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A technical question to get you thinking: Do oxygen analysers require an=20
altitude correction factor?

I am lead to believe that analysers detect the amount of oxygen present=20
through an electrical resistance process, and use this input to determine=
=20
a percentage. For example, at sea level in air the sensor should detect a=
=20
concentration equal to a partial pressure of about 0.21 bar. If it is=20
assumed that atmospheric pressure is about 1 bar, the percentage oxygen=20
is 21%. The big question is whether or not the meter actually measures=20
ambient pressure for the calculation, or whether it simply assumes a sea=20
level pressure.

Supposing it doesn=92t actually measure ambient pressure, and that we are=
=20
at an altitude where ambient pressure is 0.8 bar. If we measure the=20
oxygen content of an EAN40 mix, the partial pressure of oxygen will be=20
0.4 x 0.8 =3D 0.32 bar. If the meter is assuming a 1 bar ambient pressure=
,=20
the EAN40 mix will be shown as EAN32!

I do realise that most meters come with a "zero" adjustment, but this may=
=20
not be enough in this case. Suppose we "zero" the meter to read 21% in=20
air at  an ambient pressure of 0.8 bar. The actual partial pressure of=20
oxygen is 0.21 x 0.8 =3D 0.168 (approx.  0.17 bar). If we then analyse an=
=20
EAN40 mix, the partial pressure is 0.32 bar, as above. The meter thus=20
reads an increase of 0.15 bar over the initial reading, and assumes the=20
partial pressure (accounting for the zero point change) to be 0.15 + 0.21=
=20
=3D 0.36 bar. If the meter does not sense ambient pressure, it should the=
n=20
display 36%!

Can anyone shed any light on this subject? It may seem unimportant to=20
most of you, but we dive at an altitude of 5000 ft above sea level here.

Thanks

Mike Beresford
mikeb@gl*.co*.za*



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