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"Shimell, David (shimell)" wrote:

> All
>
> I recently travelled from the UK to South Africa bringing over my O2 Analyser
> - a Vandergraph with Teledyne O2 sensor.  On arriving home in Johannesburg I
> started playing with my equipment. (as one does).  I noticed that I had to
> adjust the calibration dial to about the 2 O'clock position whereas
> previously it would calibrate at the 12 O'clock position.  Any ideas as to
> why?
>
> I was at around sea level in the UK, and Johannesburg is at about 1500m I
> think.  The equipment was carried as hand luggage on the plane and the 2
> O'clock calibration was repeated and verified 24 hours later.
>
> I wondered whether the quantity of O2 was lower at altitude but someone I
> spoke to said this was insufficient to make a difference.  I can only think
> that the ambient pressure is the reason but cannot understand why.
> Alternatively, the galvanic cell could have been affected by the flight.
> Just curious.

The usual galvanic cells do not measure the O2 percentage they meassure the
partial pressure of the oxygen. When you travel to altitude the ambient pressure
and therefore the ppO2 of the air drops. At this altitude one might expect a
ppO2 of 0.850bar*21% = 0.178 bar.
That means your analyzer should read about 17.8 % on the 12o'clock position.

Regards,

Frank


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