All of you would be familiar with standard weight boxes used for calibration of analytical balances from your school days. Such weights are supplied in wooden boxes and are placed in individual slots inside the box along with a pair of tweezers for handling purpose. The weights provided meet weighing requirements of most analytical laboratories and include following weights: 10mg(0.01gm),20mg(0.02gm),100mg(0.1gm),200mg(0.2gm), 500mg(0.5gm),0.10gm,0.20gm,0.50gm, 1.00gm, 2.00gm,5.00gm,10.00gm,20.00gm,50gm and 100gm.
The observed balance reading should be within +/- 0.1% of market value of standard weight as per USP 41 requirements. As per the mandatory requirements, a weighing balance should be calibrated on a daily, weekly and monthly basis.
Traceability of calibration weight standards
Calibration weight standards are accompanied by a certificate of traceability. Such certificates are issued by national or international laboratories accredited by global bodies such as NIST, ASTM or OIMIL. Such certificates contain information such as the type of weight, weight class, material density, limit of uncertainty and environmental controls at the time of calibration. The traceability reports also give acceptable tolerance and uncertainty limits. During regular use, the standard weights should be calibrated periodically at assigned intervals and should be sent to a national accredited calibration laboratory and the issued certifications should be preserved for records purpose.
Handling & care
All standard weights comprise of a single piece of metal or alloy with no air cavities or foreign adjusting materials for mass stability. However, the weight readings can vary over time due to mishandling and lack of care. The commonly observed changes are the appearance of scratches or corrosion at microscopic levels due to contact with humid air or fingerprint impressions. Persons handling standard weights should take precautions like wearing gloves to avoid contamination from hands or fingerprint impressions and hair net covers to prevent fall of hair and dandruff. Further weights should be lifted only with ivory-tipped forceps to prevent scratches. Wearing face masks also prevents disturbance due to breath air drafts.
It is also essential to keep standard weight boxes under controlled temperature and humidity even when not in use to preserve their certified characteristic parameters.