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At 01:51 AM 5/14/99 -0700, Shimell, David (shimell) wrote:

Hello David,

>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.

Absolutely the sensor is affected by altitude.  The sensor
responds to variations in PO2.  Even though the FO2 in
Johannesburg is the same as in the UK, the PO2 is different
and that's really what the sensor is measuring.  The reading
from the sensor is normalized to an FO2 for the display.
That's one of the reasons for the calibration control.  The
system is so sensitive that many people use a calibrated
flow-meter or fixed orifice to make sure that the pressure
over the sensor is always consistent.  Even slight changes
in the pressure over the sensor can cause significant changes
in the reading.

There's no reason that the system would be damaged by your
flight, but if you're not confident that the system is working
correctly, try calibrating it on a tank known to be filled
with air, then try measuring a tank known to be filled with
some enriched air mix or even with 100% O2 and confirm that
it's operating properly.  Note that most O2 sensors have an
asymptotic response.  If you're measuring 100% O2, the reading
will initially converge on 100% very quickly, then it will
slow down and may take a long time to finally reach 100% or
it might even settle around 99% or so and never reach 100%.
This is a normal behavior.

-Mike Rodriguez
<mikey@ma*.co*>

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