprinted on the glassware as a plus or minus tolerance range.