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Saturation Vapor Pressure

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Saturation vapor pressure is a unique function of temperature as given in Table 5.6. Each temperature in the table may be interpreted as a dew point temperature, because as the ground cools, dew will begin to form at the temperature corresponding to the vapor pressure.

Table 5.6: Saturated Vapor Pressure of Air at Various Temperature
(C) Temp (F) | Sat Vapor Pressure (mb) | (C) Temp (F) | Sat Vapor Pressure (mb)
-18
-15
-12
-09
-07
-04
-01
02
04
07
10
13
16 00
05
10
15
20
25
30
35
40
45
50
55
60 1.5
1.9
2.4
3.0
3.7
4.6
5.6
6.9
8.4
10.2
12.3
14.8
17.7 |
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|
|
|
|
|
|
|
|
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|
| 18
21
24
27
29
32
35
38
41
43
46
49
52 65
70
75
80
85
90
95
100
105
110
115
120
125 21.0
25.0
29.6
35.0
41.0
48.1
56.2
65.6
76.2
87.8
101.4
116.8
134.2
Chart
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5.4.4 Absolute saturation (humidity) or percentage saturation (humidity)
This is define as the ratio of the number of moles of vapor per number of moles of vapor-free gas to the number of moles of vapor that would be present per mole of vapor-free gas if the mixture were completely saturated at a given temperature and total pressure.

Absolute Saturation (AS)=((number of moles of vapor)/(number of moles of vapor-free gas ))_actual/((number of moles of vapor)/(number of moles of vapor-free gas ))_saturated … (5.34)

Let subscripts 1 and 2 represent vapor and vapor-free gas respectively, then we have:

Percentage Absolute Saturation=(n_1/(n_2 ))_actual/(n_1/(n_2 ))_saturated ×100=(p_1/(p_2 ))_actual/(p_1/(p_2 ))_saturated ×100 … (5.35)

Let p1 saturated be represented by p_1^* or p_t=p_1+p_2 so that:

% AS=((p_1/(p_t-p_1 )))/(((p_1^*)/(p_t-p_1^* )) )×100=p_1/(p_1^* ) ((p_t-p_1^*)/(p_t-p_1 ))×100 …
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(5.37). Figure 5.5 show the relationships between relative humidity, wet and dry
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