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| '''Pressure head''' is a term used in [[fluid mechanics]] to represent the [[internal energy]] of a [[fluid]] due to the [[pressure]] exerted on its container. It may also be called '''static pressure head''' or simply '''static head''' (but not '''static head pressure'''). It is mathematically expressed as:
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| :<math>\psi = \frac{p}{\gamma} = \frac{p}{\rho \, g}</math>
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| where
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| :<math>\psi</math> is pressure head ([[Length]], typically in units of m);
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| :<math>p</math> is fluid [[pressure]] ([[force]] per unit [[area]], often as [[Pascal (unit)|Pa]] units); and
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| :<math>\gamma</math> is the [[specific weight]] ([[force]] per unit [[volume]], typically N/m<sup>3</sup> units)
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| :<math>\rho</math> is the [[density]] of the fluid ([[mass]] per unit [[volume]], typically kg/m<sup>3</sup>)
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| :<math>g</math> is [[standard gravity|acceleration due to gravity]] (rate of change of velocity, given in m/s<sup>2</sup>)
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| Note that in this equation, the pressure term may be [[gauge pressure]] or [[absolute pressure]], depending on the design of the container and whether it is open to the ambient air or sealed without air.
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| ==Practical uses for pressure head==
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| [[Image:Venturifixed2.PNG|left|thumb|300px|A [[Venturi meter]] with two pressure instruments open to the ambient air. (<math>p>0</math> and <math>g>0</math>) If the meter is turned upside down, we say by convention that <math>g<0</math> and the fluid inside the vertical columns will pour out the two holes. See discussion below.]]
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| [[flow measurement|Fluid flow is measured]] with a wide variety of instruments. The [[venturi meter]] in the diagram on the left shows two columns of a measurement fluid at different heights. The height of each column of fluid is proportional to the pressure of the fluid. To demonstrate a classical measurement of pressure head, we could hypothetically replace the [[working fluid]] with another fluid having different [[physical properties]].
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| For example, if the original fluids were [[water]] and we replaced it with [[Mercury (element)|mercury]] at the same pressure, we would expect to see a rather different value for pressure head. In fact, the [[specific weight]] of water is 9.8 kN/m<sup>3</sup> and the specific weight of mercury is 133 kN/m<sup>3</sup>. So, for any particular measurement of pressure head, the height of the column of water will be about 13.6 times taller than the column of mercury would be (133/9.8 = 13.6). So if a water column meter reads "13.6 cm H<sub>2</sub>O", then an equivalent measurement is "1.00 cm Hg".
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| This example demonstrates why there is some confusion surrounding pressure head and its relationship to pressure. Scientists frequently use columns of water (or mercury) to measure pressure, since for a given fluid, pressure head is proportional to pressure. Measuring pressure in units of "mm of mercury" or "inches of water" makes sense for [[instrumentation]], but these raw measurements of head must frequently be converted to more convenient pressure units using the equations above to solve for pressure.
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| In summary, '''pressure head''' is a measurement of length, which can be converted to the units of pressure, as long as strict attention is paid to the density of the measurement fluid and the local value of g.
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| ==Implications for gravitational anomalies on <math>\psi\,</math>==
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| We would normally use pressure head calculations in areas in which <math>g</math> is constant. However, if the gravitational field fluctuates, we can prove that pressure head fluctuates with it.
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| * If we consider what would happen if gravity ''decreases'', we would expect the fluid in the venturi meter shown above to withdraw from the pipe ''up'' into the vertical columns. ''Pressure head is increased''.
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| * In the case of [[weightlessness]], the pressure head approaches ''[[infinity]]''. Fluid in the pipe may "leak out" of the top of the vertical columns (assuming <math>p>0</math>).
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| * To simulate negative gravity, we could turn the venturi meter shown above upside down. In this case gravity is negative, and we would expect the fluid in the pipe to "pour out" the vertical columns. Pressure head is negative (assuming <math>p>0</math>).
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| * If <math>p<0</math> and <math>g>0</math>, we observe that the ''pressure head is also negative'', and the ambient air is sucked into the columns shown in the venturi meter above. This is called a ''[[siphon]]'', and is caused by a partial [[vacuum]] inside the vertical columns. In many venturis, the column on the left has fluid in it (<math>\psi>0</math>), while only the column on the right is a siphon (<math>\psi<0</math>).
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| * If <math>p<0</math> and <math>g<0</math>, we observe that the pressure head is again positive, predicting that the venturi meter shown above would look the same, only upside down. In this situation, gravity causes the working fluid to plug the siphon holes, but the fluid does not leak out because the ambient pressure is greater than the pressure in the pipe.
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| * The above situations imply that the [[Bernoulli's principle|Bernoulli equation]], from which we obtain static pressure head, is extremely versatile.
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| ==Applications==
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| [[Image:VenturiFlow.png|right|thumb|A flow of air through a [[venturi meter]], showing the columns connected in a U-shape (a [[manometer]]) and partially filled with water. The meter is "read" as a differential pressure head in cm or inches of water.]]
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| ===Static===
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| A [[Barometer#Mercury_barometers|mercury barometer]] is one of the classic uses of [[static pressure]] head. Such barometers are an enclosed column of mercury standing vertically with gradations on the tube. The lower end of the tube is bath in a pool of mercury open to the ambient to measure the local [[atmospheric pressure]]. The reading of a [[mercury (element)|mercury]] barometer (in [[torr|mm of Hg]], for example) can be converted into an [[absolute pressure]] using the above equations.
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| If we had a column of mercury 767 mm high, we could calculate the atmospheric pressure as (767 mm)•(133 kN/m³) = 102 kPa. See the [[Torr#Manometric units of pressure|torr]] and [[pascal (unit)]] articles for barometric pressure measurements at standard conditions.
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| ===Differential===
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| The [[venturi meter]] and [[manometer]] (shown on the right) is a common type of [[flow meter]] which can be used in many liquid applications to convert differential pressure heads into [[volumetric flow rate]], linear fluid [[speed]], or [[mass flow rate]] using [[Bernoulli's principle]]. The reading of these meters (in inches of water, for example) can be converted into a differential, or [[gauge pressure]], using the above equations.
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| ==See also==
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| *[[Pressure measurement]]
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| *[[Bernoulli's_principle#Derivations_of_Bernoulli_equation|Derivations of Bernoulli equation]]
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| *[[Hydraulic head]] or [[velocity head]], which includes a component of pressure head
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| *[[Venturi effect]]
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| ==References==
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| <references/>
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| ==External links==
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| *[http://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.html Engineering Toolbox article on Specific Weight]
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| *[http://www.engineeringtoolbox.com/static-pressure-head-d_610.html Engineering Toolbox article on Static Pressure Head]
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| [[Category:Fluid dynamics]]
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| [[Category:Pressure]]
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The writer is recognized by the title of Figures Wunder. One of the extremely best issues in the world for me is to do aerobics and I've been performing it for fairly a while. North Dakota is where me and my husband live. Since she was 18 she's been working as a meter reader but she's always needed her own business.
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