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.