The kilogram is the only basic unit of the Système International
still defined by a material artefact, namely a prototype made from platinum
iridium alloy deposited at the Bureau International des Poids et Mesures
(BIPM), Sèvres (France).
The French national mass standard, linked directly to the international
prototype is used to calibrate transfer standards made from either Pt-Ir
alloy or out of Alacrite XSH ( a non-magnetic, stainless super-alloy
whose high surface hardness enables one to achieve a mirror quality
polish).
These transfer standards guarantee the tracability of national references
ranging from 1 mg up to 5000 kg belonging to the BNM-LNE (Laboratoire
National d’Essais) which form part of the national mass calibration
chain approved by the BNM-COFRAC (Comité Français d’Accréditation).
Research activities:
The studies and research projects underway in the laboratories of
the mass and related quantities group at BNM-INM/CNAM have been selected
by the scientific committee of the Bureau National de Métrologie.
- Watt balance experiment
The two groups mass and related quantities and length are involved
in a large-scale research project supported by the Bureau National
de Métrologie (BNM), the objective of which is to link the
basic mass unit to Planck’s constant with a relative uncertainty
better than 10-8. The experiment, known as the watt balance, could
lead to a new definition of the mass unit, based on a fundamental
physical constant, thenceforth considered fixed. This contrasts with
the present definition, dating from over a century ago, based on a
single material prototype.
In addition to a permanent multidisciplinary nucleus formed by members
of three BNM national laboratories (BNM-INM/CNAM, BNM-LNE, BNM-SYRTE),
the project relies on significant collaborations both at the national
(ENSAM, ENSMP, LEMHE, LPCnam, LESIR, LGEP, LIRIS, LPUB,…) and
international levels (BIPM, METAS (Suisse), NIST (USA), NPL (Royaume
Uni), PTB (Allemagne), SP (Suède)).
Following feasability studies and development carried out in 2001
and 2002, the 2003 is geared towards two aspects of the project. On
the one hand we have made a demonstration model to verify the different
technological choices and study the bringing together of different
parts of the apparatus. On the other hand, a purpose-built laboratory,
destined to house the ultimate version of the balance has been constructed
on the site of the Laboratoire National d’Essais, located in
the Parisian suburbs at Trappes.
- Mass standards
The BNM-INM has acquired expertise in the realisation
of mass standards made from platinum-iridium alloy (like the prototype
of the kilogram), gold platinum alloys (low magnetic susceptibility,
for the watt balance), and the super-alloy Alacrite (used for the
realisation of transfer mass standards). To this end, it has developed
a polishing technique allowing one to reach a surface roughness
of about 10 nm in terms of root mean square height, while at the
same time guaranteeing an adjustment of the final 1 kg mass to within
±1 mg (i.e. a relative uncertainty of only 10-6). In parallel
with this studies, using tools developed at the institute, the group
has studied the influence of conditions of storage of mass standards,
their transfer between air and vacuum as well as tracing the origin
of surface contamination before and after the application of specific
cleaning and washing techniques.
- Influence of air composition upon the stability of mass standards
and the air buoyancy correction
The water vapour content of air plays a fundamental
role in mass metrology. It affects surface contamination of standards
and is one of the limiting factors in measurements of air vapour
density. One aspect of the study underway in our group is the use
of an optical gauge aimed at real-time monitoring of the water vapour
content in a mass comparator housing.
 
Other goals include a better knowledge of the relative
composition of ambiant air, notably, the influence of the molar fraction
of argon on the CIPM empirical formula used for the determination
of the air density. Lastly, we are investigating correlations between
high levels of carbon dioxide and the presence of methane so as to
improve our understanding of contamination of mass standards kept
in air.
- Mass standard comparator
The BNM-INM has built a single-pan kilogram comparator
allowing one to calibrate platinum-iridium kilograms with a combined
uncertainty close to 10 µg and a repeatability better than
1µg.
The group has designed and built a kilogram comparator
based on a flexure strip one-piece beam made from titanium-vanadium.
Calibrations and traceability:
As a national metrology institute, the BNM-INM/CNAM
is able to carry out calibrations linked to French national references.
The laboratories concerned by these links are, the
other NMIs that have no reference of their own, the laboratories of
the GIP BNM* and accredited laboratories for which a need for a link
to national references is stipulated.
* GIP BNM = the public sector umbrella group of laboratories
and ministries which make up the BNM
Calibration laboratories and people from industry should get in touch
with one of the accredited laboratories of which a list is available
from the COFRAC (French national accreditation committee).
Brief descriptions of calibrations available:
- Calibration of reference kilograms
- Calibration of microphones
International
collaborations:
The "mass and related quantities" group at BNM-INM is involved
in several international collaborations, in which it plays in some
cases the role of steering laboratory.
-
Steering of the project EUROMET Mass 519 "Correlations between
air humidity and anomalous of refractive index of air".
-
Steering of the project EUROMET Mass 551 "Study of several
roughness measurement techniques for the characterisation of mass
standard surface quality". This involves comparing and discussing
advantages and limitations specific to techniques used to characterize
surface roughness of mass standards.
-
Steering of the project EUROMET Mass 734 "Study of materials
for the realisation of mass standards”. This project is
linked with the watt balance experiments.
-
Participation in the project EUROMET Mass 509 Comparison of kilograms
made from platinum iridium, steered by NPL (UK).
-
Participation in the electro-acoustic activities CEI/CE29.
-
Participation in the absolute pressure comparison CCM.P-K2. Comparison
of absolute pressures from 10 kPa to 120 kPa, steered by NPL (UK).
Contacts:
- Michel LECOLLINET, lecturer, head of group michel.lecollinet
- Chouki ZERROUKI, lecturer, surface studies of mass standardszerrouki
- Naceur KHELIFA, research scientist, study of influence of environmental
conditions naceur.khelifa
- Patrick PINOT, research scientist, various studies and
coordination for BNM-INM of the BNM watt balance experiment patrick.pinot
- François VILLAR, research scientist, mechanical enginerring
studies for the watt balance francois.villar
- Stéphane MACE, research assistant, mechanical design and construction
BNM-INM stephane.mace
- Zaccari SILVESTRI, doctoral student, physico-chemical studies of mass
standard surfaces silvestri
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