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This section provides information on the conversion of water vapor pressure values to more commonly
used terms, such as parts per million by volume, dew point, mg/NL, and lbs/MMscf.
Water Vapor Pressure and Temperature
Most tables used to express the concentration of water vapor are based on vapor pressure data of liquid
and solid H
O (water and ice). The concentration of water vapor above the liquid or solid is known as the
2
water vapor pressure. Tables expressing the vapor pressure above ice and water at various temperatures
may be found in the International Critical Tables, chemical handbooks, or other engineering handbooks.
At saturation pressures above 301 kPa absolute (45 psia), non-ideal gas corrections are required.
parts per million by volume =
Table I: Saturation Vapor Pressure of Water Vapor with Respect to Temperature
0
Temp °C
– 70
0.0019
– 60
0.0080
– 50
0.0294
– 40
0.0964
– 30
0.2878
– 20
0.7834
– 10
1.9643
– 0
4.5802
0
4.5802
10
9.210
20
17.548
30
31.860
40
55.396
50
92.639
60
149.57
70
233.91
80
355.40
90
525.97
100
760.00
Absolute Humidity
The absolute humidity of water vapor in air or other gases is defined as the pressure of water vapor or as
the weight per volume of water vapor. It is expressed in various units such as grams per liter, grams per
cubic foot, or millimeters of mercury.
Numerical values of water vapor pressure above water and ice are slightly higher if the system contains
air or another atmospheric gas. The difference of water vapor pressure above pure liquid water or solid
ice is in the same system, with air present, is small. When this small difference is ignored, the absolute
humidity values of water vapor pressures in air is equal to the water vapor pressures above water and ice.
Various Unit Conversions
water partial pressure (Pv) psia @ temperature
————————————————————— x 10
total pressure (Pt) psia
Pressure (mm Hg)
1
2
3
------
------
------
0.0070
0.0061
0.0053
0.0260
0.0229
0.0202
0.0859
0.0765
0.0680
0.2591
0.2231
0.2094
0.7115
0.6456
0.5854
1.7979
1.6444
1.5029
4.2199
3.8868
3.5775
4.924
5.921
5.682
9.84
10.521
11.235
18.665
19.844
21.087
33.735
35.705
37.775
58.417
61.580
64.889
97.33
102.23
107.33
156.62
163.96
171.59
244.21
254.88
265.96
370.03
385.16
400.81
546.26
567.20
588.80
4
5
6
------
------
------
0.0046
0.0040
0.0035
0.0178
0.0156
0.137
0.0605
0.0537
0.0476
0.1880
0.1686
0.1511
0.5303
0.4800
0.4341
1.3726
1.2525
1.1421
3.2907
3.0248
2.7785
6.098
6.541
7.012
11.992
12.794
13.642
22.398
23.780
25.235
39.947
42.227
44.619
68.350
71.968
75.751
112.66
118.20
123.98
179.52
187.75
196.31
277.43
289.32
301.63
416.99
433.71
540.99
611.08
634.06
657.75
6
7
8
9
------
------
------
0.0030
0.0026
0.0022
.0120
0.0105
0.0092
0.0421
0.0373
0.0329
0.1352
0.1209
0.1080
0.3923
0.3541
0.3194
1.0406
0.9474
0.8618
2.5505
2.3395
2.1445
7.513
8.045
8.610
14.539
15.487
16.489
26.767
28.380
30.076
47.127
49.756
52.510
79.703
83.830
88.140
130.00
136.26
142.78
205.19
214.41
223.98
314.38
327.59
341.25
468.84
487.28
506.32
682.18
707.35
733.28
Software Confjiruration D-1
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