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 MASS CALIBRATION Mass commanly known as Weight has altogether different meaning in science. Mass is defined as the amount of substance or matter contained in a body or object where as Weight is defined as the force acting on an object or mass by the gravity of that planet where it is measured. Mass is an scaler quantity where as weigt is vector quantity. Mass can not be zero where as weight can be zero where there is no gravity pull such as in space. Unit of Mass is kilogram where as unit of weight is Newton. Standard of mass is a prototype cylindrical object (alloy) made of platinum-iridium kept at controlled environment at BIPM, France. This is accepted Internationally as 1 kg. On 16th Nov. 2018, mass is redefined in terms of Plank’s constant but for all practical purpose earlier definition will prevail for the time being. Calibration of Mass is done normally by substitution method utilising a single pan balance and standard weight of the same denomination having accuracy of at least one class higher. Selection of weighing balance for calibration of particular class of weight is also critical and can be referenced to OIML R 111-1 and OIML R-76. Substitution method commanly known as ABBA method where A is conventional mass value of standard and B is the mass under calibration. Conventional mass value of the subject mass is determined. After estimation of measurement uncertainty, classification of weight can be done which is based on te tolerances and the uncertainty in measurements. It has been made mandatory to classify the weight after calibration. Since the density (and thus volume) of the two masses even of the same material may be different and the weiging is done in air, there will be buoyancy effect acting differently on the two masses. Hence air buoyancy correction need to be done. Since the air buoyancy depends on the air teperature, humidity and air pressure, these parameters need to be determined/monitored very precisely and accounted for. Since mass calibration involves use of weiging balance, calibration of mass can be done only at accredited permanent lab.