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 ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________