# SI units in Finland, mass and pressure

This page is part of a series detailing the SI units and our calibration services. This article centers around mass and pressure.

For more information about mass and pressure calibrations, click here.

## 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 m^{2} s^{−1}, where the metre and the second are defined in terms of *c* and ∆*ν*_{Cs}.

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/m^{2}, 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.

## Simple facts about units of mass and pressure

**Q1: What is the SI system?**

The International System of units SI or the SI system, often just called "SI" (short for "Système International d'Unités," which is French for "International System of Units."), is a universal system for measurements.

It's a set of standard units that scientists and people all over the world use to measure things like length, mass, but also temperature. Using the same system makes it easier to understand and share information, just like when people share a common language.

**Q2: How is the kilogram defined now, and has the definition changed over time?**

At present, the kilogram is defined using a constant from physics called the Planck constant. Before, it was defined by a physical metal object called the "International Prototype of the Kilogram." The definition has changed from a physical object to a constant in nature.

**Q3:Why was the definition changed?**

It was changed because using a constant in nature has several benefits:

- Stability: Constants in nature don't change over time, so the measurement remains consistent.
- Universality: Natural constants are the same everywhere in the universe. This means measurements based on them can be replicated accurately anywhere.
- No Physical Degradation: Unlike physical objects that can wear out, get damaged, or change over time, natural constants remain unchanged.
- Precision: Using constants allows for more precise and accurate measurements.
- Independence: There's no need to rely on a single physical object as a reference, which can be lost or altered.

Using a constant in nature ensures that measurements are stable, universal, and precise.

**Q4: What is the unit of pressure in the SI system?**

The unit of pressure in the SI system is pascal. It’s symbol is Pa. As a derived unit, it can also be expressed in terms of SI base units: kg m^{-1} s^{-2}. In simple terms, pressure reading tells how much force is applied over a certain area. Imagine pressing your hand against a wall; the pressure given in terms of pascals would tell how hard you're pushing and over how big of a spot.

**Q5: Why is pressure measured?**

Pressure shows how force is distributed over an area. To ensure safety of various structures and systems, it is vital to know the pressure affecting them to avoid breakdowns and explosions. As an example, too low air pressure in a car tyre increase fuel consumption and cause damage for tyre while too high pressure may explode the whole tyre.