Editing Pressure measurement (section)

===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>