Mass is the only base unit, which is represented as a primary standard in the form of artifact for more than 125 years. International prototype of kilogram (IPK) is kept at the Bureau International des Poids et Mesures (BIPM), Paris and serves as the international standard of kilogram. It is made of 90% platinum and 10% iridium and as a cylinder of 39 mm diameter and 39 mm height. Replicas of the IPK are made of the same material and used at BIPM as reference or working standards and national prototype of kilogram (NPK), kept at different National Metrology Institutes (NMIs). NPK-57, kept at CSIR-National Physical Laboratory, is sent periodically to BIPM for calibration
