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> Date: Thu, 17 Apr 97 09:14:46 -0700
> From: Mike Beresford <mikeb@gl*.co*.za*>
> To: techdiver@aquanaut.com
> Subject: O2 Analysers : Altitude Correction required?
Mike,
Oxygen analysers don't require an altitude correction and
I am under the impression this is why:
The reason the analyser can read correctly regardless of altitude is because
the fuel cell is a liquid filled cell with a flexible membrane where the gas
goes
through the fuel cell. When there is no gas flow the pressure on either
side of the membrane is the same because the liquid is not
compressable and the membrane is flexible. The calibrating of the
analyser is then just setting a reference point on a linear scale.
Once that is done, all readings should be correct and linear.
Ted Green
> A technical question to get you thinking: Do oxygen analysers require an
> altitude correction factor?
>
> I am lead to believe that analysers detect the amount of oxygen present
> through an electrical resistance process, and use this input to determine
> a percentage. For example, at sea level in air the sensor should detect a
> concentration equal to a partial pressure of about 0.21 bar. If it is
> assumed that atmospheric pressure is about 1 bar, the percentage oxygen
> is 21%. The big question is whether or not the meter actually measures
> ambient pressure for the calculation, or whether it simply assumes a sea
> level pressure.
>
> Supposing it doesn't actually measure ambient pressure, and that we are
> at an altitude where ambient pressure is 0.8 bar. If we measure the
> oxygen content of an EAN40 mix, the partial pressure of oxygen will be
> 0.4 x 0.8 = 0.32 bar. If the meter is assuming a 1 bar ambient pressure,
> the EAN40 mix will be shown as EAN32!
>
> I do realise that most meters come with a "zero" adjustment, but this may
> not be enough in this case. Suppose we "zero" the meter to read 21% in
> air at an ambient pressure of 0.8 bar. The actual partial pressure of
> oxygen is 0.21 x 0.8 = 0.168 (approx. 0.17 bar). If we then analyse an
> EAN40 mix, the partial pressure is 0.32 bar, as above. The meter thus
> reads an increase of 0.15 bar over the initial reading, and assumes the
> partial pressure (accounting for the zero point change) to be 0.15 + 0.21
> = 0.36 bar. If the meter does not sense ambient pressure, it should then
> display 36%!
>
> Can anyone shed any light on this subject? It may seem unimportant to
> 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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>
Ted Green (owner)
Tidewater Aquatics (Dive Store)
Salisbury Maryland USA
TDI IT #029
SSI MI #178
The world contains but three types of people:
1. Those who make things happen.
2. Those who watch things happen.
3. Those who wonder what happened.
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