National Physical Laboratory

What are the differences between mass, true mass and conventional mass values? (FAQ - Mass & Density)

Mass and True Mass

The mass of a body relates to the amount of material it contains (see full definition) and there is no difference between mass and true mass. The prefix true is sometimes added to the word mass where it is important to make it clear that a particular value of mass being considered is not a conventional mass value and it is particularly important to avoid this potential ambiguity when, for example, specifying the value of weights (see below).

The international prototype kilogram, on which the mass scale throughout the world is realized, is defined as a true mass of exactly 1 kilogram. Most high accuracy comparisons are performed on a true mass basis but the values are usually converted to conventional mass values when quoted on a certificate.

Conventional Mass

When a weight is calibrated the mass value quoted on its certificate of calibration is normally a conventional mass value - appropriate where the value is determined by weighing the item in air in accordance with International Recommendation OIML R 33. This Recommendation says formally: For a weight at 20 °C, the conventional mass is the mass of a reference weight of a density of 8 000 kg/m3 that it balances in air of density 1.2 kg/m3.

Some background to conventional mass

Air is a fluid and, as such, exerts a buoyancy force on all objects weighed in it. The value of buoyancy will depend on the volume of the object being weighed and the density of the air surrounding it - which itself changes with temperature, humidity, composition and pressure (see air density change with altitude). The buoyancy effect cancels out if a weight is calibrated on a two-pan balance by comparison with a reference weight of identical density. In practice though, the weight being calibrated will undoubtedly have a density value that is different to that of the reference weight (even if only slightly) and a correction for the differential buoyancy effect will need to be applied.

Every day hundreds of thousands of weighings are performed where the accuracies required make buoyancy effects very significant. It is not realistic to perform such measurements in vacuum - where there is no buoyancy effect - but, even if this were possible, most weights are used in air and the mass values attributed to them must reflect the conditions under which they will be used.

International Recommendation OIML R 33 essentially provides an in-air 'datum', or set of reference conditions, which defines the conventional mass of a weight, at 20 °C, as the (true) mass of a reference weight of a density of 8 000 kg/m3 that just 'balances' it when in air of density 1.2 kg/m3 (see above for exact wording). In practice these conditions will not be realized exactly and small corrections for temperature, material density and air density have to be applied to allow for this.

Completely unofficial text for conveying the spirit of what is meant by conventional mass might therefore be a value, slightly different from the true mass value, which an object appears to have when it is used in normal-ish ambient conditions - but please don't quote us on this one!

Last Updated: 25 Mar 2010
Created: 9 Aug 2007


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