Editing Pressure measurement (section)

====Cold cathode====
[[File:Penning 01.jpg|thumb|Penning vacuum gauge (cut-away)]]

There are two subtypes of [[Cold cathode|cold-cathode]] ionization gauges: the '''Penning gauge''' (invented by [[Frans Michel Penning]]), and the '''inverted magnetron''', also called a '''Redhead gauge'''. The major difference between the two is the position of the [[anode]] with respect to the [[cathode]]. Neither has a filament, and each may require a [[direct current|DC]] potential of about 4 [[volt|kV]]  for operation. Inverted magnetrons can measure down to 1{{e|−12}} [[Torr]].

Likewise, cold-cathode gauges may be reluctant to start at very low pressures, in that the near-absence of a gas makes it difficult to establish an electrode current - in particular in Penning gauges, which use an axially symmetric magnetic field to create path lengths for electrons that are of the order of metres. In ambient air, suitable ion-pairs are ubiquitously formed by cosmic radiation; in a Penning gauge, design features are used to  ease the set-up of a discharge path. For example, the electrode of a Penning gauge is usually finely tapered to facilitate the field emission of electrons.

Maintenance cycles of cold cathode gauges are, in general, measured in years, depending on the gas type and pressure that they are operated in. Using a cold cathode gauge in gases with substantial organic components, such as pump oil fractions, can result in the growth of delicate carbon films and shards within the gauge that eventually either short-circuit the electrodes of the gauge or impede the generation of a discharge path.

{| class="wikitable mw-collapsible"
|+ Comparison of pressure measurement instruments<ref name="Harris1989">{{cite book|author=Nigel S. Harris|title=Modern Vacuum Practice|url=https://books.google.com/books?id=3nbVAAAAMAAJ|year=1989|publisher=McGraw-Hill|isbn=978-0-07-707099-1}}</ref>
! Physical phenomena
! Instrument
! Governing equation
! Limiting factors
! Practical pressure range
! Ideal accuracy
! Response time
|-
| Mechanical
| Liquid column manometer
| <math>\Delta P = \rho g h</math>
|
| atm. to 1 mbar
|
|
|-
| Mechanical
| Capsule dial gauge
|
| Friction
| 1000 to 1 mbar
| ±5% of full scale
| Slow
|-
| Mechanical
| Strain gauge
|
|
| 1000 to 1 mbar
|
| Fast
|-
| Mechanical
| Capacitance manometer
|
| Temperature fluctuations
| atm to 10<sup>−6</sup> mbar
| ±1% of reading
| Slower when filter mounted
|-
| Mechanical
| McLeod
| Boyle's law
|
| 10 to 10<sup>−3</sup> mbar
| ±10% of reading between 10<sup>−4</sup> and 5⋅10<sup>−2</sup> mbar
|
|-
| Transport
| Spinning rotor ([[Drag (physics)|drag]])
|
|
| 10<sup>−1</sup> to 10<sup>−7</sup> mbar
| ±2.5% of reading between 10<sup>−7</sup> and 10<sup>−2</sup> mbar
2.5 to 13.5% between 10<sup>−2</sup> and 1 mbar
|
|-
| Transport
| Pirani ([[Wheatstone bridge]])
|
| Thermal conductivity
| 1000 to 10<sup>−3</sup> mbar (const. temperature)
10 to 10<sup>−3</sup> mbar (const. voltage)
| ±6% of reading between 10<sup>−2</sup> and 10 mbar
| Fast
|-
| Transport
| Thermocouple ([[Seebeck effect]])
|
| Thermal conductivity
| 5 to 10<sup>−3</sup> mbar
| ±10% of reading between 10<sup>−2</sup> and 1 mbar
|
|-
| Ionization
| Cold cathode (Penning)
|
| Ionization yield
| 10<sup>−2</sup> to 10<sup>−7</sup> mbar
| +100 to -50% of reading
|
|-
| Ionization
| Hot cathode (ionization induced by thermionic emission)
|
| Low current measurement; parasitic x-ray emission
| 10<sup>−3</sup> to 10<sup>−10</sup> mbar
| ±10% between 10<sup>−7</sup> and 10<sup>−4</sup> mbar
±20% at 10<sup>−3</sup> and 10<sup>−9</sup> mbar
±100% at 10<sup>−10</sup> mbar
|
|}