Editing Vacuum (section)

== Measurement ==
{{Main|Pressure measurement}}

The quality of a vacuum is indicated by the amount of matter remaining in the system, so that a high quality vacuum is one with very little matter left in it. Vacuum is primarily measured by its [[absolute pressure]], but a complete characterization requires further parameters, such as [[temperature]] and chemical composition. One of the most important parameters is the [[mean free path]] (MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with each other. As the gas density decreases, the MFP increases, and when the MFP is longer than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of [[fluid mechanics]] do not apply. This vacuum state is called ''high vacuum'', and the study of fluid flows in this regime is called particle gas dynamics. The MFP of air at atmospheric pressure is very short, 70&nbsp;[[nanometer|nm]], but at 100&nbsp;[[millipascal|mPa]] (≈{{val|e=-3|u=[[Torr]]}}) the MFP of room temperature air is roughly 100&nbsp;mm, which is on the order of everyday objects such as [[vacuum tube]]s. The [[Crookes radiometer]] turns when the MFP is larger than the size of the vanes.

Vacuum quality is subdivided into ranges according to the technology required to achieve it or measure it. These ranges were defined in ISO 3529-1:2019 as shown in the following table (100&nbsp;Pa corresponds to 0.75&nbsp;Torr; Torr is a non-SI unit):
{| class="wikitable"
! scope=col | Pressure range
! scope=col | Definition
! scope=col | The reasoning for the definition of the ranges is as follows (typical circumstances):
|-
|Prevailing atmospheric pressure (31&nbsp;kPa to 110&nbsp;kPa) to 100&nbsp;Pa
|low  (rough) vacuum
|Pressure can be achieved by simple materials (e.g. regular steel) and positive displacement vacuum pumps; viscous flow regime for gases
|-
|<100 Pa to 0.1 Pa
|medium  (fine) vacuum
|Pressure can be achieved by elaborate materials (e.g. stainless steel) and positive displacement vacuum pumps; transitional flow regime for gases
|-
|<0.1 Pa to {{val|1|e=-6|u=Pa}}
|high vacuum (HV)
|Pressure can be achieved by elaborate materials (e.g. stainless steel), elastomer sealings and high vacuum pumps; molecular flow regime for gases
|-
|<{{val|1|e=-6|u=Pa}} to {{val|1|e=-9|u=Pa}}
|ultra-high vacuum (UHV)
|Pressure can be achieved by elaborate materials (e.g. low-carbon stainless steel), metal sealings, special surface preparations and cleaning, bake-out and high vacuum pumps; molecular flow regime for gases
|-
|below  {{val|1|e=-9|u=Pa}}
|extreme-high vacuum (XHV)
|Pressure can be achieved by sophisticated materials (e.g. vacuum fired low-carbon stainless steel, aluminium, copper-beryllium, titanium), metal sealings, special surface preparations and cleaning, bake-out and additional getter pumps; molecular flow regime for gases
|}
* '''Atmospheric pressure''' is variable but {{convert|101.325|and|100|kPa|mbar|comma=off}} are common [[Standard temperature and pressure|standard or reference pressures]].
* '''Deep space''' is generally much more empty than any artificial vacuum. It may or may not meet the definition of high vacuum above, depending on what region of space and astronomical bodies are being considered. For example, the MFP of interplanetary space is smaller than the size of the Solar System, but larger than small planets and moons. As a result, solar winds exhibit continuum flow on the scale of the Solar System, but must be considered a bombardment of particles with respect to the Earth and Moon.
* '''Perfect vacuum''' is an ideal state of no particles at all. It cannot be achieved in a [[laboratory]], although there may be small volumes which, for a brief moment, happen to have no particles of matter in them. Even if all particles of matter were removed, there would still be [[photon]]s, as well as [[dark energy]], [[virtual particle]]s, and other aspects of the [[quantum vacuum]].

=== Relative versus absolute measurement ===
Vacuum is measured in units of [[pressure]], typically as a subtraction relative to ambient atmospheric pressure on Earth. But the amount of relative measurable vacuum varies with local conditions. On the surface of [[Venus]], where ground-level atmospheric pressure is much higher than on Earth, much higher relative vacuum readings would be possible. On the surface of the Moon with almost no atmosphere, it would be extremely difficult to create a measurable vacuum relative to the local environment.

Similarly, much higher than normal relative vacuum readings are possible deep in the Earth's ocean. A [[submarine]] maintaining an internal pressure of 1 atmosphere submerged to a depth of 10 atmospheres (98 metres; a 9.8-metre column of seawater has the equivalent weight of 1 atm) is effectively a vacuum chamber keeping out the crushing exterior water pressures, though the 1&nbsp;atm inside the submarine would not normally be considered a vacuum.

Therefore, to properly understand the following discussions of vacuum measurement, it is important that the reader assumes the relative measurements are being done on Earth at sea level, at exactly 1 atmosphere of ambient atmospheric pressure.

=== Measurements relative to 1&nbsp;atm ===
[[File:McLeod gauge 01.jpg|upright|thumb|A glass McLeod gauge, drained of mercury]]
The [[SI]] unit of pressure is the [[pascal (unit)|pascal]] (symbol Pa), but vacuum is often measured in [[torr]]s, named for an Italian physicist Torricelli (1608–1647). A torr is equal to the displacement of a millimeter of mercury ([[mmHg]]) in a [[manometer]] with 1&nbsp;torr equaling 133.3223684 pascals above absolute zero pressure. Vacuum is often also measured on the [[barometer|barometric]] scale or as a percentage of [[atmospheric pressure]] in [[bar (unit)|bars]] or [[atmosphere (unit)|atmospheres]]. Low vacuum is often measured in [[millimeters of mercury]] (mmHg) or pascals (Pa) below standard atmospheric pressure. "Below atmospheric" means that the absolute pressure is equal to the current atmospheric pressure.

In other words, most low vacuum gauges that read, for example 50.79&nbsp;Torr. Many inexpensive low vacuum gauges have a margin of error and may report a vacuum of 0&nbsp;Torr but in practice this generally requires a two-stage rotary vane or other medium type of vacuum pump to go much beyond (lower than) 1&nbsp;torr.

=== Measuring instruments ===
Many devices are used to measure the pressure in a vacuum, depending on what range of vacuum is needed.<ref>{{cite book| first=Moore | last=John H. | author2=Christopher Davis | author3=Michael A. Coplan | author4=Sandra Greer | name-list-style=amp | title=Building Scientific Apparatus | publisher=Westview Press | location=Boulder, Colorado | date=2002 | isbn=978-0-8133-4007-4 | oclc=50287675 }}{{page needed|date=May 2013}}</ref>

'''Hydrostatic''' gauges (such as the mercury column [[manometer]]) consist of a vertical column of liquid in a tube whose ends are exposed to different pressures. The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube. The simplest design is a closed-end U-shaped tube, one side of which is connected to the region of interest. Any fluid can be used, but [[Mercury (element)|mercury]] is preferred for its high density and low vapour pressure. Simple hydrostatic gauges can measure pressures ranging from 1&nbsp;torr (100&nbsp;Pa) to above atmospheric. An important variation is the [[McLeod gauge]] which isolates a known volume of vacuum and compresses it to multiply the height variation of the liquid column. The McLeod gauge can measure vacuums as high as 10<sup>−6</sup>&nbsp;torr (0.1&nbsp;mPa), which is the lowest direct measurement of pressure that is possible with current technology. Other vacuum gauges can measure lower pressures, but only indirectly by measurement of other pressure-controlled properties. These indirect measurements must be calibrated via a direct measurement, most commonly a McLeod gauge.<ref name=measure>{{cite book| first=Thomas G. | last=Beckwith | author2=Roy D. Marangoni | author3=John H. Lienhard V | name-list-style=amp | date=1993 | title=Mechanical Measurements | edition=Fifth | publisher=Addison-Wesley | location=Reading, Massachusetts | isbn=978-0-201-56947-6 | pages=591–595 | chapter=Measurement of Low Pressures }}</ref>

The kenotometer is a particular type of hydrostatic gauge, typically used in power plants using steam turbines. The kenotometer measures the vacuum in the steam space of the condenser, that is, the exhaust of the last stage of the turbine.<ref>{{cite web | url=http://www.ephf.ca/blog.asp?id=74 | title=Kenotometer Vacuum Gauge | publisher=Edmonton Power Historical Foundation | date=22 November 2013 | access-date=3 February 2014}}</ref>

'''Mechanical''' or '''elastic''' gauges depend on a Bourdon tube, diaphragm, or capsule, usually made of metal, which will change shape in response to the pressure of the region in question. A variation on this idea is the '''capacitance manometer''', in which the diaphragm makes up a part of a capacitor. A change in pressure leads to the flexure of the diaphragm, which results in a change in capacitance. These gauges are effective from 10<sup>3</sup>&nbsp;torr to 10<sup>−4</sup>&nbsp;torr, and beyond.

'''Thermal conductivity''' gauges rely on the fact that the ability of a gas to conduct heat decreases with pressure. In this type of gauge, a wire filament is heated by running current through it. A  [[Thermocouple#Thermocouple as vacuum gauge|thermocouple]] or [[Resistance Temperature Detector]] (RTD) can then be used to measure the temperature of the filament. This temperature is dependent on the rate at which the filament loses heat to the surrounding gas, and therefore on the thermal conductivity. A common variant is the [[Pirani gauge]] which uses a single platinum filament as both the heated element and RTD. These gauges are accurate from 10&nbsp;torr to 10<sup>−3</sup>&nbsp;torr, but they are sensitive to the chemical composition of the gases being measured.

'''[[Ionization gauge]]s''' are used in ultrahigh vacuum. They 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>−3</sup>&nbsp;torr to 10<sup>−10</sup> torr. The principle behind [[cold cathode]] version is the same, except that electrons are produced in a discharge created by a high voltage electrical discharge. Cold cathode gauges are accurate from 10<sup>−2</sup>&nbsp;torr to 10<sup>−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 = 978-0-442-00522-1 | pages = 1278–1284 | article=Vacuum Techniques}}</ref>