Vacuum plays a central role in many industries, from manufacturing to research. In this guide, we explain the methods, units and scales for measuring vacuum, to help you understand vacuum better.

Our vacuum converter makes it easy to quickly convert from kilopascals (kPa) to other common units such as millimeters of mercury (mmHg), Torr, PSI and bar.

Vacuum is defined as a state where the pressure is lower than atmospheric pressure. This results in the amount of air or gas being reduced, creating a negative pressure relative to the surroundings.

High vacuum is a state of extremely low pressure, between 10⁻³ mbar (0.001 mbar) and 10⁻⁷ mbar (0.0000001 mbar).

In high vacuum, there are very few gas molecules, creating an environment of low air density and where collisions between molecules are rare. The low molecular density results in long mean free paths for particles, and gas release from surfaces (desorption) can affect stability.

To create a high vacuum, a two-stage process is often used. First, the pressure is reduced by forepumps, such as rotary vane pumps, to a low vacuum of around 1 mbar. Then, high vacuum pumps, such as turbomolecular pumps, take over and reduce the pressure to high vacuum levels.

High vacuum is useful in industries and research where purity and low pressures are crucial:

• Semiconductor production: The manufacture of microchips and electronic components requires extremely clean environments that can only be achieved in high vacuum.

• Surface coating (PVD and CVD): Thin films of materials such as metal or ceramics are applied to surfaces in a vacuum to create protective or functional layer.

• Research and particle accelerators: Vacuum environments are used to minimize interference from gas molecules in physics and chemistry experiments.

• Space technology: Simulation of the vacuum conditions of space in test chambers for satellites and spacecraft.

• Electron microscopy: High vacuum enables high-resolution images in advanced microscopes.
  

Vacuum is measured using different instruments depending on the vacuum level and the desired precision. Since vacuum represents a pressure lower than atmospheric pressure, the measurement requires adapted methods for low vacuum, medium vacuum, and high vacuum. 

Common units for vacuum measurement:

• Pascal (Pa): The SI unit used for pressure, where lower values indicate higher vacuum.     

• Millibar (mbar): Common in industrial applications.

• Torr (mmHg): Traditional unit mainly used in scientific applications.

• Bar (bar): General unit for pressure levels.
  

The choice of vacuum gauge depends on the vacuum level, application, and required accuracy.

• Low vacuum (Atmosphere to 1 mbar): Manometric gauges.

• Medium vacuum (1 mbar to 10⁻³ mbar): Mechanical gauges.

• High and ultra-high vacuum (10⁻³ to 10⁻¹⁰ mbar): Electronic gauges.
  

Manometric vacuum gauges use a column of liquid to measure pressure differences. An example is the U-tube manometer, where the difference in liquid level between two tubes – often filled with mercury – indicates the pressure. These gauges are robust and require no power, but they are unsuitable for high vacuums and require careful handling of liquids.

Mechanical vacuum gauges measure vacuum by registering the deformation of a flexible component, such as a diaphragm or Bourdon tube. When the pressure difference occurs, the component bends or deforms, and this movement is converted into a pressure reading. Mechanical gauges are mainly used in low and medium vacuums, from about 1 mbar to atmospheric pressure. These gauges are easy to use and cost-effective, but lack the accuracy required for high vacuums.

Electronic vacuum gauges are advanced instruments used to measure high and ultra-high vacuum. These gauges convert vacuum levels into electrical signals and operate based on principles such as heat transfer, electrical conductivity, or ionization of gas molecules. They offer high precision and a wide measurement range, from medium vacuum to ultra-high vacuum (a few Pascals to 10⁻¹⁰ Torr). The drawbacks are that they require regular calibration and are relatively expensive.

Common types of electronic vacuum gauges:

• Pirani gauge: Utilizes the thermal conductivity of gases. A heated wire loses heat to the surroundings, and this loss decreases at lower pressures because fewer gas molecules are available to carry away the heat.

• Cold cathode gauge: Creates plasma in the measurement chamber by ionizing gas molecules. The resulting ion current is proportional to the gas pressure.

• Hot cathode gauge: Uses thermionic emission from a heated filament. The emitted electrons collide with gas molecules and generate a measurable ion current that is linear with pressure.
  

To make it easier to compare and convert between units used to measure vacuum, we have compiled a clear conversion table.

The comparison table below makes it easy to convert between different pressure units and vacuum levels. It is divided into columns with different units, such as atmospheric pressure (ATM), water column, mercury column (mmHg), PSI and percent vacuum.

The International System of Units, or SI (Système International d'Unités), is the most widely used system of units of measurement globally, establishing standardized units for measuring physical quantities such as length, weight, time, and pressure. For pressure, the Pascal (Pa) is the official SI unit and is defined as 1 Newton per square meter (N/m²). The SI system ensures that scientific and technical measurements are consistent and comparable worldwide.