Mike,
Analyzers will display the approximate O2 percentage of air at sea level =
to be 20.9 (+/- 1%).
When you take the analyzer to altitude however, (such as in an airplane =
that is not equipped with a pressurized cabin) it will display the =
partial pressure equivalent of air at a given altitude. i.e.:
On a flight I made to Houston from Ft. Lauderdale in a Cessna 210 =
averaging 19,000ft altitude my mini-ox read 12.0  in the cabin. When I =
used the sensor in my O2 mask to confirm a sufficient supply of =
supplemental O2 was present it read 64.4. Hmmmmmmmmm. I was feeding the =
mask 100% O2 at a 5 liter per minute flow however the ambient pressure =
was so much lower than sea level pressure it read a much lesser =
percentage than 100%. So, if you calibrate from two significantly =
different gas mixtures and consider the partial pressure for the =
altitude you are at you can get reliable information. Underwater the =
partial pressure will increase quite rapidly with your descent,  so the =
slight surface discrepancy should only be a problem after you surface.
Use Voyager software to run some "what if" scenarios before you make any =
decisions. If I can be of any other help feel free to contact me by =
e-mail :  divetek@oc*.co* or call 800-874-6888.
Sempre Deep,
Capt. Jim Mims   =20
IANTD/ B.O.A.
-----Original Message-----
From:	Ted Green [SMTP:scuba@md*.co*]
Sent:	Thursday, April 17, 1997 1:25 PM
To:	techdiver@aquanaut.com; Mike Beresford
Subject:	Re: O2 Analysers : Altitude Correction required?

> 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,=20
        Oxygen analysers don't require an altitude correction and=20
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=20
through the fuel cell. When there is no gas flow the pressure on either=20
side of the membrane is the same because the liquid is not=20
compressable and the membrane is flexible. The calibrating of the=20
analyser is then just setting a reference point on a linear scale.=20
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=20
> altitude correction factor?
>=20
> 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 =

> ambient pressure for the calculation, or whether it simply assumes a =
sea=20
> level pressure.
>=20
> Supposing it doesn't 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 =

> 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!
>=20
> 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 =

> air at  an ambient pressure of 0.8 bar. The actual partial pressure of =

> 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 =
then=20
> display 36%!
>=20
> 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.
>=20
> Thanks
>=20
> Mike Beresford
> mikeb@gl*.co*.za*
>=20
>=20
>=20
> --
> Send mail for the `techdiver' mailing list to =
`techdiver@aquanaut.com'.
> Send list subscription requests to `techdiver-request@aquanaut.com'.
>=20
>=20
Ted Green (owner)               =20
Tidewater Aquatics (Dive Store) =20
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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