Editing Pressure measurement (section)

==Electronic pressure instruments{{anchor|Electronic}}==
; Metal strain gauge
:The [[strain gauge]] is generally glued (foil strain gauge) or deposited (thin-film strain gauge) onto a membrane. Membrane deflection due to pressure causes a resistance change in the strain gauge which can be electronically measured.
; Piezoresistive strain gauge
:Uses the [[piezoresistive]] effect of bonded or formed strain gauges to detect strain due to applied pressure.
; Piezoresistive silicon pressure sensor
:The sensor is generally a temperature compensated, [[piezoresistive]] silicon pressure sensor chosen for its excellent performance and long-term stability. Integral temperature compensation is provided over a range of 0–50&nbsp;°C using [[Laser trimming|laser-trimmed]] resistors. An additional laser-trimmed resistor is included to normalize pressure sensitivity variations by programming the gain of an external differential amplifier. This provides good sensitivity and long-term stability. The two ports of the sensor, apply pressure to the same single transducer, please see pressure flow diagram below.
[[File:Piezoresistive Silicon Pressure Sensor.png|thumb]]

This is an over-simplified diagram, but you can see the fundamental design of the internal ports in the sensor. The important item here to note is the "diaphragm" as this is the sensor itself. Is it slightly convex in shape (highly exaggerated in the drawing); this is important as it affects the accuracy of the sensor in use.

The shape of the sensor is important because it is calibrated to work in the direction of air flow as shown by the RED arrows. This is normal operation for the pressure sensor, providing a positive reading on the display of the digital pressure meter. Applying pressure in the reverse direction can induce errors in the results as the movement of the air pressure is trying to force the diaphragm to move in the opposite direction. The errors induced by this are small, but can be significant, and therefore it is always preferable to ensure that the more positive pressure is always applied to the positive (+ve) port and the lower pressure is applied to the negative (-ve) port, for normal 'gauge pressure' application. The same applies to measuring the difference between two vacuums, the larger vacuum should always be applied to the negative (-ve) port.
The measurement of pressure via the Wheatstone Bridge looks something like this....

[[File:Measuring Circuit.png|thumb|Application schematic]]
The effective electrical model of the transducer, together with a basic signal conditioning circuit, is shown in the application schematic. The pressure sensor is a fully active Wheatstone bridge which has been temperature compensated and offset adjusted by means of thick film, laser trimmed resistors. The excitation to the bridge is applied via a constant current. The low-level bridge output is at +O and -O, and the amplified span is set by the gain programming resistor (r). The electrical design is microprocessor controlled, which allows for calibration, the additional functions for the user, such as Scale Selection, Data Hold, Zero and Filter functions, the Record function that stores/displays MAX/MIN. 
; Capacitive
:Uses a diaphragm and pressure cavity to create a variable [[capacitor]] to detect strain due to applied pressure.
; Magnetic
:Measures the displacement of a diaphragm by means of changes in [[inductance]] (reluctance), [[LVDT]], [[Hall effect]], or by [[eddy current]] principle.
; Piezoelectric
:Uses the [[Piezoelectricity|piezoelectric]] effect in certain materials such as quartz to measure the strain upon the sensing mechanism due to pressure.
; Optical
:Uses the physical change of an optical fiber to detect strain due to applied pressure.
; Potentiometric
:Uses the motion of a wiper along a resistive mechanism to detect the strain caused by applied pressure.
; Resonant
:Uses the changes in [[resonant frequency]] in a sensing mechanism to measure stress, or changes in gas density, caused by applied pressure.

===Thermal conductivity===
Generally, as a [[real gas]] increases in density -which may indicate an increase in [[pressure]]- its ability to conduct heat increases. In this type of gauge, a wire [[Electrical filament|filament]] is heated by running current through it. A [[thermocouple]] or [[resistance thermometer]] (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<sup>&minus;3</sup>&nbsp;Torr to 10 [[Torr]], but their calibration is sensitive to the chemical composition of the gases being measured.

====Pirani (one wire)====
[[File:Pirani 02.jpg|thumb|Pirani vacuum gauge (open)]]
{{Main|Pirani gauge}}
A [[Pirani gauge]] consists of a metal wire open to the pressure being measured.  The wire is [[Joule heating|heated]] by a current flowing through it and cooled by the gas surrounding it.  If the gas pressure is reduced, the cooling effect will decrease, hence the equilibrium temperature of the wire will increase.  The [[Electrical resistance|resistance]] of the wire is a [[Electrical resistance#Temperature dependence|function of its temperature]]: by measuring the [[volt]]age across the wire and the [[Electric current|current]] flowing through it, the resistance (and so the gas pressure) can be determined. This type of gauge was invented by [[Marcello Pirani]].

====Two-wire====
In two-wire gauges, one wire coil is used as a heater, and the other is used to measure temperature due to [[convection]]. '''Thermocouple gauges''' and '''thermistor gauges''' work in this manner using a [[thermocouple]] or [[thermistor]], respectively, to measure the temperature of the heated wire.

===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
|
|}