Rob,

Was going to add this in previous note, but neglected.

If the fog has cleared from my noggin' this morning, the atmosphere contains
~21% oxygen.  That is ~210,000 PPM

I have read various numbers for what minimum allowable percent oxygen is
allowable for human safety.  I believe that it was 18-19.5% for a "no
effect" minimum permissible oxygen level.  So that is 180-195,000 PPM
oxygen.  That is why the sensors used for determining whether an atmosphere
is breathable or not, are not suitable for what you desire to do - they are
just not anywhere near sensitive enough.

Hope this helps


Steve Creswick
Sr Associate - Balanced Enterprise Solutions
http://www.linkedin.com/in/stevencreswick
                         616 834 1883




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