**Name of Standard: **National weight standard

**Code designation:** ECM 120-1/00-007

**Year of publication:** 2000

**Department:** nr. 6012 CMI RI 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·m

^{2}·s

^{-1}, where metre and second are defined in terms of

*c*a Δν

_{Cs}.

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.

Standard number | 67 |

Mass | 1 kg + 0,148 mg |

Uncertainty | 0,021 mg |

Volume at 0 °C | 46,4352 cm^{3} |

Density at 0 °C | 21535,39 kg/m^{3} |

Coefficient of thermal bulk expansion | (25,869 + 0,00565 t) Î 10^{-6} °C^{-1} |

Alloy composition | 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 |

In 2021, a regular calibration was carried out at the International Bureau of Weights and Measures. The currently used weight value is 1 kg + 0.148 mg +- 0.021 mg. This weight value is similar to previous data and thus there is no significant change in weight with use. The magnitude of the uncertainty is significantly higher than the previous value of 0,004 mg, which is due to the gradual introduction of the new definition of the unit of mass into metrological practice.

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. |

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.