# Vacuum Measurement

## Basics of Vacuum Measurement

• The importance of vacuum measurement lies in monitoring and controlling the vacuum inside a system.
• The pressure of a vacuum is typically measured in units of Torr, millibar (mbar), or Pascal (Pa), with 1 atmosphere of pressure equating to approximately 760 Torr, 1000 mbar, or 101,325 Pa.
• The states of vacuum are often categorised into low vacuum (also known as rough vacuum), medium vacuum, and high vacuum (also referred as ultra-high and extreme-high vacuum).

## Vacuum Gauges

• Instruments used to measure the pressure in a vacuum system are called vacuum gauges.
• Vacuum gauges operate on different principles and cover different ranges of vacuum pressure. Hence, there can be multiple gauges used in one vacuum system to cover the whole pressure range.

## Mechanical Gauges

• Mechanical gauges measure vacuum pressure by detecting the force exerted by the vacuum on a mechanical system.
• An example of a mechanical gauge is the Bourdon gauge, where pressure changes cause a flexible metallic tube to contract or expand - this movement is then converted into a dial or digital readout.
• Mechanical gauges are often used for rough vacuum measurement.

## Thermal Conductivity Gauges

• Thermal conductivity gauges rely on the fact that the rate of heat loss from a hot body is dependent on the density of the surrounding gas.
• The Pirani gauge and thermocouple gauge are examples of thermal conductivity gauges.
• These gauges are suitable for low to medium vacuum ranges.

## Ionisation Gauges

• Ionisation gauges operate based on the ionisation of gas molecules by an electron stream.
• There are two main types of ionisation gauges: hot cathode ionisation gauges and cold cathode ionisation gauges.
• Ionisation gauges are used to measure high and ultra-high vacuums.

## Vacuum Quality

• The quality of a vacuum is determined by various factors: the pressure, the quantity and type of gas molecules present, and the level of outgassing - the release of trapped or absorbed gases from materials inside the vacuum.
• A vacuum system must be carefully designed and correctly operated to minimise outgassing and maintain a high-quality vacuum.
• Vacuum quality is essential in applications such as Vacuum deposition, used in the creation of thin films and coatings in materials science and semiconductor manufacturing.