14
Appendix A: Calculating the %O
2
in Air
The oxygen concentration in dry air (zero humidity) is 20.945 and decreases with increasing
humidity. The %O
2
in air can be calculated from the barometric pressure (P
b
, in mmHg), the
relative humidity (Rh), and the saturated water vapor pressure (P
ws
, in mmHg) by using the
following formula:
%O
2
= 20.945% x (P
b
- P
ws
x (Rh/100)) / P
b
The saturated water vapor pressure (P
ws
) is a function of the ambient temperature (T
a
) and is
given in the table below. For example, at 21
C, P
ws
= 18.65 mmHg.
T
a
(
C)
0
1
2
3
4
5
6
7
8
9
P
ws
(mm Hg)
0
4.579
4.926
5.294
5.685
6.101
6.543
7.013
7.513
8.045
8.609
10
9.209
9.844
10.518
11.231
11.987
12.788
13.634
14.530
15.477
16.477
20
17.535
18.650
19.827
21.068
22.377
23.756
25.209
26.739
28.349
30.043
30
31.824
33.695
35.663
37.729
39.898
42.175
44.563
47.067
49.692
52.442
40
55.324
58.34
61.50
64.8
68.26
71.88
75.65
79.60
83.71
88.02
50
92.51
97.2
102.09
107.2
112.51
118.04
123.80
129.82
136.08
142.60
60
149.38
156.43
163.77
171.38
179.31
187.54
196.09
204.96
214.17
223.73
70
233.7
243.9
254.6
265.7
277.2
289.1
301.4
314.1
327.3
341.0
80
355.1
369.7
384.9
400.6
416.8
433.6
450.9
468.7
487.1
506.1
90
525.76
546.05
566.99
588.60
610.90
633.9
657.62
682.07
707.27
733.24
1 mmHg = 0.01934 lbf/in2 = 1 torr
1 atm = 14.696 lbf/in2 = 760 torr = 101325 N/m2