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National vacuum standard

Name of Standard: National vacuum standard

Code designation: ECM 170-4/06-033

Year of publication: 2019

Department: nr. 6013 CMI RI Brno

Guarantor: Mgr. Martin Vičar, Ph.D.

Number of CMC lines to be provided:  2

Measurement modeMeasurement rangeUnitUncertainty of generated pressure p [Pa]
Gauge pressure1 to 15 000Pa0,008 Pa + 2,2·10⁻⁵ · p
Differential pressure1 to 15 000Pa0,006 Pa + 2,2·10⁻⁵ · p
Absolute pressure1 to 15 000Pa0,007 Pa + 2,2·10⁻⁵ · p

(p is the value of the generated pressure in pascals)

CMC magnitudes are expressed in terms of expanded uncertainty using an expansion factor of k = 2 (coverage probability of approximately 95%).
In the case of these instruments, the pressure force acting on the piston is compensated by an electronic dynamometer, which is calibrated by an internal reference weight before measurement. The weight of the piston and connecting parts is tared before measurement, so that only the forces due to the pressure action on the piston are registered during the measurement. The rotation of the piston in conventional digital piston pressure gauges causes periodic pressure fluctuations of the order of units of pascals, which already have a significant effect on the achievable uncertainty when measuring very low pressures. However, the piston pressure gauge, which uses a connection between a non-rotating piston and an electronic dynamometer, makes it possible to register even very small changes in the equilibrium state of the piston. However, while the rotating piston is self-centring during the measurement, in this case it is necessary to ensure its centring, e.g. by means of the gas flow in the conical gap between the piston and the housing. For the measurement of very small pressure differentials, the most suitable arrangement is a slot forming a gap extending from both ends of the housing to the centre where the lubricating gas is forced into the slot. When operating in absolute mode, the reference side of the gauge is pumped by a vacuum pump and the reference vacuum is measured by a suitable vacuum gauge. The national standard od rough vacuum is based on the above principle and can measure absolute pressures, gauge pressures and pressure differentials. Its range in absolute mode is 15 kPa to 1 Pa. It was built between 2002 and 2019.
The pressure generated by the national standard is calculated according to the definition equation:

Where stands for:

p .......…....


Kcal ........….

the instrument sensitivity constant,

Nm .....….....

comparator indication during measurement,

N0 .....….....

comparator indication during zeroing,

Ni .....….....

correction of comparator indication,


coefficient of thermal expansion of the piston,

αc ......….....

coefficient of thermal expansion of the cylinder,

pV …….…

residual vacuum pressure (in absolute mode),

T0 .......….....

reference temperature,

A20 ....

the size of the effective area of the piston-cylinder at the reference temperature.

The standard has successfully participated in one key comparison under the KCDB (two others are currently underway), two additional comparisons under the KCDB and one additional published international comparison.

Obrázková galerie: