Editing Pressure measurement (section)

==Applications==
[[File:Датчик давления.JPG|thumb|right|300px|Industrial wireless pressure sensor]]
There are many applications for pressure sensors:

*'''Pressure sensing'''
This is where the measurement of interest is [[pressure]], expressed as a [[force]] per unit area. This is useful in weather instrumentation, aircraft, automobiles, and any other machinery that has pressure functionality implemented.

*'''Altitude sensing'''
{{further|Pressure altimeter}}
This is useful in aircraft, rockets, satellites, weather balloons, and many other applications. All these applications make use of the relationship between changes in pressure relative to the altitude. This relationship is governed by the following equation:<ref>http://www.wrh.noaa.gov/slc/projects/wxcalc/formulas/pressureAltitude.pdf {{Webarchive|url=https://web.archive.org/web/20170703083618/http://www.wrh.noaa.gov/slc/projects/wxcalc/formulas/pressureAltitude.pdf |date=2017-07-03 }} National Oceanic and Atmospheric Administration</ref>
<math display="block">h = (1-(P/P_\mathrm{ref})^{0.190284}) \times 145366.45\mathrm{ ft}</math>
This equation is calibrated for an [[altimeter]], up to 36,090&nbsp;feet (11,000&nbsp;m). Outside that range, an error will be introduced which can be calculated differently for each different pressure sensor. These error calculations will factor in the error introduced by the change in temperature as we go up.

Barometric pressure sensors can have an altitude resolution of less than 1&nbsp;meter, which is significantly better than GPS systems (about 20&nbsp;meters altitude resolution). In navigation applications altimeters are used to distinguish between stacked road levels for car navigation and floor levels in buildings for pedestrian navigation.

*'''Flow sensing'''
This is the use of pressure sensors in conjunction with the [[venturi effect]] to measure flow. Differential pressure is measured between two segments of a venturi tube that have a different aperture. The pressure difference between the two segments is directly proportional to the flow rate through the venturi tube. A low pressure sensor is almost always required as the pressure difference is relatively small.

*'''Level / depth sensing''' {{anchor|Level}}
{{further|Level sensor#Pressure|Depth gauge}}
A pressure sensor may also be used to calculate the level of a fluid. This technique is commonly employed to measure the depth of a submerged body (such as a diver or submarine), or level of contents in a tank (such as in a water tower). For most practical purposes, fluid level is directly proportional to pressure. In the case of fresh water where the contents are under atmospheric pressure, 1psi = 27.7 inH<sub>2</sub>O / 1Pa = 9.81 mmH<sub>2</sub>O. The basic equation for such a measurement is
<math display="block">P = \rho gh</math>
where ''P'' = pressure, ''ρ'' = density of the fluid, ''g'' = standard gravity, ''h'' = height of fluid column above pressure sensor

*'''Leak testing'''
A pressure sensor may be used to sense the decay of pressure due to a system leak. This is commonly done by either comparison to a known leak using differential pressure, or by means of utilizing the pressure sensor to measure pressure change over time.

*'''Groundwater measurement'''
[[File:Zone de l'union6476.jpg|thumb|right|Above-ground casing of a piezometer]]
[[File:Pore pressure measurement.svg|thumb|100px|Symbol used in drawings]]
A '''piezometer''' is either a device used to measure liquid [[pressure]] in a system by measuring the height to which a column of the liquid rises against gravity, or a device which measures the pressure (more precisely, the [[Hydraulic head|piezometric head]]) of [[groundwater]]<ref name="Ch 9 Measurement of Groundwater Pressure">{{cite book | last=Dunnicliff|first=John|title=Geotechnical Instrumentation for Monitoring Field Performance|orig-year=1988 | year=1993 | publisher=Wiley-Interscience|isbn=0-471-00546-0|page=117}}</ref> at a specific point. A piezometer is designed to measure static pressures, and thus differs from a [[pitot tube]] by not being pointed into the fluid flow. Observation wells give some information on the water level in a formation, but must be read manually. Electrical pressure [[transducer]]s of several types can be read automatically, making data acquisition more convenient.

The first piezometers in [[geotechnical engineering]] were open wells or standpipes (sometimes called '''Casagrande piezometers''')<ref>{{cite book|last=Casagrande|first=A|title=Soil Mechanics in the design and Construction of the Logan Airport | year=1949|publisher=J. Boston Soc. Civil Eng., Vol 36, No. 2|pages=192–221}}</ref>  installed into an [[aquifer]]. A Casagrande piezometer will typically have a solid casing down to the depth of interest, and a slotted or screened casing within the zone where water pressure is being measured. The casing is sealed into the drillhole with clay, bentonite or concrete to prevent surface water from contaminating the groundwater supply. In an unconfined aquifer, the water level in the piezometer would not be exactly coincident with the [[water table]], especially when the vertical component of flow velocity is significant. In a [[artesian aquifer|confined aquifer under artesian conditions]], the water level in the piezometer indicates the pressure in the aquifer, but not necessarily the water table.<ref>''Manual on Suburface Investigations'', 1988, [[American Association of State Highway and Transportation Officials]] page 182</ref> Piezometer wells can be much smaller in diameter than production wells, and a 5&nbsp;cm diameter standpipe is common.

Piezometers in durable casings can be buried or pushed into the ground to measure the groundwater pressure at the point of installation. The pressure gauges (transducer) can be vibrating-wire, pneumatic, or strain-gauge in operation, converting pressure into an electrical signal. These piezometers are cabled to the surface where they can be read by [[data logger]]s or portable readout units, allowing faster or more frequent reading than is possible with open standpipe piezometers.