The International Avogadro project relates the kilogram to the mass of a fixed number of atoms by measuring the number of atoms in a sphere of silicon.
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Mass
Mass is unique amongst the base units of the International System of Units in being defined in terms of a physical artefact.
A cylinder of platinum iridium alloy, held at the BIPM near Paris, is defined as being exactly one kilogram in mass. All mass measurements undertaken in the World should be traceable to this artefact via National Measurement Institutes and accredited laboratories. The United Kingdom’s national standard of mass, Kilogram 18, is held by the National Physical Laboratory and is the basis of the entire mass scale in the UK.
Accurate measurement of mass is important in a wide range of applications from product design to ensuring compliance with health and safety requirements. NPL provides a calibration service for weights of all OIML classes, as well as providing ‘hands-on’ training in mass metrology.
The world-wide equivalence of mass measurements is vital to trade and commerce. NPL actively participates in a wide range of international comparisons and collaborations to ensure that measurements made in the UK are equivalent to those made elsewhere.
Mass metrology is an active area for research, and NPL carries out a series of projects designed to increase our knowledge and understanding in this area. Requirements for traceability are extending beyond the traditional lower limit of 1 milligram. Additionally, demands for lower uncertainties are driving the move away from the use of a physical artefact to provide the World’s standard.
See also: The Weighing Panel of the Institute of Measurement and Control (InstMC)
See also: EURAMET e.V. Technical Committee - Mass and Related Quantities
Mass research
- Scientists at the National Physical Laboratory are developing small mass standards with values below a milligram.
- The NPL watt balance project relates the kilogram to the Planck constant, the metre and the second, using quantum-based measurements of voltage and resistance.
- Quartz crystal microbalances (QCMs) are used in a wide range of applications and NPL is working towards a means of making such measurements traceable to national standards.
- Mass is the only base SI unit that is defined in terms of a physical artefact.
- Storage of mass standards in inert gas/vacuum to achieve improved mass stability compared with storage in air.
- Cleaning of platinum-iridium mass standards using ultra violet light and ozone provides a non-contact alternative to solvent/steam cleaning.
Mass products & services
- NPL’s wide range of facilities and knowledge means that NPL is well positioned to meet your calibration and training requirements.
- The National Physical Laboratory has two mass balances designed to compare masses under vacuum.
Mass FAQs
- The International Prototype Kilogram is not perfectly stable (its mass changes with time), the amount it changes cannot be known perfectly (there is no 'perfect' reference against which to judge it) and the values of the national copies cannot be monitored at the highest level of accuracy without being compared directly with it.
- Up to a point yes, but unless a weight is of suitable design and material and in appropriate condition it will not be possible to give it a meaningful calibration and it would certainly be a waste of money.
- Give the weight a general inspection to check its construction, surface finish and the suitability of its magnetic properties.
- Yes, magnetic fields - and indeed magnetically permeable materials close to a balance - can effect a weighing result.
- Historically there have been a variety of units of mass and density, and approximate conversion factors to some of these are given below.