# SI unit: Mass and pressure

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VTT

## Unit of mass: kilogram (kg)

​The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10−34 when expressed in the unit J s, which is equal to kg m2 s−1, where the metre and the second are defined in terms of c and ∆νCs.

Information on the practical realisation of the definition of the kilogram can be found in the ​Mise en pratique for the definition of the kilogram in the SI.

## Unit of pressure: pascal (Pa)

Pressure is the amount of force applied perpendicular to the surface of an object per unit area and the for it is p  (or P).

The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N/m2, or kg·m−1·s−2). Pascal is a so-called coherent derived unit in the SI with a special name and symbol.

## Calibration services for mass and pressure

### Calibration of weights

The measurement range of mass at VTT MIKES is 1 mg ... 2000 kg. The calibrations of weights are performed by using generally accepted weighing methods: the direct comparison method and the subdivision method. In the first method, the weight is directly compared to a standard and in the latter a set of weights is calibrated by using one or several weight standards.

MIKES is capable to calibrate weights of OIML classes E1, E2, and F1, whose nominal masses are at most 20 kg (E1), 50 kg (E2) and 2000 kg (F1). In addition, MIKES offers calibration services for weights of lower OIML classes, whose masses are between 10 kg and 2000 kg. MIKES also calibrates other weights such as weights of pressure balances. In the calibration certificate, the masses are given as conventional masses or as true masses. The smallest achievable measurement uncertainties in mass calibrations are presented in table 1. Weights whose nominal mass is 50 kg or bigger are calibrated at VTT MIKES Kajaani.

### Calibration of pressure measuring devices

MIKES has good capabilities to calibrate different measuring devices of pressure. The measuring range for gauge pressure is 0 ... 500 MPa and for absolute pressure 0.0005 Pa ... 1.75 MPa. The best measurement standards at MIKES are pressure balances, which are used to realise pressure according to its definition p = F / A, i.e. pressure is force divided by area. Pressure balances are used for gauge and negative gauge pressure measurements and for absolute pressure measurements.

In pressure ranges below the range of pressure balances, capacitive sensors and spinning rotor gauges are used as measurement standards. The lowest pressures (absolute pressures 0.0005 Pa ... 2 Pa) are calibrated by using spinning rotor gauges.

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### Heikki Kajastie

Research Scientist

### Kari Kyllönen

Research Technician

### Sari Saxholm

Senior Scientist
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