Name of Standard: National weight standard
Code designation: ECM 120-1/00-007
Year of publication: 2000
Department: odd. 6012 ČMI OI Brno
Guarantor: Mgr. Jaroslav Zůda, Ph.D.
Number of CMC lines provided: 6
Among the basic quantities of the SI system of units is mass, the unit of which is the kilogram.
The kilogram, symbol kg, is the SI unit of mass. It is defined by fixing the numerical value of Planck's constant equal to 6,626 070 15 × 10-34, when expressed in units of J-s, which is equal to kg·m2·s-1, where metre and second are defined in terms of c a ΔνCs.
All weights are tied to this prototype through various calibrations. The national weight standards are closest to the prototype within each country.
After the division of the Czechoslovak Federal Republic into two independent states, the Czech Metrology Institute had to build a completely new workplace for primary mass standardization, which would provide the demanding tasks associated with ensuring continuity in the field of mass, which until then had been provided for the entire federation by the workplace in Bratislava. The Primary Mass Laboratory was equipped with standard equipment in 1994 as a result of the PHARE G24 project for the construction of a primary mass laboratory with the assistance of the Swiss Government. At that time it was about ensuring continuity at the secondary level. In 1995, the metrological continuity of the standards for the Czech Republic was resolved with the help of the Swiss institute OFMET (now METAS), which ensured the weight calibration of two 1 kg nominal weight standards made of austenitic steel.
In 1999, the platinum-iridium prototype of the kilogram with the designation 67 was obtained from the BIPM, which was declared the state weight standard of the Czech Republic in 2000 and thus became the starting point of the scheme of continuity of the Czech Republic's weight gauges. The standard is defined as the weight of a prototype platinum-iridium alloy whose weight value is determined by comparison with the international prototype kilogram, which is deposited with the International Bureau of Weights and Measures. The purpose of the standard is to transfer the unit of weight by means of comparator scales to working standards of austenitic steel. The standard has the shape of an equilateral cylinder with the following parameters:
|Mass||1 kg + 0,165 mg|
|Volume at 0 °C||46,4352 cm3|
|Density at 0 °C||21535,40 kg/m3|
Coefficient of thermal bulk expansion
|(25,869 + 0,00565 t) Î 10-6 °C-1|
|90 % Pt + 10 % Ir|
|Admixture||Rh + Pl + Ru < 0,2 %, Fe < 0,05 %, ostatní < 0,02 %|
|Shaepe|| Rovnostranný válec (h = d = 39 mm)|
|Surface roughness||Ra = 0,01 µm|
The etalon is kept in a special double glass cover on a base with three centering screws for fixing the top cover. For handling, special pliers with contact surfaces covered with material preventing damage to the surface of the prototype during gripping are provided. The standard is transported in a special tube which allows the prototype to be attached at five points. This prevents movement of the prototype during transport. The touch surfaces are covered with a special material.
In 2010, a regular calibration was carried out by the International Bureau of Weights and Measures. The currently used weight value is 1 kg + 0.188 mg ± 0.006 mg. Comparing with the previous value, we find that the weight value has increased slightly, which may be due to the gradual contamination of the surface. The higher uncertainty in the mass reading is due to the different calibration method of the International Bureau.
The national weight standard was used to calibrate two austenitic steel weights using a Mettler Toledo M-One comparator balance after transport back to the Czech Republic. The relationship used in the calibration is of the form
Where stands for:
weight of stainless steel weights,
the weight of the national standard,
indicates the difference in volume of the two weights,
the air density at which the calibration takes place and,.
the density of the weight that was used in calculating the mass scale of the comparator.
The density and volume of a state standard are indicated in its calibration certificate issued by the International Bureau of Weights and Measures. The density of a stainless steel weight can be determined by hydrostatic weighing, i.e. measuring the weight in another medium of known density. The weight is then subject to a different buoyant force and therefore has a different apparent mass. Knowing the density of the medium, we are then able to determine the density of the weight.
Using the new vacuum comparator, only one instrument can be used for such measurements. This is because this instrument allows measurements in both air and vacuum, where we can assume a zero density environment. This also allows us to measure the difference in actual masses. We can also measure in a pure gas environment, and thus without disturbing the surface layers due to, for example, air humidity. Further information about the instrument and the measurement method is given in a separate chapter.
Using such an accurate comparator, an uncertainty in the weight of the stainless steel weight of up to 0.04 mg for k = 1 can be achieved. The best value so far given in the CMC tables is 0.17 mg for k = 2, which is also the best value given in the calibration sheets for customers.
With the further development of mass measurement in vacuum, we can expect further improvements in the calibration capabilities of the mass laboratory with emphasis on a potential new definition of the unit of mass, where the primary realization of the unit is assumed to be in a vacuum environment.