
35
Appendix F:
Calculating the %O
2
in Air
The oxygen concentration in dry air (zero humidity) is 20.945 and decreases with increasing
humidity. The %O2 in air can be calculated from the barometric pressure (Pb), the relative
humidity (Rh), and the saturated water vapor pressure (Pws) by using the following formula:
%O2 = 20.945% x (Pb - Pws x (Rh/100)) / Pb
The saturated water vapor pressure (Pws) is a function of the ambient temperature (Ta) and is
given in the table below.
Ta
(
°
C)
0
1
2
3
4
5
6
7
8
9
Pws
(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 58.340 61.500 64.800 68.260 71.880 75.650 79.600 83.710 88.020
50
92.510 97.200 102.09 107.20 112.51 118.04 123.80 129.82 136.08 142.60
60 149.38 156.43 163.77 171.38 179.31 187.54 196.09 204.96 214.17 223.73
70 233.70 243.90 254.60 265.70 277.20 289.10 301.40 314.10 327.30 341.00
80
355.10 369.70 384.90 400.60 416.80 433.60 450.90 468.70 487.10 506.10
90 525.76 546.05 566.99 588.60 610.90 633.90 657.62 682.07 707.27 733.24
Example: At Ta = 19
°
C, Pws = 16.477mmHg.