How often do I need to calibrate my weights?

The frequency with which calibrations should be carried out is an important, if sometimes difficult, question; there is no hard and fast rule but there are two main considerations.

Firstly, all weights change mass with time; the issue is how much they change and whether this is important to a given application. New weights should be calibrated immediately and then relatively frequently (typically annually) in order to establish their reproducibility - essentially their metrological stability or the change in their mass between calibrations. Initial estimates of reproducibility are sometimes made using type-test data from earlier calibration results of similar calibrated weights but the resultant uncertainty of measurement has to be cautiously higher, until real data is available.

Secondly, the required uncertainty of measurement should be assessed. If the weight's reproducibility is shown, by successive calibrations, to be substantially better than the uncertainty required then the interval between calibrations can be extended - perhaps up to somewhere between 2 years and 4 years. But at the other extreme - where the the weight's reproducibility approaches the uncertainty needed - the calibration intervals should be much shorter.

Weight construction

Weights should be made of a material that is chemically unreactive, non-magnetic, hard enough to resist scratching and of a density that meets the OIML R 111 Recommendations for its Class. Austenitic stainless steel is generally used in the construction of Class E₁ and E₂ weights. Lower accuracy weights may be manufactured from brass, iron or other suitable materials. Class E₁ and E₂ weights must be integral in construction, ie be made of a single piece of material.