International Organisation for Legal Metrology

The International Organisation for Legal Metrology (OIML from the French “Organisation Internationale de Métrologie Légale”) is housed in a non-descript office in the Rue Turgot in the 9th arrondissement of Paris and is about 700 metres the from the Gare du Nord. Like the BIPM, the OIML is an inter-governmental organisation which gives its staff quasi-diplomatic status, though, like the BIPM, French nationals pay French income tax. The OIML works closely with BIPM, the International Standards Organization (ISO) and other international bodies in coordinating metrology around the globe, with each organisation having its own specific role.


Logo of the OIML

MV readers are no doubt aware of the role of BIPM is to define units of measure and the means to realise those definitions to the highest degree possible. This might be good for science, but what about the practical world? If I buy a kilogram of sugar, am I really bothered whether it is a few grains short or whether it is a few grains over the specified weight? The cost of getting that packet correct to the nearest microgram will far exceed the cost of the sugar. This is where the OIML come in. The role of the OIML is best explained by an example. In the years leading up to 1976, the OIML and the EEC worked together to produce a specification (Directive CEE 76/211) which resulted in the “e” (for “estimated”) symbol that now appears on packets of sugar (and many other goods).  In the case of 1 kg packets of sugar, the EU directive requires that the average weight per packet in a batch shall be at least 1 kg and that the maximum short weight allowed for any one packet in that batch is 7.5 grams.  A number of countries outside the EU including Australia and South Africa have included the EEC specification into their domestic legislation. The UK, on leaving the EU has retained the legislation that was produced under the directive.

OIML Establishment

The OIML is an inter-governmental organisation established in 1955 to facilitate and coordinate legal metrology on an international scale. “Legal Metrology” is the discipline of making and recording measurements that have a legal implication. This includes but is not restricted situations where the amount of money that changes hands is dependent on a measurement, where a party might have a liability for their product which is measurement-dependant or where a measurement will determine whether an offence (such as speeding, drink-drive etc) has been committed. Part of the OIML’s brief is to assist in the international harmonisation of legislation in such matters.

The OIML has a three-tier structure – the “Conference” which consists of representatives from the member states, and which meets once every four years, the “International Committee” which oversees technical work, and which drafts formal output from such work and the “Bureau”, a secretariat based in Paris. Much of the CIPM’s work is done through specialist committees whose members are employed by metrology-oriented organisations in participating member states.

The three principal activities of the OIML are:

  • Overseeing of and issuing of internationally recognised (“Mutual recognition”) certificates to laboratories around the world certifying that their work is in accordance with international standards.
  • Publication of recommended standards pertaining to legal metrology and production of expert examinations from a metrology point of view of proposed legislation.
  • Joint publications with other organisations such as ISO and BIPM to ensure that there are no contradictions between outputs from the various organisations.

OIML Outputs

The principal outputs from the OIML are sets of publications. Those of interest to readers of MV are:

Joint activities with other organisations which include:

  • The International Vocabulary of Metrology (VIM) which is a joint production between eight organisations.
  • Liaison with the BIPM’s Consultative Committee on Units, the committee which, amongst other things, produces the SI Brochure.

Recommended standards that member states are advised to incorporate into their own laws. To date 150 recommendations have been published. Published recommendations are revisited once every few years at which time some are revised while others might be deprecated. At the current time there are 104 current recommendations. Recommendations that have been published that are of interest to MV readers include:

  • R 52-en – “Hexagonal weights – Metrological and technical requirements”. Hexagonal weights have been used for metric units for over a century. This recommendation describes standard sizes so that the user can mix-and-match weights from different manufacturers.               
  • R 76-en – “non-automatic weighing instruments” – This recommendation describes the specifications for four different classes of weighing devices – defined as Class I, Class II, Class III and Class IIII respectively. Supermarkets and similar establishments are usually required to have Class III devices (ability to weigh objects up to 1 part in 10 000) whereas Class IIII devices (up to 1 part in 1500) are designed for use in mobile medical health clinics.
  • R 111-en – “Weights of classes E1, E2, F1, F2, M1, M1-2, M2, M2-3 and M3”. This recommendation defines eight different classes of weights, each class having different accuracy requirements. If goes without saying that cheaper weights have less accuracy but are often more robust.
  • R 138-en – “Vessels for commercial transactions”. This recommendation describes, amongst other things, the allowances made when calibrating glasses for use in pubs.

Other recommendations include taxi meters, speech audiometry equipment, automatic rail weighbridges, vehicle exhaust measuring instruments, speedometers and so on.

Finally, the OIML is sometimes asked to produce expert reports of certain topics. Seven such reports have been produced including the report that led to the EEC (as it was) introducing the “e” mark for an “estimated quantity”.

Conclusions

Rather than summarising the work done by the OIML, let me give two examples from their various outputs which I believe will give MV readers food for thought:

  • Imperial pint glasses are variously described as being equivalent to 568 ml or 570 ml. OIML recommendation R 138-en allows a maximum error of (10 ml + 2.5%V) for glasses where the volume shown by a gauge mark (V is the nominal volume of the glass). This is far greater than the 2 ml variation in describing the size of the glasses. Thus, if a manufacturer exercises reasonable care in manufacturing such glasses, their products can fit either description of the pint.
  • The SI is often described as a “System of measurement” which is a shorthand for “System of units of measurement”. VIM describes the “International System of Units” as a “system of units based on the …” and it describes a “Measurement system” as “set of one or more measuring instruments …”.  VIM also draws to attention that in French, the translation of “measurement system” and “system of measurement” are identical – “système de mesure”. This is the underlying reason why VIM describes the SI as a “system of units” rather than a “system of measurement” – after all, “SI” translates to “International System of Units”, not “International System of Measurement”.

References

  1. OIML home page: www.oiml.org
  2. OIML analysis of EEC directive CEE 76/211:  https://www.oiml.org/en/files/pdf_e/e004-e04.pdf/view
  3. SI Brochure (9th edition): https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf (Para 1.3)
  4. List of OIML Recommendations: https://www.oiml.org/en/publications/recommendations
  5. Text of the International Vocabulary of Metrology: https://www.oiml.org/en/files/pdf_v/v002-200-e07.pdf

One thought on “International Organisation for Legal Metrology”

  1. The 568 mL amount would be known in the business as an impossible fill. It can’t be done no matter how much it is desired. Practical filling machines usually have a minimum fill capacity of 10 mL, some may go as low as 5 mL. Anything finer just doesn’t exist. The reason would be the costs to produce and maintain them. They would need to be almost constantly re-calibrated and checked.

    The 568 mL value comes only from those who try to insist that a pint is a precise amount that never changes and is just as perfect as the litre. 570 mL means the pint changes with the whims of time, something imperial proponents loathe. But, in the modern world, where machinery that does the making and the filling are all metric and designed around providing rounded metric fills and sizes, so exact equivalents to pre-SI units becomes impossible as long as complicated conversion factors persist.

    Also, 568 mL isn’t really a pint either. It is a lesser amount. To be precise to the definition, it should be 568.261 mL. Now, that would really be an impossible fill.

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