Editing Pressure measurement (section)

===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.