Editing Pressure measurement (section)

==Instruments==
[[File:Manometer anim 01.gif|thumb|A [[pressure gauge]] pointer moving over its full range]]
[[Image:druckmessumformer.jpg|right|thumb|200px|Pressure transmitter]]
[[Image:Digital Pressure Sensor.jpg|right|thumb|200px|Digital air pressure sensor]]
[[Image:Digital Barometer Sensor.jpg|right|thumb|200px|Miniature digital barometric pressure sensor]]
[[File:Pressure sensor chip.jpg|thumb|Front and back of a silicon pressure sensor chip. Note the etched depression in the front; the sensitive area is extremely thin. The back side shows the circuitry, and rectangular contact pads at top and bottom. Size: 4×4 mm.]]

A '''pressure sensor''' is a device for pressure measurement of [[gas]]es or [[liquids]]. 
Pressure sensors can alternatively be called '''pressure transducers''', '''pressure transmitters''', '''pressure senders''', '''pressure indicators''', '''piezometers''' and '''manometers''', among other names.

Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a [[transducer]]; it generates a signal as a [[Function (mathematics)|function]] of the pressure imposed. 

Pressure sensors can vary drastically in technology, design, performance, application suitability and cost. A conservative estimate would be that there may be over 50 technologies and at least 300 companies making pressure sensors worldwide. There is also a category of pressure sensors that are designed to measure in a dynamic mode for capturing very high speed changes in pressure. Example applications for this type of sensor would be in the measuring of combustion pressure in an engine cylinder or in a gas turbine. These sensors are commonly manufactured out of [[piezoelectric]] materials such as quartz.

Some pressure sensors are [[pressure switch]]es, which turn on or off at a particular pressure. For example, a water pump can be controlled by a pressure switch so that it starts when water is released from the system, reducing the pressure in a reservoir.

Pressure range, sensitivity, dynamic response and cost all vary by several orders of magnitude from one instrument design to the next. The oldest type is the liquid column (a vertical tube filled with mercury) manometer invented by [[Evangelista Torricelli]] in 1643.  The U-Tube was invented by [[Christiaan Huygens]] in 1661.

There are two basic categories of analog pressure sensors: force collector and other types.

; Force collector types: These types of electronic pressure sensors generally use a force collector (such a diaphragm, piston, Bourdon tube, or bellows) to measure strain (or deflection) due to applied force  over an area (pressure).	

* '''''Piezoresistive strain gauge''''': Uses the [[piezoresistive]] effect of bonded or formed [[strain gauge]]s to detect strain due to an applied pressure, electrical resistance increasing as pressure deforms the material. Common technology types are silicon (monocrystalline), polysilicon thin film, bonded metal foil, thick film, [[Silicon on sapphire|silicon-on-sapphire]] and sputtered thin film. Generally, the strain gauges are connected to form a [[Wheatstone bridge]] circuit to maximize the output of the sensor and to reduce sensitivity to errors. This is the most commonly employed sensing technology for general purpose pressure measurement.	
*'''''Capacitive''''': Uses a diaphragm and pressure cavity to create a variable capacitor to detect strain due to applied pressure, capacitance decreasing as pressure deforms the diaphragm. Common technologies use metal, ceramic, and silicon diaphragms. Capacitive pressure sensors are being integrated into [[CMOS]] technology<ref>{{Cite journal |last1=Nagata |first1=Tomio |last2=Terabe |first2=Hiroaki |last3=Kuwahara |first3=Sirou |last4=Sakurai |first4=Shizuki |last5=Tabata |first5=Osamu |last6=Sugiyama |first6=Susumu |last7=Esashi |first7=Masayoshi |date=1992-08-01 |title=Digital compensated capacitive pressure sensor using CMOS technology for low-pressure measurements |url=https://dx.doi.org/10.1016/0924-4247%2892%2980189-A |journal=Sensors and Actuators A: Physical |language=en |volume=34 |issue=2 |pages=173–177 |doi=10.1016/0924-4247(92)80189-A |issn=0924-4247}}</ref> and it is being explored if thin [[2d materials|2D materials]] can be used as diaphragm material.<ref>{{Cite journal |last1=Lemme |first1=Max C. |last2=Wagner |first2=Stefan |last3=Lee |first3=Kangho |last4=Fan |first4=Xuge |last5=Verbiest |first5=Gerard J. |last6=Wittmann |first6=Sebastian |last7=Lukas |first7=Sebastian |last8=Dolleman |first8=Robin J. |last9=Niklaus |first9=Frank |last10=van der Zant |first10=Herre S. J. |last11=Duesberg |first11=Georg S. |last12=Steeneken |first12=Peter G. |date=2020-07-20 |title=Nanoelectromechanical Sensors Based on Suspended 2D Materials |journal=Research |language=en |volume=2020 |pages=1–25 |doi=10.34133/2020/8748602|pmid=32766550 |pmc=7388062 |bibcode=2020Resea202048602L }}</ref>
*'''''Electromagnetic''''': Measures the displacement of a diaphragm by means of changes in [[inductance]] (reluctance), [[LVDT|linear variable differential transformer (LVDT)]], [[Hall effect]], or by [[eddy current]] principle.
*'''''Piezoelectric''''': Uses the [[piezoelectric]] effect in certain materials such as quartz to measure the strain upon the sensing mechanism due to pressure. This technology is commonly employed for the measurement of highly dynamic pressures. As the basic principle is dynamic, no static pressures can be measured with piezoelectric sensors.
*'''''Strain-Gauge''''': Strain gauge based pressure sensors also use a pressure sensitive element where metal strain gauges are glued on or thin-film gauges are applied on by sputtering. This measuring element can either be a diaphragm or for metal foil gauges measuring bodies in can-type can also be used. The big advantages of this monolithic can-type design are an improved rigidity and the capability to measure highest pressures of up to 15,000 bar. The electrical connection is normally done via a Wheatstone bridge, which allows for a good amplification of the signal and precise and constant measuring results.<ref>{{Cite web|url=https://www.hbm.com/en/7646/what-is-a-pressure-sensor/|title=What is a Pressure Sensor?|website=HBM|language=en|access-date=2018-05-09}}</ref>
*'''''Optical''''': Techniques include the use of the physical change of an optical fiber to detect strain due to applied pressure. A common example of this type utilizes [[Fiber Bragg Grating]]s. This technology is employed in challenging applications where the measurement may be highly remote, under high temperature, or may benefit from technologies inherently immune to electromagnetic interference.  Another analogous technique utilizes an elastic film constructed in layers that can change reflected wavelengths according to the applied pressure (strain).<ref>Elastic hologram' pages 113-117, Proc. of the IGC 2010, {{ISBN|978-0-9566139-1-2}} here: http://www.dspace.cam.ac.uk/handle/1810/225960</ref>
*'''''Potentiometric''''': Uses the motion of a wiper along a resistive mechanism to detect the strain caused by applied pressure.

[[File:Ruska Instrument - Cat. No. 10 (image a).jpg|thumb|A force-balanced fused quartz Bourdon tube pressure sensor. The mirror that should be mounted to the armature is absent.]]
* '''''Force balancing''''': Force-balanced fused quartz Bourdon tubes use a spiral Bourdon tube to exert force on a pivoting armature containing a mirror, the reflection of a beam of light from the mirror senses the angular displacement and current is applied to electromagnets on the armature to balance the force from the tube and bring the angular displacement to zero, the current that is applied to the coils is used as the measurement. Due to the extremely stable and repeatable mechanical and thermal properties of fused quartz and the force balancing which eliminates most non-linear effects these sensors can be accurate to around 1[[Parts per million|PPM]] of full scale.<ref>{{cite journal |url=https://www.researchgate.net/publication/230966593_Characterization_of_quartz_Bourdon-type_high-pressure_transducers |title=Characterization of quartz Bourdon-type high-pressure transducers |journal=Metrologia |date=November 2005 |doi=10.1088/0026-1394/42/6/S20}}</ref> Due to the extremely fine fused quartz structures which are made by hand and require expert skill to construct these sensors are generally limited to scientific and calibration purposes. Non force-balancing sensors have lower accuracy and reading the angular displacement cannot be done with the same precision as a force-balancing measurement, although easier to construct due to the larger size these are no longer used.

; Other types: These types of electronic pressure sensors use other properties (such as density) to infer pressure of a gas, or liquid.  
	
*'''''Resonant''''': Uses the changes in [[resonant frequency]] in a sensing mechanism to measure stress, or changes in gas density, caused by applied pressure. This technology may be used in conjunction with a force collector, such as those in the category above. Alternatively, resonant technology may be employed by exposing the resonating element itself to the media, whereby the resonant frequency is dependent upon the density of the media. Sensors have been made out of vibrating wire, vibrating cylinders, quartz, and silicon [[Microelectromechanical systems|MEMS]]. Generally, this technology is considered to provide very stable readings over time. The squeeze-film pressure sensor is a type of MEMS resonant pressure sensor that operates by a thin membrane that compresses a thin film of gas at high frequency. Since the compressibility and stiffness of the gas film are pressure dependent, the resonance frequency of the squeeze-film pressure sensor is used as a measure of the gas pressure.<ref>{{Cite journal |last1=Andrews |first1=M. K. |last2=Turner |first2=G. C. |last3=Harris |first3=P. D. |last4=Harris |first4=I. M. |date=1993-05-01 |title=A resonant pressure sensor based on a squeezed film of gas |url=https://dx.doi.org/10.1016/0924-4247%2893%2980196-N |journal=Sensors and Actuators A: Physical |language=en |volume=36 |issue=3 |pages=219–226 |doi=10.1016/0924-4247(93)80196-N |issn=0924-4247}}</ref><ref>{{Cite journal |last1=Dolleman |first1=Robin J. |last2=Davidovikj |first2=Dejan |last3=Cartamil-Bueno |first3=Santiago J. |last4=van der Zant |first4=Herre S. J. |last5=Steeneken |first5=Peter G. |date=2016-01-13 |title=Graphene Squeeze-Film Pressure Sensors |url=https://pubs.acs.org/doi/10.1021/acs.nanolett.5b04251 |journal=Nano Letters |language=en |volume=16 |issue=1 |pages=568–571 |doi=10.1021/acs.nanolett.5b04251 |pmid=26695136 |arxiv=1510.06919 |bibcode=2016NanoL..16..568D |s2cid=23331693 |issn=1530-6984}}</ref>
*'''''Thermal''''': Uses the changes in [[thermal conductivity]] of a gas due to density changes to measure pressure. A common example of this type is the [[Pirani gauge]].
*'''''Ionization''''': Measures the flow of charged gas particles (ions) which varies due to density changes to measure pressure. Common examples are the Hot and Cold Cathode gauges.

A pressure sensor, a resonant [[quartz crystal]] [[strain gauge]] with a [[Bourdon tube]] force collector, is the critical sensor of [[Deep-ocean Assessment and Reporting of Tsunamis|DART]].<ref>{{cite web |last1=Milburn |first1=Hugh |title=The NOAA DART II Description and Disclosure |url=https://www.ndbc.noaa.gov/dart/dart_ii_description_6_4_05.pdf |website=noaa.gov |publisher=NOAA, U.S. Government |access-date=4 April 2020}}</ref> DART detects [[tsunami]] waves from the bottom of the open ocean. It has a pressure resolution of approximately 1mm of water when measuring pressure at a depth of several kilometers.<ref>{{cite web |last1=Eble |first1=M. C. |last2=Gonzalez |first2=F. I. |title=Deep-Ocean Bottom Pressure Measurements in the Northeast Pacific |url=https://nctr.pmel.noaa.gov/Dart/Pdf/Eble_J_atmo_91.pdf |website=noaa.gov |publisher=NOAA, U.S. Government |access-date=4 April 2020}}</ref>

===Hydrostatic===
'''Hydrostatic''' gauges (such as the mercury column manometer) compare pressure to the hydrostatic force per unit area at the base of a column of fluid. Hydrostatic gauge measurements are independent of the type of gas being measured, and can be designed to have a very linear calibration. They have poor dynamic response.

====Piston====
Piston-type gauges counterbalance the pressure of a fluid with a spring (for example [[tire-pressure gauge]]s of comparatively low accuracy) or a solid weight, in which case it is known as a [[deadweight tester]] and may be used for calibration of other gauges.

====Liquid column (manometer)====
[[File:Utube.svg|thumb|upright|The difference in fluid height in a liquid-column manometer is proportional to the pressure difference: <math>h = \frac{P_a - P_o}{g \rho}</math>]]
[[File:Ring balance manometer VEB Junkalor Dessau.jpg|thumb|[[Ring balance manometer]]]]
Liquid-column gauges consist of a column of liquid in a tube whose ends are exposed to different pressures. The column will rise or fall until its weight (a force applied due to gravity) is in equilibrium with the pressure differential between the two ends of the tube (a force applied due to fluid pressure). A very simple version is a U-shaped tube half-full of liquid, one side of which is connected to the region of interest while the [[reference]] pressure (which might be the [[atmospheric pressure]] or a vacuum) is applied to the other. The difference in liquid levels represents the applied pressure. The pressure exerted by a column of fluid of height ''h'' and density ''ρ'' is given by the hydrostatic pressure equation, ''P'' = ''hgρ''. Therefore, the pressure difference between the applied pressure ''P<sub>a</sub>'' and the reference pressure ''P''<sub>0</sub> in a U-tube manometer can be found by solving {{nowrap|''P<sub>a</sub>'' − ''P''<sub>0</sub> {{=}} ''hgρ''}}. In other words, the pressure on either end of the liquid (shown in blue in the figure) must be balanced (since the liquid is static), and so {{nowrap|''P<sub>a</sub>'' {{=}}  ''P''<sub>0</sub> + ''hgρ''}}.

In most liquid-column measurements, the result of the measurement is the height ''h'', expressed typically in mm, cm, or inches. The ''h'' is also known as the [[pressure head]]. When expressed as a pressure head, pressure is specified in units of length and the measurement fluid must be specified. When accuracy is critical, the temperature of the measurement fluid must likewise be specified, because liquid density is a function of [[temperature]]. So, for example, pressure head might be written "742.2&nbsp;mm<sub>Hg</sub>" or "4.2&nbsp;in<sub>H<sub>2</sub>O</sub> at 59&nbsp;°F" for measurements taken with mercury or water as the manometric fluid respectively. The word "gauge" or "vacuum" may be added to such a measurement to distinguish between a pressure above or below the atmospheric pressure. Both mm of mercury and inches of water are common pressure heads, which can be converted to S.I. units of pressure using [[unit conversion]] and the above formulas.

If the fluid being measured is significantly dense, hydrostatic corrections may have to be made for the height between the moving surface of the manometer working fluid and the location where the pressure measurement is desired, except when measuring differential pressure of a fluid (for example, across an [[orifice plate]] or venturi), in which case the density ρ should be corrected by subtracting the density of the fluid being measured.<ref>{{cite book |title=Methods for the Measurement of Fluid Flow in Pipes, Part 1. Orifice Plates, Nozzles and Venturi Tubes |date=1964 |publisher=[[British Standards Institute]] |page=36}}</ref>

Although any fluid can be used, [[Mercury (element)|mercury]] is preferred for its high density (13.534&nbsp;g/cm<sup>3</sup>) and low [[Vapor pressure|vapour pressure]]. Its convex [[meniscus (liquid)|meniscus]] is advantageous since this means there will be no pressure errors from [[wetting]] the glass, though under exceptionally clean circumstances, the mercury will stick to glass and the barometer may become stuck (the mercury can sustain a [[Pressure#Negative pressures|negative absolute pressure]]) even under a strong vacuum.<ref>{{cite book |title=Manual of Barometry (WBAN) |date=1963 |publisher=U.S. Government Printing Office |pages=A295–A299 |url=https://www.analogweather.com/uploads/7/7/7/5/77750690/manual_of_barometry_1.pdf}}</ref> For low pressure differences, light oil or water are commonly used (the latter giving rise to units of measurement such as [[Inch of water|inches water gauge]] and [[Millimeters, water gauge|millimetres H<sub>2</sub>O]]). Liquid-column pressure gauges have a highly linear calibration. They have poor dynamic response because the fluid in the column may react slowly to a pressure change.

When measuring vacuum, the working liquid may evaporate and contaminate the vacuum if its [[vapor pressure]] is too high. When measuring liquid pressure, a loop filled with gas or a light fluid can isolate the liquids to prevent them from mixing, but this can be unnecessary, for example, when mercury is used as the manometer fluid to measure differential pressure of a fluid such as water. Simple hydrostatic gauges can measure pressures ranging from a few [[torr]]s (a few 100&nbsp;Pa) to a few atmospheres (approximately {{val|1,000,000|u=Pa}}).

A single-limb liquid-column manometer has a larger reservoir instead of one side of the U-tube and has a scale beside the narrower column. The column may be inclined to further amplify the liquid movement. Based on the use and structure, following types of manometers are used<ref name=manometer_types>[Was: "fluidengineering.co.nr/Manometer.htm". At 1/2010 that took me to bad link. Types of fluid Manometers]</ref>
# Simple manometer
# Micromanometer
# Differential manometer
# Inverted differential manometer

====McLeod gauge====
[[File:McLeod gauge.jpg|thumb|upright|A McLeod gauge, drained of mercury]]

A [[McLeod gauge]] isolates a sample of gas and compresses it in a modified mercury manometer until the pressure is a few [[Millimeter of mercury|millimetres of mercury]]. The technique is very slow and unsuited to continual monitoring, but is capable of good accuracy. Unlike other manometer gauges, the McLeod gauge reading is dependent on the composition of the gas, since the interpretation relies on the sample compressing as an [[ideal gas]]. Due to the compression process, the McLeod gauge completely ignores partial pressures from non-ideal vapors that condense, such as pump oils, mercury, and even water if compressed enough.

{{block indent | em = 1.5 | text =  '''Useful range''': from around 10<sup>−4</sup>&nbsp;Torr<ref>{{cite web |url=http://www.tau.ac.il/~phchlab/experiments/vacuum/Techniques_of_high_vacuum/Vacuum5.html |title=Techniques of High Vacuum |archive-url=https://web.archive.org/web/20060504124236/http://www.tau.ac.il/~phchlab/experiments/vacuum/Techniques_of_high_vacuum/Vacuum5.html |website=[[Tel Aviv University]] |date=2006-05-04 |archive-date=2006-05-04}}</ref>  (roughly 10<sup>−2</sup>&nbsp;Pa) to vacuums as high as 10<sup>−6</sup>&nbsp;Torr (0.1&nbsp;mPa),}}

0.1&nbsp;mPa 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-dependent properties. These indirect measurements must be calibrated to SI units by a direct measurement, most commonly a McLeod gauge.<ref>{{Cite book | first1=Thomas G. | last1=Beckwith | first2=Roy D. | last2=Marangoni | first3=John H. | last3=Lienhard V | name-list-style=amp | date=1993 | title=Mechanical Measurements | edition=Fifth | publisher=[[Addison-Wesley]] | location=Reading, MA | isbn=0-201-56947-7 | pages=591–595 | chapter=Measurement of Low Pressures }}</ref>

===Aneroid===
{{see also|Aneroid altimeter}}
'''Aneroid''' gauges are based on a metallic pressure-sensing element that flexes elastically under the effect of a pressure difference across the element. "Aneroid" means "without fluid", and the term originally distinguished these gauges from the hydrostatic gauges described above. However, aneroid gauges can be used to measure the pressure of a liquid as well as a gas, and they are not the only type of gauge that can operate without fluid. For this reason, they are often called '''mechanical''' gauges in modern language. Aneroid gauges are not dependent on the type of gas being measured, unlike thermal and ionization gauges, and are less likely to contaminate the system than hydrostatic gauges. The pressure sensing element may be a '''Bourdon tube''', a diaphragm, a capsule, or a set of bellows, which will change shape in response to the pressure of the region in question. The deflection of the pressure sensing element may be read by a linkage connected to a needle, or it may be read by a secondary transducer. The most common secondary transducers in modern vacuum gauges measure a change in capacitance due to the mechanical deflection. Gauges that rely on a change in capacitance are often referred to as capacitance manometers.

====Bourdon tube{{Anchor|Bourdon gauge}}====
[[File:Manometer 104026.jpg|thumb|Membrane-type manometer]]

The Bourdon pressure gauge uses the principle that a flattened tube<ref>{{Cite book |last=Hiscox |first=Gardner, D. |url=https://lccn.loc.gov/14011376 |title=Mechanical movements, powers and devices; a treatise describing mechanical movements and devices used in constructive and operative machinery and the mechanical arts, being practically a mechanical dictionary, commencing with a rudimentary description of the early known mechanical powers and detailing the various motions, appliances and inventions used in the mechanical arts to the present time, including a chapter on straight line movements, by Gardner D. Hiscox, |publisher=The Norman W. Henley publishing co. |year=1914 |edition=14 |location=New York |page=50 |lccn=14011376 |oclc=5069239}}</ref> tends to straighten or regain its circular form in cross-section when pressurized. (A [[party horn]] illustrates this principle.) This change in cross-section may be hardly noticeable, involving moderate [[Stress (mechanics)|stress]]es within the elastic range of easily workable materials. The [[Deformation (mechanics)|strain]] of the material of the tube is magnified by forming the tube into a C shape or even a helix, such that the entire tube tends to straighten out or uncoil elastically as it is pressurized. [[Eugène Bourdon]] patented his gauge in France in 1849, and it was widely adopted because of its superior simplicity, linearity, and accuracy; Bourdon is now part of the Baumer group and still manufacture Bourdon tube gauges in France.  Edward Ashcroft purchased Bourdon's American patent rights in 1852 and became a major manufacturer of gauges.  Also in 1849, Bernard Schaeffer in Magdeburg, Germany patented a successful diaphragm (see below) pressure gauge, which, together with the Bourdon gauge, revolutionized pressure measurement in industry.<ref>[http://www.archivingindustry.com/Indicator/discspring.htm ''The Engine Indicator'' Canadian Museum of Making]</ref> But in 1875 after Bourdon's patents expired, his company [[Budenberg Gauge Company|Schaeffer and Budenberg]] also manufactured Bourdon tube gauges.

[[File:Bourdon pressure gauge drawing.png|thumb|left|Illustration of a Bourdon pressure gauge by Gardner Dexter Hiscox, 1899]]

[[File:E Bourdons Patent Compound Gauge.jpg|thumb|upright|An original 19th century Eugene Bourdon compound gauge, reading pressure both below and above atmospheric with great sensitivity]]

In practice, a flattened thin-wall, closed-end tube is connected at the hollow end to a fixed pipe containing the fluid pressure to be measured. As the pressure increases, the closed end moves in an arc, and this motion is converted into the rotation of a (segment of a) gear by a connecting link that is usually adjustable. A small-diameter pinion gear is on the pointer shaft, so the motion is magnified further by the [[gear ratio]]. The positioning of the indicator card behind the pointer, the initial pointer shaft position, the linkage length and initial position, all provide means to calibrate the pointer to indicate the desired range of pressure for variations in the behavior of the Bourdon tube itself. Differential pressure can be measured by gauges containing two different Bourdon tubes, with connecting linkages (but is more usually measured via diaphragms or bellows and a balance system).

Bourdon tubes measures [[gauge pressure]], relative to ambient atmospheric pressure, as opposed to [[absolute pressure]]; vacuum is sensed as a reverse motion. Some aneroid barometers use Bourdon tubes closed at both ends (but most use diaphragms or capsules, see below).  When the measured pressure is rapidly pulsing, such as when the gauge is near a [[reciprocating pump]], an [[:wikt:orifice|orifice]] restriction in the connecting pipe is frequently used to avoid unnecessary wear on the gears and provide an average reading; when the whole gauge is subject to mechanical vibration, the case (including the pointer and dial) can be filled with an oil or [[glycerin]].  Typical high-quality modern gauges provide an accuracy of ±1% of span (Nominal diameter 100mm, Class 1 EN837-1), and a special high-accuracy gauge can be as accurate as 0.1% of full scale.<ref>{{Cite book|last = Boyes |first = Walt |title = Instrumentation Reference Book |edition = Fourth |publisher = [[Butterworth-Heinemann]] |date = 2008 |pages = 1312}}</ref>

Force-balanced fused quartz Bourdon tube sensors work on the same principle but uses the reflection of a beam of light from a mirror to sense the angular displacement and current is applied to electromagnets to balance the force of the tube and bring the angular displacement back to zero, the current that is applied to the coils is used as the measurement. Due to the extremely stable and repeatable mechanical and thermal properties of quartz and the force balancing which eliminates nearly all physical movement these sensors can be accurate to around 1&nbsp;[[Parts per million|PPM]] of full scale.<ref>{{Cite web|url=https://www.researchgate.net/publication/230966593|title=Characterization of quartz Bourdon-type high-pressure transducers|website=ResearchGate|access-date=2019-05-05}}</ref> Due to the extremely fine fused quartz structures which must be made by hand these sensors are generally limited to scientific and calibration purposes.

In the following illustrations of a compound gauge (vacuum and gauge pressure), the case and window has been removed to show only the dial, pointer and process connection. This particular gauge is a combination vacuum and pressure gauge used for automotive diagnosis:
[[File:WPGaugeFace-2.jpg|thumb|left|Indicator front with pointer and dial]]
[[File:WPPressGaugeMech-2.jpg|thumb|Mechanical side with Bourdon tube]]
* The left side of the face, used for measuring vacuum, is calibrated in [[inHg|inches of mercury]] on its outer scale and [[torr|centimetres of mercury]] on its inner scale
* The right portion of the face is used to measure [[Fuel pump (engine)|fuel pump]] pressure or [[turbocharger#boost|turbo boost]] and is scaled in  [[pound-force per square inch|pounds per square inch]] on its outer scale and kg/[[square centimeter|cm<sup>2</sup>]] on its inner scale.

{{Clear}}

Mechanical details include stationary and moving parts.
[[File:WPPressGaugeDetailHC.jpg|thumb|Mechanical details]]

Stationary parts:
{{ordered list | list-style-type = upper-alpha
| 1 = Receiver block. This joins the inlet pipe to the fixed end of the Bourdon tube (1) and secures the chassis plate (B). The two holes receive screws that secure the case.
| 2 = Chassis plate. The dial is attached to this. It contains bearing holes for the axles.
| 3 = Secondary chassis plate. It supports the outer ends of the axles.
| 4 = Posts to join and space the two chassis plates.
}}

Moving parts:
# Stationary end of Bourdon tube. This communicates with the inlet pipe through the receiver block.
# Moving end of Bourdon tube. This end is sealed.
# Pivot and pivot pin
# Link joining pivot pin to lever (5) with pins to allow joint rotation
# Lever, an extension of the sector gear (7)
# Sector gear axle pin
# Sector gear
# Indicator needle axle. This has a spur gear that engages the sector gear (7) and extends through the face to drive the indicator needle. Due to the short distance between the lever arm link boss and the pivot pin and the difference between the effective radius of the sector gear and that of the spur gear, any motion of the Bourdon tube is greatly amplified. A small motion of the tube results in a large motion of the indicator needle.
# Hair spring to preload the gear train to eliminate gear lash and [[hysteresis]]

====Diaphragm (membrane) {{anchor|Diaphragm|Membrane}}====
A second type of aneroid gauge uses [[Deflection (engineering)|deflection]] of a flexible [[Artificial membrane|membrane]] that separates regions of different pressure. The amount of deflection is repeatable for known pressures so the pressure can be determined by using calibration. The deformation of a thin diaphragm is dependent on the difference in pressure between its two faces.  The reference face can be open to atmosphere to measure gauge pressure, open to a second port to measure differential pressure, or can be sealed against a vacuum or other fixed reference pressure to measure absolute pressure. The deformation can be measured using mechanical, optical or capacitive techniques. Ceramic and metallic diaphragms are used.
The useful range is above 10<sup>−2</sup> [[Torr]] (roughly 1 [[Pascal (unit)|Pa]]).<ref name=schoonoverinc>[https://web.archive.org/web/20040102031317/http://schoonoverinc.com/PDFs/Sky%20Brochure.pdf Product brochure from Schoonover, Inc]</ref>
For absolute measurements, welded pressure capsules with diaphragms on either side are often used.
Membrane shapes include:
* Flat
* Corrugated
* Flattened tube
* Capsule

====Bellows====
[[File:Barograph 03.jpg|thumb|A pile of pressure capsules with corrugated diaphragms in an aneroid [[barograph]]]]

In gauges intended to sense small pressures or pressure differences, or require that an absolute pressure be measured, the gear train and needle may be driven by an enclosed and sealed bellows chamber, called an '''aneroid'''. (Early [[barometer]]s used a column of liquid such as water or the liquid metal [[mercury (element)|mercury]] suspended by a [[vacuum]].) This bellows configuration is used in aneroid barometers (barometers with an indicating needle and dial card), [[altimeter]]s, altitude recording [[barograph]]s, and the altitude [[telemetry]] instruments used in [[weather balloon]] [[radiosonde]]s. These devices use the sealed chamber as a reference pressure and are driven by the external pressure. Other sensitive aircraft instruments such as [[airspeed indicator|air speed indicators]] and rate of climb indicators ([[variometer]]s) have connections both to the internal part of the aneroid chamber and to an external enclosing chamber.

====Magnetic coupling====
These gauges use the attraction of two magnets to translate differential pressure into motion of a dial pointer. As differential pressure increases, a magnet attached to either a piston or rubber diaphragm moves. A rotary magnet that is attached to a pointer then moves in unison. To create different pressure ranges, the spring rate can be increased or decreased.

===Spinning-rotor gauge===
The spinning-rotor gauge works by measuring how a rotating ball is slowed by the viscosity of the gas being measured. The ball is made of steel and is magnetically levitated inside a steel tube closed at one end and exposed to the gas to be measured at the other. The ball is brought up to speed (about 2500 or 3800&nbsp;[[radian|rad]]/s), and the deceleration rate is measured after switching off the drive, by electromagnetic transducers.<ref>A. Chambers, ''Basic Vacuum Technology'', pp. 100–102, CRC Press, 1998. {{ISBN|0585254915}}.</ref> The range of the instrument is 5<sup>−5</sup> to 10<sup>2</sup>&nbsp;Pa (10<sup>3</sup>&nbsp;Pa with less accuracy). It is accurate and stable enough to be used as a [[secondary standard]]. During the last years this type of gauge became much more user friendly and easier to operate. In the past the instrument was famous for requiring some skill and knowledge to use correctly. For high accuracy measurements various corrections must be applied and the ball must be spun at a pressure well below the intended measurement pressure for five hours before using. It is most useful in calibration and research laboratories where high accuracy is required and qualified technicians are available.<ref>John F. O'Hanlon, ''A User's Guide to Vacuum Technology'', pp. 92–94, John Wiley & Sons, 2005. {{ISBN|0471467154}}.</ref> Insulation vacuum monitoring of cryogenic liquids is a well suited application for this system too. With the inexpensive and long term stable, weldable sensor, that can be separated from the more costly electronics, it is a perfect fit to all static vacuums.