Editing Pressure measurement (section)

===Ionization gauge===
'''Ionization gauges''' are the most sensitive gauges for very low pressures (also referred to as hard or high vacuum). They sense pressure indirectly by measuring the electrical ions produced when the gas is bombarded with electrons. Fewer ions will be produced by lower density gases. The calibration of an ion gauge is unstable and dependent on the nature of the gases being measured, which is not always known. They can be calibrated against a [[McLeod gauge]] which is much more stable and independent of gas chemistry.

[[Thermionic emission]] generates electrons, which collide with gas atoms and generate positive [[ion]]s.  The ions are attracted to a suitably [[Bias (electrical engineering)|biased]] electrode known as the collector.  The current in the collector is proportional to the rate of ionization, which is a function of the pressure in the system.  Hence, measuring the collector current gives the gas pressure.  There are several sub-types of ionization gauge.
{{block indent | em = 1.5 | text = '''Useful range''': 10<sup>−10</sup> - 10<sup>−3</sup> torr (roughly 10<sup>−8</sup> - 10<sup>−1</sup> Pa)}}

Most ion gauges come in two types: hot [[cathode]] and cold cathode. In the [[Hot-filament ionization gauge|hot cathode]] version, an electrically heated filament produces an electron beam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current depends on the number of ions, which depends on the pressure in the gauge. Hot cathode gauges are accurate from 10<sup>&minus;3</sup>&nbsp;Torr to 10<sup>&minus;10</sup>&nbsp;Torr. The principle behind [[cold cathode]] version is the same, except that electrons are produced in the discharge of a high voltage. Cold cathode gauges are accurate from 10<sup>&minus;2</sup>&nbsp;[[Torr]] to 10<sup>&minus;9</sup>&nbsp;Torr. Ionization gauge calibration is very sensitive to construction geometry, chemical composition of gases being measured, corrosion and surface deposits. Their calibration can be invalidated by activation at atmospheric pressure or low vacuum. The composition of gases at high vacuums will usually be unpredictable, so a [[mass spectrometer]] must be used in conjunction with the ionization gauge for accurate measurement.<ref>{{cite encyclopedia | editor=Robert M. Besançon | encyclopedia=The Encyclopedia of Physics | edition=3rd | date=1990 | publisher=Van Nostrand Reinhold, New York | isbn = 0-442-00522-9 | pages = 1278–1284 | article=Vacuum Techniques}}</ref>

====Hot cathode====
[[File:Bayard-Alpert gauge.jpg|thumb|Bayard–Alpert hot-cathode ionization gauge]]

A [[Hot-filament ionization gauge|hot-cathode ionization gauge]] is composed mainly of three electrodes acting together as a [[triode]], wherein the [[cathode]] is the filament. The three electrodes are a collector or plate, a [[Electrical filament|filament]], and a [[electrical grid|grid]]. The collector current is measured in [[picoampere]]s by an [[electrometer]]. The filament voltage to ground is usually at a potential of 30 volts, while the grid voltage at 180–210 volts DC, unless there is an optional [[electron bombardment]] feature, by heating the grid, which may have a high potential of approximately 565 volts.

The most common ion gauge is the hot-cathode '''Bayard–Alpert gauge''', with a small ion collector inside the grid. A glass envelope with an opening to the vacuum can surround the electrodes, but usually the '''nude gauge''' is inserted in the vacuum chamber directly, the pins being fed through a ceramic plate in the wall of the chamber. Hot-cathode gauges can be damaged or lose their calibration if they are exposed to atmospheric pressure or even low vacuum while hot. The measurements of a hot-cathode ionization gauge are always logarithmic.

Electrons emitted from the filament move several times in back-and-forth movements around the grid before finally entering the grid. During these movements, some electrons collide with a gaseous molecule to form a pair of an ion and an electron ([[electron ionization]]). The number of these [[ions]] is proportional to the gaseous molecule density multiplied by the electron current emitted from the filament, and these ions pour into the collector to form an ion current. Since the gaseous molecule density is proportional to the pressure, the pressure is estimated by measuring the ion current.

The low-pressure sensitivity of hot-cathode gauges is limited by the photoelectric effect. Electrons hitting the grid produce x-rays that produce photoelectric noise in the ion collector. This limits the range of older hot-cathode gauges to 10<sup>−8</sup>&nbsp;Torr and the Bayard–Alpert to about 10<sup>−10</sup>&nbsp;Torr. Additional wires at cathode potential in the line of sight between the ion collector and the grid prevent this effect. In the extraction type the ions are not attracted by a wire, but by an open cone. As the ions cannot decide which part of the cone to hit, they pass through the hole and form an ion beam. This ion beam can be passed on to a:
* [[Faraday cup]]
* [[Microchannel plate detector]] with Faraday cup
* [[Quadrupole mass analyzer]] with Faraday cup
* [[Quadrupole mass analyzer]] with microchannel plate detector and Faraday cup
* [[Ion len]]s and acceleration voltage and directed at a target to form a [[sputtering|sputter gun]]. In this case a valve lets gas into the grid-cage.

{{See also|Electron ionization}}

====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
|
|}