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| The '''vapour pressure of [[properties of water|water]]''' is the pressure at which [[water vapour]] is [[saturated vapor|in thermodynamic equilibrium with its condensed state]]. At higher pressures water would [[condense]]. The water [[vapour pressure]] is the [[partial pressure]] of [[water vapour]] in any gas mixture in equilibrium with solid or liquid water. As for other substances, water vapour pressure is a function of [[temperature]] and can be determined with [[Clausius–Clapeyron relation]].
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| ==Approximation formula ==
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| The vapour pressure of water may be approximated by the following relations (in order of increasing accuracy):
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| *:<math>P = \exp\left(20.386-\frac{5132\,\mathrm{K}}{T}\right)\,\mathrm{mmHg}</math>
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| *::where {{mvar|P}} is the vapour pressure ([[mmHg]]) and {{mvar|T}} is the temperature in [[kelvin]].
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| * Using the [[Antoine equation]]
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| *:<math>\log_{10}P = A - \frac{B}{C + T}</math>
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| *::where the temperature {{mvar|T}} is in degrees [[Celsius]] and the vapour pressure {{mvar|P}} is in mmHg. The constants are given as
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| {| cellpadding="4" rules="all" style="margin: 1em 0em; background: #ffffff; border: 2px solid #aaa;"
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| |- align="center" bgcolor="#f0f0f0"
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| !
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| !{{mvar|A}}
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| !{{mvar|B}}
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| !{{mvar|C}}
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| !{{math|''T''<sub>min</sub>}}, °C
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| !{{math|''T''<sub>max</sub>}}, °C
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| |- align="center"
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| | bgcolor="#f0f0f0" |Water||8.07131||1730.63||233.426||1||100
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| |- align="center"
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| | bgcolor="#f0f0f0" |Water||8.14019||1810.94||244.485||99||374
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| |}
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| ==Graphical pressure dependency on temperature==
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| [[File:Vapor Pressure of Water.png|thumb|center|800px|Vapour pressure diagrams of water; data taken from [[Dortmund Data Bank]]. Graphics shows [[triple point]], [[critical point (thermodynamics)|critical point]] and [[boiling point]] of water.]]
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| <div style="clear: right;" />
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| ==Table of water vapour pressures==
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| The following table list water vapour pressure in units of kPa and mmHg as a function of temperature in degrees [[Celsius]].<ref>http://www.fordhamprep.org/gcurran/sho/sho/reference/table74a.htm Water vapour pressure data</ref>
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| {| class="wikitable"
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| |-
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| ! Temperature<br>(°C) !! Vapour<br>pressure<br>(kPa) !! Vapour<br>pressure<br>(mmHg)
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| |-
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| | 0 || 0.6 || 4.5
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| |-
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| | 3 || 0.8 || 6.0
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| |-
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| | 5 || 0.9 || 6.8
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| |-
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| | 8 || 1.1 || 8.3
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| |-
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| | 10 || 1.2 || 9.0
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| |-
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| | 12 || 1.4 || 10.5
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| |-
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| | 14 || 1.6 || 12.0
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| |-
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| | 16 || 1.8 || 13.5
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| |-
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| | 18 || 2.1 || 15.8
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| |-
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| | 19 || 2.2 || 16.5
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| |-
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| | 20 || 2.3 || 17.5
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| |-
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| | 21 || 2.5 || 18.7
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| |-
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| | 22 || 2.6 || 19.8
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| |-
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| | 23 || 2.8 || 21.1
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| |-
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| | 24 || 3.0 || 22.4
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| |-
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| | 25 || 3.2 || 23.8
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| |-
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| | 26 || 3.4 || 25.2
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| |-
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| | 27 || 3.6 || 26.7
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| |-
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| | 28 || 3.8 || 28.4
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| |-
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| | 29 || 4.0 || 30.0
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| |-
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| | 30 || 4.2 || 31.5
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| |-
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| | 32 || 4.8 || 36.0
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| |-
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| | 35 || 5.6 || 42.0
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| |-
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| | 40 || 7.4 || 55.5
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| |-
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| | 50 || 12.3 || 92.3
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| |-
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| | 60 || 19.9 || 149.3
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| |-
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| | 70 || 31.2 || 234.1
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| |-
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| | 80 || 47.3 || 354.9
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| |-
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| | 90 || 70.1 || 525.9
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| |-
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| | 100 || 101.3 || 760.0
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| |}
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| === ITS-90 table ===
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| The following table is derived from [[ITS-90]] formulations of water vapour pressure.<ref>Hardy, R. 1998, ITS-90 formulations for vapor pressure, frostpoint temperature, dewpoint temperature and enhancement factors in the range -100 to +100C.</ref>
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| {| class="wikitable"
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| |-
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| ! ITS-90 Temperature<br>(°C) !! Vapour<br>pressure<br>(Pa)
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| | 0 || 611.213
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| |-
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| | 0.01 || 611.657
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| |-
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| | 2 || 705.973
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| |-
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| | 4 || 813.522
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| |-
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| | 6 || 935.316
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| |-
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| | 8 || 1072.944
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| |-
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| | 10 || 1228.139
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| |-
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| | 12 || 1402.782
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| |-
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| | 14 || 1598.914
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| |-
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| | 16 || 1818.747
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| |-
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| | 18 || 2064.670
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| |-
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| | 20 || 2339.262
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| |-
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| | 22 || 2645.302
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| |-
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| | 24 || 2985.777
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| |-
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| | 26 || 3363.893
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| |-
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| | 28 || 3783.090
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| |-
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| | 30 || 4247.046
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| |-
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| | 32 || 4759.694
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| |-
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| | 34 || 5325.230
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| |-
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| | 36 || 5948.122
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| |-
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| | 38 || 6633.128
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| |-
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| | 40 || 7385.299
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| |-
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| | 42 || 8209.997
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| |-
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| | 44 || 9112.902
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| |-
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| | 46 || 10100.025
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| |-
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| | 48 || 11177.719
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| |-
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| | 50 || 12352.690
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| |-
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| | 52 || 13632.007
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| |-
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| | 54 || 15023.115
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| |-
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| | 56 || 16533.844
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| |-
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| | 58 || 18172.420
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| |-
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| | 60 || 19947.476
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| |-
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| | 62 || 21868.064
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| |-
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| | 64 || 23943.660
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| |-
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| | 66 || 26184.180
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| |-
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| | 68 || 28599.
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| |-
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| | 70 || 31201.894
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| |-
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| | 72 || 34001.191
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| |-
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| | 74 || 37009.635
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| |-
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| | 76 || 40239.470
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| |-
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| | 78 || 43703.430
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| |-
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| | 80 || 47414.751
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| |-
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| | 82 || 51387.177
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| |-
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| | 84 || 55634.969
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| |-
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| | 86 || 60172.910
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| |-
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| | 88 || 65016.317
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| |-
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| | 90 || 70181.042
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| |-
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| | 92 || 75683.482
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| |-
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| | 94 || 81540.588
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| |-
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| | 96 || 87769.863
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| |-
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| | 98 || 94389.378
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| |-
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| | 99.97436 || 101325.000
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| |-
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| | 100 || 101417.770
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| |}
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| ==See also==
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| *[[Gas laws]]
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| *[[Molar mass]]
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| *[[Goff-Gratch equation]]
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| 100 degree
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| ==References==
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| {{reflist}}
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| ==Further reading==
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| *{{cite book
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| |author=Garnett, Pat; Anderton, John D; Garnett, Pamela J
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| |title=Chemistry Laboratory Manual For Senior Secondary School | publisher = Longman | year = 1997 | isbn = 0-582-86764-9}}
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| * Murphy, D. M. and Koop, T. (2005): Review of the vapour pressures of ice and supercooled water for atmospheric applications, Quarterly Journal of the Royal Meteorological Society 131(608): 1539–1565. {{doi|10.1256/qj.04.94}}
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| ==External links==
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| * [http://cires.colorado.edu/~voemel/vp.html Web Page listing various vapour pressure equations]
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| [[Category:Thermodynamic properties]]
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| [[Category:Water in gas]]
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| [[Category:Atmospheric thermodynamics]]
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