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**Mass and related quantities

In the field of mass and related quantities (MRQ), the quantities covered by the national references are as follows :

Note: the quantities followed by an asterisk are not quantities deriving from mass (please refer to the table of units derived from mass; and related quantities for the expression of the units and the relationship diagram of the quantities units in the MRQ field to the base SI units as shown in figure 1.

Mass

Materialization of the mass unit

Materialization of the mass unit

The kilogram is the mass unit. It is the only unit that is still defined with one single material standard, the international prototype of the kilogram (written ) maintained at the Bureau International des Poids et Mesures (BIPM).

In France dissemination of the mass unit is carried out through the national prototype n° 35 (one of the forty copies of the international prototype made around 1885 and attributed to France in 1889 by drawing lots). Prototype n° 35 has participated in the three periodical verifications involving in 1889, 1946 and 1989.

Keeping, maintaining and disseminating the national prototype has required the implementation of one set of intermediate mass standards – ideally of different types – designed to reduce at the minimum its use.

High performance means for comparison

The LNE is equipped with one M-One Mass comparator (figure 2) developed by the Mettler company. This comparator has a range of 1 kg and resolution of 0,1 µg that allows making comparisons of masses in vacuum and under controlled atmosphere between 100 g and 1 kg.

This Comparator has been installed in a room dedicated to the mass references of the best level, between 1 mg and 10 kg. This room has, among other features, temperature maintained at (20 ± 0,1) °C, and mean air velocity at 0,07 m/s and one ISO 6 (# class 1 000) level of cleanliness.

Fig. 2. - M-One Mass Comparator in ISO 6 clean room
Fig. 2. - M-One Mass Comparator in ISO 6 clean room

Means for studying surfaces

Even though the mass standards are maintained and handled with great care, they are exposed to many sources of contaminations and alterations. Concerning this aspect, the topography of surfaces is a key parameter, as a hilly relief is likely to increase unnecessarily the active surface of the mass standards. To bring this parameter under control, the LNE-INM has developed jointly with the CNAM’s physics laboratory, one X reflectometer (figure 3) and one optical surface roughness standard. Both instruments that are based on the measurements of X-rays scattering on the one hand and visible rays scattering on the other hand, bring complementary performances and allow exploring different topographies.

Fig. 3. - X Reflectometer without its protective cage
Fig. 3. - X Reflectometer without its protective cage

In order to study the sorption (adsorption) phenomena, the LNE-INM has developed two benches

Fig. 4. - View of the introduction lock (left) and of the introduction chamber (right) of the thermal desorption spectrometry (TDS) device
Fig. 4. - View of the introduction lock (left) and of the introduction chamber (right) of the thermal desorption spectrometry (TDS) device

All these benches allow for instance, defining how to better realize or clean one mass standard

Transfer to the users

The LNE has implemented an entire suite of comparators in order to materialise and disseminate the multiple and sub-multiples of the kilogram. The equipment allows calibrating the standard masses between 1 mg and 5 000 kg.

Masses metrology is closely related to the measurement of density and volumes. At the best uncertainty level, the laboratory uses two benches based on the principle of hydrostatic weighing :

Force and torque

The force unit - the newton (N) – derives from mass, length and time. The method used to obtain the best uncertainties consists in hanging masses in the gravitation field which acceleration will have been measured.

In France the force reference comprises five benches (traction and compression):

Fig. 5. - Reference bench of force with a pyramid of sensors
Fig. 5. - Reference bench of force with a pyramid of sensors

The torque unit - the newton metre (N·m) – derives from mass, length and time. The method that allows obtaining the best uncertainties consists in hanging masses, placed in the gravitation field, at the end of a leverage of force.

The current references are made up of one bench with deadweight and leverage of force with capacity of 2 kN·m and one calibrating bench of very large torque with capacity of 200 kN·m (figure 6).

Fig. 6. - Reference bench with  200 kN·m torque
Fig. 6. - Reference bench with 200 kN·m torque

Viscosity

When a viscous fluid flows, internal friction appears: fluid viscosity is a measurement of this internal friction. Dynamic viscosity is generally expressed in milliPascal second (mPa·s), and kinematic viscosity – deducted from the previous by dividing it by the density of the fluid – in square millimetre by square second (mm²/s).

Viscosity scale is based on viscosity of distilled water at 20°C: the viscosity scale is built from this value by successive extrapolation using several capillary viscometers and fluids with different viscosities. This scale is consolidated at LNE through one metrology redundancy by doubling the number of viscometers.

Owing to the fragility of this scale and to the significant increase in the uncertainties in relation with the extrapolations, the LNE has developed one absolute falling ball viscometer. (figure 7). It allows to improve the uncertainties for dynamic viscosities in excess of 1 Pa·s.

Fig. 7. - Absolute viscometer: view of the tube and the falling ball device
Fig. 7. - Absolute viscometer: view of the tube and the falling ball device

Pressure

The unit of pressure - the pascal (Pa) – derives from mass, length and time. Traceability to the base quantities of the SI is carried out using the absolute pressure standard made up of two pressure balances with piston-cylinder units (figure 8) that comprises a range of pressures extending from 10 kPa to 1 MPa.

Fig. 8. - Reference balance of absolute pressure
Fig. 8. - Reference balance of absolute pressure

Based on this primary standard, the LNE has implemented a whole suite of references that cover a range of static pressures extending between 10-6 Pa and 1 GPa :

A certain number of industrial processes require measuring the dynamic pressures. In order to provide the traceability required by these measures, the LNE-ENSAM (Laboratory of dynamic metrology) has developed references for dynamic pressures. These are made up of :

The figure 9 shows the ranges of pressures and frequencies covered by the references of dynamic pressures at the LNE-ENSAM.

Fig. 9. - Field covered by the references of dynamic pressure
Fig. 9. - Field covered by the references of dynamic pressure

Flow

Fluid flow (water)

In this field, the LNE-Cetiat has implemented the national reference. It consists in one calibration bench based on the gravimetric method. The bench is made up of three measurement lines assembled in parallel, each fitted with one weighing tank (figure 10). The operating regime of this bench extends from 0,008 m3/h à 36 m3/h with temperatures ranging from 15 °C to 90 °C and pressures extending from 1 bar (0,1 MPa) and 3 bar (0,3 MPa).

Fig. 10. - Fluid (water) flowmetering bench with the three weighing tanks (front)
Fig. 10. - Fluid (water) flowmetering bench with the three weighing tanks (front)

Gas flow

In the field of gas flowmetering, the specificity of the traceability chain set up in France relies on the use of the cylindrical-throat Critical Flow Venturi Nozzle (figure 11).

Fig. 11. - Cross section of Cylindrical-throat Critical Flow Venturi Nozzle
Fig. 11. - Cross section of Cylindrical-throat Critical Flow Venturi Nozzle

At the best level of uncertainty, the nozzles are individually calibrated on the primary “PISCINE” bench (figure 12) of the LNE-LADG using natural gas between 9 m3/h and 10 000 m3/h with pressures ranging from 6 bar (0,6 MPa) to 55 bar (5,5 MPa). Nozzles are calibrated according to the primary PVTt (Pressure Volume Temperature time) method that consists in defining the discharge coefficient of the nozzles (CD) from thermodynamic parameters and time.

Fig. 12. - View of the primary calibrating bench and the cylindrical-throat nozzles
Fig. 12. - View of the primary calibrating bench and the cylindrical-throat nozzles

Following calibration at best level, the nozzles are used as transfer standards in air and gas on the secondary benches of the LADG to disseminate the “gas flow rate” quantity. The means that have been used allow covering flow rates ranging between 0,01 m3/h et 90 000 m3/h ou de 3,410-6 kg/s to 30 kg/s.

The LNE has also implemented references to cover the low gas flow rates, more particularly, to include the needs linked wit the dynamic preparation of the gaseous mixtures. Three gravimetric benches have been constructed and allow to cover a range of mass flows between 0,03 mg/s and 700 mg/s (cf. figure 13).

Fig. 13. - Calibration bench for mass flows of gases
Fig. 13. - Calibration bench for mass flows of gases

Air speed

In the field of anemometry, the national reference has been implemented by the LNE-Cetiat. It comprises two benches:

Fig. 14. - View of the test section of the “high speed” wind tunnel showing the probe of the laser Doppler anemometer (front)
Fig. 14. - View of the test section of the “high speed” wind tunnel showing the probe of the laser Doppler anemometer (front)

Sound pressure

In the field of sound pressure, focus is generally placed on the variations of pressure. Between the threshold of audibility and the threshold of painful hearing, the human ear perceives a range of pressures covering 6 decades. That’s why one logarithmic scale is used to define the sound pressure level : Lp = 20 x log (p / p0) with p lmeasured pressure and p0 lreference sound pressure that approximately corresponds to the lowest pressure that can be detected at the frequency of 1 kHz by an individual with good hearing.

In acoustics, the basic sensor is the microphone. It converts the variations of the incident pressure into one electrical signal. These sensors are calibrated in cavity through the reciprocity method. The LNE has also set up the means for free field calibration of the microphones inside one anechoic chamber (cf. figure 15).

Fig. 15. - Large anechoic chamber
Fig. 15. - Large anechoic chamber

Acceleration

The LNE has implemented the national reference of accelerometry. It is structured around one sinusoidal exciter and one Michelson interferometer (figure 16). The field of measure covers frequencies ranging from 10 Hz to 10 kHz.

Fig. 16. - Sinusoidal vibration bench using the absolute mean and high frequencies method
Fig. 16. - Sinusoidal vibration bench using the absolute mean and high frequencies method